Flatten the 9P control tree and move it out of fs.zig

The served tree loses the self/ level: /index /ctl /new /log /screen
/listeners /pane/<n>/... /os, with /src only in -Dembed-sources=true builds
(default off, on for esp32p4). ctl speaks the editor's own language with two
lowercase verbs, look TEXT and exec TEXT, plus acme's addr verbs; the new/
factory directory becomes one clone file; cons is gone (exec Msg); name and
sel are files; stats report real lengths, modes and mtimes; /log streams
pane new/del/rename/save events. The tree code lives in src/ninep/
(tree, pane, ctl, addr, pty, events, screen, sources); fs.zig keeps host
access, mounts, resolution and find/grep. Same engine and transports.
README (fs-help.txt) and docs rewritten; tests updated and extended.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

9 files changed, 4053 insertions, 0 deletions

diff --git a/src/ninep/addr.zig b/src/ninep/addr.zig
new file mode 100644
index 00000000..f7642b9e
--- /dev/null
+++ b/src/ninep/addr.zig
@@ -0,0 +1,276 @@
+//! The address language of a pane's `addr` file: acme's, with mvzr regexps.
+const std = @import("std");
+const mvzr = @import("mvzr");
+const modal = @import("../modal.zig");
+const pane_files = @import("pane.zig");
+
+pub const Range = pane_files.State.Range;
+
+fn clip(n: usize) u32 {
+    return std.math.cast(u32, n) orelse std.math.maxInt(u32);
+}
+
+fn safePattern(pat: []const u8) bool {
+    var i: usize = 0;
+    while (i < pat.len) : (i += 1) {
+        if (pat[i] != '\\') continue;
+        if (i + 1 >= pat.len) return false;
+        i += 1;
+    }
+    return true;
+}
+
+pub const Addr = struct {
+    text: []const u8,
+    lim: ?Range,
+    expr: []const u8,
+    i: usize = 0,
+    depth: u8 = 0,
+
+    const max_depth = 32;
+    const Size = enum { char, line };
+
+    pub fn address(a: *Addr, ar_in: Range) ?Range {
+        const start = a.i;
+        var ar = ar_in;
+        var r = ar_in;
+        var dir: u8 = 0;
+        var size: Size = .line;
+        var c: u8 = 0;
+        while (a.i < a.expr.len) {
+            const prevc = c;
+            c = a.expr[a.i];
+            a.i += 1;
+            switch (c) {
+                ',', ';' => {
+                    if (c == ';') ar = r;
+                    if (prevc == 0) r.q0 = 0; // lhs defaults to 0
+                    if (a.i >= a.expr.len) {
+                        r.q1 = clip(a.text.len); // rhs defaults to $
+                    } else {
+                        if (a.depth >= max_depth) return null;
+                        a.depth += 1;
+                        const nr = a.address(ar) orelse return null;
+                        a.depth -= 1;
+                        r.q1 = nr.q1;
+                    }
+                    return r;
+                },
+                '+', '-' => {
+                    if (prevc == '+' or prevc == '-') {
+                        const nc = if (a.i < a.expr.len) a.expr[a.i] else 0;
+                        if (nc != '#' and nc != '/' and nc != '?')
+                            r = a.number(r, 1, prevc, .line) orelse return null;
+                    }
+                    dir = c;
+                },
+                '.', '$' => {
+                    if (a.i != start + 1) {
+                        a.i -= 1;
+                        return r;
+                    }
+                    r = if (c == '.') ar else .{ .q0 = clip(a.text.len), .q1 = clip(a.text.len) };
+                    dir = if (a.i < a.expr.len) '+' else 0;
+                },
+                '#', '0'...'9' => {
+                    var digit = c;
+                    if (c == '#') {
+                        if (a.i >= a.expr.len or a.expr[a.i] < '0' or a.expr[a.i] > '9') {
+                            a.i -= 1;
+                            return r;
+                        }
+                        digit = a.expr[a.i];
+                        a.i += 1;
+                        size = .char;
+                    }
+                    var n: u64 = digit - '0';
+                    while (a.i < a.expr.len) : (a.i += 1) {
+                        const d = a.expr[a.i];
+                        if (d < '0' or d > '9') break;
+                        n = @min(n * 10 + (d - '0'), std.math.maxInt(u32));
+                    }
+                    r = a.number(r, @intCast(n), dir, size) orelse return null;
+                    dir = 0;
+                    size = .line;
+                },
+                '/', '?' => {
+                    const back = c == '?';
+                    r = a.regexp(r, a.pattern(c), back) orelse return null;
+                    dir = 0;
+                    size = .line;
+                },
+                else => {
+                    a.i -= 1;
+                    return r;
+                },
+            }
+        }
+        if (dir != 0) r = a.number(r, 1, dir, .line) orelse return null;
+        return r;
+    }
+
+    fn pattern(a: *Addr, delim: u8) []const u8 {
+        const s = a.i;
+        while (a.i < a.expr.len) {
+            const c = a.expr[a.i];
+            if (c == '\n') break;
+            a.i += 1;
+            if (c == '\\') {
+                if (a.i < a.expr.len) a.i += 1;
+                continue;
+            }
+            if (c == delim) return a.expr[s .. a.i - 1];
+        }
+        return a.expr[s..a.i];
+    }
+
+    fn number(a: *Addr, r_in: Range, n: u32, dir: u8, size: Size) ?Range {
+        var r = r_in;
+        if (size == .char) {
+            var off: i64 = n;
+            if (dir == '+') {
+                off = @as(i64, r.q1) + n;
+            } else if (dir == '-') {
+                if (r.q0 == 0 and n > 0) r.q0 = clip(a.text.len);
+                off = @as(i64, r.q0) - n;
+            }
+            if (off < 0 or off > @as(i64, @intCast(a.text.len))) return null;
+            const g = clip(modal.graphemeStart(a.text, @intCast(off)));
+            return .{ .q0 = g, .q1 = g };
+        }
+        var line: i64 = n;
+        var q0: usize = r.q0;
+        var q1: usize = r.q1;
+        switch (dir) {
+            '-' => {
+                if (q0 < a.text.len) while (q0 > 0 and a.text[q0 - 1] != '\n') {
+                    q0 -= 1;
+                };
+                q1 = q0;
+                while (line > 0 and q0 > 0) {
+                    if (a.text[q0 - 1] == '\n') {
+                        line -= 1;
+                        q1 = q0;
+                    }
+                    q0 -= 1;
+                }
+                if (line > 1) return null;
+                while (q0 > 0 and a.text[q0 - 1] != '\n') q0 -= 1;
+                return .{ .q0 = clip(q0), .q1 = clip(q1) };
+            },
+            '+' => {
+                if (q1 > 0) while (q1 < a.text.len and a.text[q1 - 1] != '\n') {
+                    q1 += 1;
+                };
+                q0 = q1;
+            },
+            else => {
+                q0 = 0;
+                q1 = 0;
+            },
+        }
+        while (line > 0 and q1 < a.text.len) {
+            const ch = a.text[q1];
+            q1 += 1;
+            if (ch == '\n' or q1 == a.text.len) {
+                line -= 1;
+                if (line > 0) q0 = q1;
+            }
+        }
+        if (line > 0) return null;
+        return .{ .q0 = clip(q0), .q1 = clip(q1) };
+    }
+
+    fn regexp(a: *Addr, r: Range, pat: []const u8, back: bool) ?Range {
+        if (pat.len == 0 or !safePattern(pat)) return null;
+        const re = mvzr.compile(pat) orelse return null;
+        if (back) {
+            const hi = @min(@as(usize, r.q0), a.text.len);
+            var best: ?mvzr.Match = null;
+            var at: usize = 0;
+            while (at < hi) {
+                const m = re.matchPos(at, a.text[0..hi]) orelse break;
+                best = m;
+                at = if (m.end > m.start) m.end else m.end + 1;
+            }
+            const m = best orelse return null;
+            return .{ .q0 = clip(m.start), .q1 = clip(m.end) };
+        }
+        const hi = if (a.lim) |l| @min(@as(usize, l.q1), a.text.len) else a.text.len;
+        const from = @min(@as(usize, r.q1), hi);
+        const m = re.match(a.text[from..hi]) orelse return null;
+        return .{ .q0 = clip(from + m.start), .q1 = clip(from + m.end) };
+    }
+};
+
+const testing = std.testing;
+const th = @import("testing.zig");
+const tree = @import("tree.zig");
+const Node = tree.Node;
+const E = tree.E;
+const Status = tree.Status;
+
+test "the address language, form by form" {
+    const gpa = testing.allocator;
+    const p = try th.withFile(gpa, "one\ntwo\nthree\n");
+    defer p.deinit();
+    const serial = th.serialOf(p);
+    const addr = Node.of(serial, .addr);
+
+    const Case = struct { expr: []const u8, q0: u32, q1: u32 };
+    for ([_]Case{
+        .{ .expr = "#0", .q0 = 0, .q1 = 0 },
+        .{ .expr = "#5", .q0 = 5, .q1 = 5 },
+        .{ .expr = "0", .q0 = 0, .q1 = 0 },
+        .{ .expr = "1", .q0 = 0, .q1 = 4 },
+        .{ .expr = "2", .q0 = 4, .q1 = 8 },
+        .{ .expr = "$", .q0 = 14, .q1 = 14 },
+        .{ .expr = ",", .q0 = 0, .q1 = 14 },
+        .{ .expr = "1,2", .q0 = 0, .q1 = 8 },
+        .{ .expr = "#1,#4", .q0 = 1, .q1 = 4 },
+        .{ .expr = "2+1", .q0 = 8, .q1 = 14 },
+        .{ .expr = "$-1", .q0 = 8, .q1 = 14 },
+        .{ .expr = "/two/", .q0 = 4, .q1 = 7 },
+        .{ .expr = "/t.o/", .q0 = 4, .q1 = 7 },
+        .{ .expr = "1\n", .q0 = 0, .q1 = 4 },
+    }) |c| {
+        _ = th.wr(p, addr, "#0");
+        const w = th.wr(p, addr, c.expr);
+        try testing.expectEqual(Status.ok, w.reply.status);
+        const got = th.rd(p, addr, 0, 64);
+        var want: [32]u8 = undefined;
+        try testing.expectEqualStrings(
+            try std.fmt.bufPrint(&want, "{d:>11} {d:>11} ", .{ c.q0, c.q1 }),
+            got.bytes,
+        );
+    }
+
+    _ = th.wr(p, addr, "1");
+    _ = th.wr(p, addr, ".");
+    try testing.expectEqual(@as(u32, 0), p.fs.panes[0].addr.q0);
+    try testing.expectEqual(@as(u32, 4), p.fs.panes[0].addr.q1);
+
+    _ = th.wr(p, addr, "$");
+    _ = th.wr(p, addr, "?o?");
+    try testing.expectEqual(@as(u32, 6), p.fs.panes[0].addr.q0); // the `o` in "two"
+    try testing.expectEqual(@as(u32, 7), p.fs.panes[0].addr.q1);
+
+    _ = th.wr(p, addr, "1");
+    _ = th.wr(p, Node.of(serial, .ctl), "limit=addr\n");
+    _ = th.wr(p, addr, "#0");
+    try testing.expectEqual(E.INVAL, th.wr(p, addr, "/three/").errno());
+    _ = th.wr(p, Node.of(serial, .ctl), "clean\n");
+    _ = th.call(p, .{ .tag = 6, .op = .open, .node = Node.of(serial, .ctl) });
+    try testing.expect(p.fs.panes[0].limit == null);
+    _ = th.wr(p, addr, "#0");
+    try testing.expectEqual(Status.ok, th.wr(p, addr, "/three/").reply.status);
+
+    for ([_][]const u8{ "zzz", "#", "//", "/nomatch/", "1 2", "99", "/a\\" }) |bad| {
+        _ = th.wr(p, addr, "#0");
+        try testing.expectEqual(E.INVAL, th.wr(p, addr, bad).errno());
+    }
+
+    const nested = "," ** 4096;
+    _ = th.wr(p, addr, "#0");
+    try testing.expectEqual(E.INVAL, th.wr(p, addr, nested).errno());
+}
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
new file mode 100644
index 00000000..d61ebf75
--- /dev/null
+++ b/src/ninep/ctl.zig
@@ -0,0 +1,503 @@
+//! The two ctl files. `look TEXT` is a right click and `exec TEXT` a middle
+//! click in the editor's own command language; a pane's ctl adds the addr
+//! verbs. Reading /ctl answers the serials the last command made or touched.
+const std = @import("std");
+const pardes = @import("../pardes.zig");
+const panes = @import("../panes.zig");
+const config = @import("../config.zig");
+const fs = @import("../fs.zig");
+const tree = @import("tree.zig");
+const pane_files = @import("pane.zig");
+
+const Pardes = pardes.Pardes;
+const Pane = pardes.Pane;
+const MAX_PANES = pardes.MAX_PANES;
+const Req = tree.Req;
+const Reply = tree.Reply;
+const E = tree.E;
+
+const Verb = enum {
+    @"addr=dot",
+    clean,
+    cleartag,
+    dirty,
+    @"dot=addr",
+    get,
+    @"limit=addr",
+    mark,
+    nomark,
+    noscroll,
+    scroll,
+    show,
+};
+
+fn verbIs(line: []const u8, word: []const u8) bool {
+    if (!std.mem.startsWith(u8, line, word)) return false;
+    return line.len == word.len or line[word.len] == ' ' or line[word.len] == '\t';
+}
+
+/// The text after `look` or `exec`, or null when the line is not that verb.
+/// Control characters other than tab are refused.
+fn textAfter(line: []const u8, verb: []const u8) ?[]const u8 {
+    if (!verbIs(line, verb)) return null;
+    const text = std.mem.trim(u8, line[verb.len..], " \t");
+    if (text.len == 0) return "";
+    for (text) |c| if (c < ' ' and c != '\t') return "";
+    return text;
+}
+
+// ---- which panes a command made or touched ----
+
+const Serials = struct {
+    list: [MAX_PANES]u32 = undefined,
+    len: usize = 0,
+
+    fn snapshot(p: *Pardes) Serials {
+        var s: Serials = .{};
+        for (p.panes) |slot| if (slot) |pane| {
+            s.list[s.len] = pane.serial;
+            s.len += 1;
+        };
+        return s;
+    }
+
+    fn has(s: *const Serials, serial: u32) bool {
+        for (s.list[0..s.len]) |known| if (known == serial) return true;
+        return false;
+    }
+};
+
+fn noteResult(p: *Pardes, serial: u32) void {
+    for (p.fs.results[0..p.fs.results_len]) |known| if (known == serial) return;
+    if (p.fs.results_len == p.fs.results.len) return;
+    p.fs.results[p.fs.results_len] = serial;
+    p.fs.results_len += 1;
+}
+
+fn activeSerial(p: *Pardes) ?u32 {
+    const pane = p.panes[p.active] orelse return null;
+    return pane.serial;
+}
+
+/// Runs one look or exec from the pane `id`, then records the panes it
+/// created; failing that, a look records the pane it focused and an exec
+/// the pane it acted on (even one it closed).
+fn run(p: *Pardes, id: usize, exec: bool, text: []const u8) void {
+    const context = p.panes[id].?.serial;
+    const before = Serials.snapshot(p);
+    if (exec) _ = p.execute(id, text) else p.lookAt(id, text);
+    const after = Serials.snapshot(p);
+    var created = false;
+    for (after.list[0..after.len]) |serial| if (!before.has(serial)) {
+        noteResult(p, serial);
+        created = true;
+    };
+    if (created) return;
+    noteResult(p, if (exec) context else activeSerial(p) orelse return);
+}
+
+fn beginResults(p: *Pardes) void {
+    p.fs.results_len = 0;
+    p.fs.results_valid = true;
+}
+
+// ---- /ctl ----
+
+pub fn writeRoot(p: *Pardes, req: Req) Reply {
+    for ([2]bool{ false, true }) |apply| {
+        if (apply) beginResults(p);
+        var it = std.mem.splitScalar(u8, req.data, '\n');
+        while (it.next()) |raw| {
+            const line = std.mem.trim(u8, raw, " \t\r");
+            if (line.len == 0) continue;
+            const exec = verbIs(line, "exec");
+            const text = textAfter(line, if (exec) "exec" else "look") orelse return Reply.fail(req.tag, E.INVAL);
+            if (text.len == 0) return Reply.fail(req.tag, E.INVAL);
+            if (!apply) continue;
+            if (p.panes[p.active] == null) return Reply.fail(req.tag, E.NOENT);
+            run(p, p.active, exec, text);
+        }
+    }
+    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
+}
+
+pub fn rootText(p: *Pardes, buf: []u8) []const u8 {
+    var w = std.Io.Writer.fixed(buf);
+    if (p.fs.results_valid) {
+        for (p.fs.results[0..p.fs.results_len]) |serial| w.print("{d}\n", .{serial}) catch break;
+        return w.buffered();
+    }
+    var count: usize = 0;
+    for (p.panes) |slot| count += @intFromBool(slot != null);
+    const pid: u32 = if (comptime pardes.hosted) @intCast(std.c.getpid()) else 0;
+    w.print("pid {d}\nversion {s}\npanes {d}\n", .{ pid, pardes.version, count }) catch {};
+    return w.buffered();
+}
+
+pub const root_text_cap = @max(MAX_PANES * 11, 64 + pardes.version.len);
+
+pub fn readRoot(p: *Pardes, req: Req) Reply {
+    var buf: [root_text_cap]u8 = undefined;
+    const text = rootText(p, &buf);
+    const out = p.fs.stage(p.gpa);
+    out.appendSlice(p.gpa, text) catch return Reply.fail(req.tag, E.NOMEM);
+    return tree.stagedReply(p, req);
+}
+
+// ---- /pane/<n>/ctl ----
+
+fn fontName(p: *Pardes) []const u8 {
+    const name = p.settings.font.effective_name.get();
+    return if (name.len == 0) "default" else name;
+}
+
+fn writeQuoted(w: *std.Io.Writer, s: []const u8) !void {
+    const plain = s.len > 0 and for (s) |c| {
+        if (c <= ' ' or c == '\'') break false;
+    } else true;
+    if (plain) return w.writeAll(s);
+    try w.writeByte('\'');
+    for (s) |c| {
+        if (c == '\'') try w.writeByte('\'');
+        try w.writeByte(c);
+    }
+    try w.writeByte('\'');
+}
+
+pub const pane_text_cap = 7 * 12 + 2 * 256 + 4;
+
+/// acme's winctlprint: serial, tag length, body length, a reserved zero, the
+/// dirty flag, then the width in cells, the font and the tab width.
+pub fn paneText(p: *Pardes, pane: *Pane, buf: []u8) []const u8 {
+    var w = std.Io.Writer.fixed(buf);
+    w.print("{d:>11} {d:>11} {d:>11} {d:>11} {d:>11} {d:>11} ", .{
+        pane.serial,
+        pane_files.tagOf(p, pane).len,
+        pane_files.bodyOf(pane).len,
+        @as(u32, 0),
+        @intFromBool(pane_files.dirtyOf(pane)),
+        pane.cols,
+    }) catch return w.buffered();
+    writeQuoted(&w, fontName(p)) catch return w.buffered();
+    w.print(" {d:>11} ", .{config.tab_width}) catch {};
+    return w.buffered();
+}
+
+pub fn readPane(p: *Pardes, req: Req, pane: *Pane) Reply {
+    var buf: [pane_text_cap]u8 = undefined;
+    const out = p.fs.stage(p.gpa);
+    out.appendSlice(p.gpa, paneText(p, pane, &buf)) catch return Reply.fail(req.tag, E.NOMEM);
+    return tree.stagedReply(p, req);
+}
+
+pub fn writePane(p: *Pardes, req: Req, serial: u32) Reply {
+    for ([2]bool{ false, true }) |apply| {
+        if (apply) beginResults(p);
+        var it = std.mem.splitScalar(u8, req.data, '\n');
+        while (it.next()) |raw| {
+            const line = std.mem.trim(u8, raw, " \t\r");
+            if (line.len == 0) continue;
+            const live = p.paneBySerial(serial) orelse if (apply) break else return Reply.fail(req.tag, E.NOENT);
+            const errno = paneVerb(p, live, line, apply);
+            if (errno != 0) return Reply.fail(req.tag, errno);
+        }
+    }
+    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
+}
+
+fn paneVerb(p: *Pardes, id: usize, line: []const u8, apply: bool) u16 {
+    const pane = p.panes[id] orelse return E.INVAL;
+    const pf = &p.fs.panes[id];
+
+    for ([_]bool{ false, true }) |exec| if (textAfter(line, if (exec) "exec" else "look")) |text| {
+        if (text.len == 0) return E.INVAL;
+        if (apply) run(p, id, exec, text);
+        return 0;
+    };
+    const v = std.meta.stringToEnum(Verb, line) orelse return E.INVAL;
+    if (!apply) return 0;
+
+    switch (v) {
+        .@"addr=dot" => pf.addr = pane_files.dotOf(pane),
+        .@"dot=addr" => {
+            pane_files.clampAddr(pf, pane_files.bodyOf(pane).len);
+            pane_files.setDot(pane, pf.addr);
+        },
+        .@"limit=addr" => {
+            pane_files.clampAddr(pf, pane_files.bodyOf(pane).len);
+            pf.limit = pf.addr;
+        },
+        .clean => if (pane_files.fileOf(pane)) |f| {
+            f.saved_revision = f.revision;
+        },
+        .dirty => if (pane_files.fileOf(pane)) |f| {
+            f.saved_revision = f.revision -% 1;
+        },
+        .cleartag => {
+            pane.tag_tail_len = 0;
+            pane.tag_init = true;
+        },
+        .get => if (pane_files.fileOf(pane)) |f| {
+            if (panes.Output.fileTraits(f.output).saves) {
+                if (fs.read(p, f.path)) |bytes| {
+                    panes.File.pushUndo(p, pane);
+                    panes.File.setContent(p, f, bytes);
+                    f.saved_revision = f.revision;
+                } else |err| return switch (err) {
+                    error.FileNotFound => E.NOENT,
+                    else => E.IO,
+                };
+            }
+        },
+        .mark => {
+            pf.nomark = false;
+            panes.File.pushUndo(p, pane);
+        },
+        .nomark => pf.nomark = true,
+        .noscroll => pf.noscroll = true,
+        .scroll => pf.noscroll = false,
+        .show => pane_files.showOffset(pane, pane_files.dotOf(pane).q0),
+    }
+    return 0;
+}
+
+// ---- tests ----
+
+const testing = std.testing;
+const th = @import("testing.zig");
+const call = th.call;
+const rd = th.rd;
+const wr = th.wr;
+const withFile = th.withFile;
+const serialOf = th.serialOf;
+const Node = tree.Node;
+const Status = tree.Status;
+const root_ctl = @intFromEnum(tree.TopFile.ctl);
+
+test "pane ctl read is index's five fields plus width in cells, font and tab width" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "x\n");
+    defer p.deinit();
+    const pane = p.panes[0].?;
+
+    const a = rd(p, Node.of(pane.serial, .ctl), 0, 4096);
+    try testing.expectEqual(Status.ok, a.reply.status);
+    var want: std.ArrayList(u8) = .empty;
+    defer want.deinit(gpa);
+    try want.print(gpa, "{d:>11} {d:>11} {d:>11} {d:>11} {d:>11} {d:>11} {s} {d:>11} ", .{
+        pane.serial, pane_files.tagOf(p, pane).len, @as(usize, 2), 0, 0, pane.cols, "default", config.tab_width,
+    });
+    try testing.expectEqualStrings(want.items, a.bytes);
+    try testing.expectEqual(@as(u64, a.bytes.len), call(p, .{ .tag = 1, .op = .getattr, .node = Node.of(pane.serial, .ctl) }).reply.attr.size);
+
+    var quoted: [64]u8 = undefined;
+    var w = std.Io.Writer.fixed(&quoted);
+    try writeQuoted(&w, "DejaVu Sans Mono");
+    try testing.expectEqualStrings("'DejaVu Sans Mono'", w.buffered());
+    w = std.Io.Writer.fixed(&quoted);
+    try writeQuoted(&w, "it's");
+    try testing.expectEqualStrings("'it''s'", w.buffered());
+}
+
+test "every pane ctl verb, and every refusal" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "one\ntwo\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const ctl_node = Node.of(serial, .ctl);
+    const pane = p.panes[0].?;
+    const pf = &p.fs.panes[0];
+
+    try testing.expectEqual(Status.ok, wr(p, ctl_node, "nomark\nnoscroll\ndirty\n").reply.status);
+    try testing.expect(pf.nomark and pf.noscroll and pane_files.dirtyOf(pane));
+    try testing.expectEqual(Status.ok, wr(p, ctl_node, "mark\nscroll\nclean\n").reply.status);
+    try testing.expect(!pf.nomark and !pf.noscroll and !pane_files.dirtyOf(pane));
+
+    _ = wr(p, ctl_node, "cleartag\n");
+    try testing.expectEqual(@as(usize, 0), pane.tag_tail_len);
+
+    _ = wr(p, Node.of(serial, .addr), "2");
+    _ = wr(p, ctl_node, "limit=addr\n");
+    try testing.expectEqual(@as(u32, 4), pf.limit.?.q0);
+    _ = wr(p, ctl_node, "dot=addr\nshow\n");
+    try testing.expectEqual(@as(i32, 1), pane.cur_row);
+
+    for ([_][]const u8{
+        "menu",       "nomenu", "dump echo hi", "dumpdir /tmp", "font Go Mono", "lock",      "unlock", "bogus", "DEL",
+        "name x.txt", "put",    "del",          "delete",       "Look x",       "Exec Save",
+    }) |bad| try testing.expectEqual(E.INVAL, wr(p, ctl_node, bad).errno());
+
+    try testing.expect(!pane_files.dirtyOf(pane));
+    try testing.expectEqual(E.INVAL, wr(p, ctl_node, "dirty\nbogus\n").errno());
+    try testing.expect(!pane_files.dirtyOf(pane));
+}
+
+test "exec runs a builtin at the pane and records the pane it acted on" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "Msg fs-ran\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const pane = p.panes[0].?;
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "exec Msg from ctl\n").reply.status);
+    try testing.expectEqualStrings("from ctl", pane.msg[0..pane.msg_len]);
+    var expected: [16]u8 = undefined;
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&expected, "{d}\n", .{serial}), rd(p, root_ctl, 0, 4096).bytes);
+
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "exec New\n").reply.status);
+    const made = p.panes[p.active].?;
+    try testing.expect(made.serial != serial);
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&expected, "{d}\n", .{made.serial}), rd(p, root_ctl, 0, 4096).bytes);
+
+    try testing.expectEqual(Status.ok, wr(p, Node.of(made.serial, .ctl), "exec Del\n").reply.status);
+    try testing.expect(p.paneBySerial(made.serial) == null);
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&expected, "{d}\n", .{made.serial}), rd(p, root_ctl, 0, 4096).bytes);
+}
+
+test "the root ctl reads editor status until a command runs and validates whole batches" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "Msg root\n");
+    defer p.deinit();
+    const status = rd(p, root_ctl, 0, 4096);
+    try testing.expectEqual(Status.ok, status.reply.status);
+    var lines = std.mem.tokenizeScalar(u8, status.bytes, '\n');
+    try testing.expect(std.mem.startsWith(u8, lines.next().?, "pid "));
+    var expected: [64]u8 = undefined;
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&expected, "version {s}", .{pardes.version}), lines.next().?);
+    try testing.expectEqualStrings("panes 1", lines.next().?);
+    try testing.expect(lines.next() == null);
+    const stat = call(p, .{ .tag = 1, .op = .getattr, .node = root_ctl });
+    try testing.expectEqual(@as(u64, status.bytes.len), stat.reply.attr.size);
+    try testing.expectEqual(@as(u16, 0o666), stat.reply.attr.mode);
+
+    const serial = p.next_serial;
+    for ([_][]const u8{ "look", "exec", "look \t", "Look /virtual/index", "exec Msg a\x00b", "exec Msg x\nbogus\n", "name x", "put" }) |invalid| {
+        try testing.expectEqual(E.INVAL, wr(p, root_ctl, invalid).errno());
+        try testing.expectEqual(serial, p.next_serial);
+    }
+    try testing.expectEqualStrings(status.bytes, rd(p, root_ctl, 0, 4096).bytes);
+
+    const pane = p.panes[0].?;
+    try testing.expectEqual(Status.ok, wr(p, root_ctl, "exec Msg hello there\n").reply.status);
+    try testing.expectEqualStrings("hello there", pane.msg[0..pane.msg_len]);
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&expected, "{d}\n", .{pane.serial}), rd(p, root_ctl, 0, 4096).bytes);
+
+    try testing.expectEqual(Status.ok, wr(p, root_ctl, "look /virtual/index\nexec New\n").reply.status);
+    var made: [MAX_PANES]u32 = undefined;
+    var n: usize = 0;
+    for (p.panes) |slot| if (slot) |q| if (q.serial != pane.serial) {
+        made[n] = q.serial;
+        n += 1;
+    };
+    try testing.expectEqual(@as(usize, 2), n);
+    const listed = rd(p, root_ctl, 0, 4096);
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&expected, "{d}\n{d}\n", .{ @min(made[0], made[1]), @max(made[0], made[1]) }), listed.bytes);
+    const opened = p.panes[p.paneBySerial(@min(made[0], made[1])).?].?;
+    try testing.expectEqualStrings("/virtual/index", opened.file.?.path);
+}
+
+test "look opens spaced paths and locations without editing the source pane" {
+    if (!fs.platform_has_fs) return error.SkipZigTest;
+    const gpa = testing.allocator;
+    var tmp = testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.writeFile(testing.io, .{ .sub_path = "two words.zig", .data = "first\nsecond\nthird\n" });
+    var path_buf: [4096]u8 = undefined;
+    const path = path_buf[0..try tmp.dir.realPathFile(testing.io, "two words.zig", &path_buf)];
+    const p = try withFile(gpa, "source stays intact\n");
+    defer p.deinit();
+    const source = p.panes[0].?;
+    const ctl_node = Node.of(source.serial, .ctl);
+    const revision = source.file.?.revision;
+    const undo_len = source.file.?.history.undo_len;
+    source.cur_col = 3;
+    var command: [4200]u8 = undefined;
+    try testing.expectEqual(Status.ok, wr(p, ctl_node, try std.fmt.bufPrint(&command, "look {s}:2:3\n", .{path})).reply.status);
+    const opened = p.panes[p.active].?;
+    try testing.expect(opened != source);
+    try testing.expectEqualStrings(path, opened.file.?.path);
+    try testing.expectEqualStrings("first\nsecond\nthird\n", opened.file.?.content);
+    try testing.expectEqual(@as(i32, 1), opened.cur_row);
+    try testing.expectEqual(@as(i32, 2), opened.cur_col);
+    try testing.expectEqualStrings("source stays intact\n", source.file.?.content);
+    try testing.expectEqual(revision, source.file.?.revision);
+    try testing.expectEqual(undo_len, source.file.?.history.undo_len);
+    try testing.expectEqual(@as(i32, 3), source.cur_col);
+    var expected: [16]u8 = undefined;
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&expected, "{d}\n", .{opened.serial}), rd(p, root_ctl, 0, 4096).bytes);
+
+    p.active = 0;
+    try testing.expectEqual(Status.ok, wr(p, root_ctl, try std.fmt.bufPrint(&command, "look {s}:3:2\n", .{path})).reply.status);
+    try testing.expect(p.panes[p.active].? == opened);
+    try testing.expectEqual(@as(i32, 2), opened.cur_row);
+    try testing.expectEqual(@as(i32, 1), opened.cur_col);
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&expected, "{d}\n", .{opened.serial}), rd(p, root_ctl, 0, 4096).bytes);
+}
+
+test "look validates the whole batch before opening virtual files" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "source\n");
+    defer p.deinit();
+    const source = p.panes[0].?;
+    const ctl_node = Node.of(source.serial, .ctl);
+    const serial = p.next_serial;
+    for ([_][]const u8{
+        "look", "look \t", "look /virtual/index\x00ignored", "look /virtual/index\nbogus\n",
+    }) |invalid| {
+        try testing.expectEqual(E.INVAL, wr(p, ctl_node, invalid).errno());
+        try testing.expectEqual(serial, p.next_serial);
+        try testing.expectEqual(@as(usize, 0), p.active);
+        try testing.expectEqualStrings("source\n", source.file.?.content);
+    }
+    try testing.expectEqual(Status.ok, wr(p, ctl_node, "look /n/self/index\n").reply.status);
+    const opened = p.panes[p.active].?;
+    try testing.expectEqualStrings("/virtual/index", opened.file.?.path);
+    try testing.expect(std.mem.indexOf(u8, opened.file.?.content, "/test.txt") != null);
+    try testing.expectEqualStrings("source\n", source.file.?.content);
+}
+
+test "get reloads the pane from disk and reports a missing file without losing dirty contents" {
+    const gpa = testing.allocator;
+    var tmp = testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.writeFile(testing.io, .{ .sub_path = "note.txt", .data = "from disk\n" });
+    var directory_buf: [4096]u8 = undefined;
+    const directory = directory_buf[0..try tmp.dir.realPath(testing.io, &directory_buf)];
+    var path_buf: [4096]u8 = undefined;
+    const path = try std.fmt.bufPrint(&path_buf, "{s}/note.txt", .{directory});
+
+    const p = try withFile(gpa, "in memory\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const ctl_node = Node.of(serial, .ctl);
+    const pane = p.panes[0].?;
+
+    var name: [std.fs.max_path_bytes + 8]u8 = undefined;
+    _ = wr(p, Node.of(serial, .name), try std.fmt.bufPrint(&name, "{s}\n", .{path}));
+    try testing.expectEqual(Status.ok, wr(p, ctl_node, "get\n").reply.status);
+    try testing.expectEqualStrings("from disk\n", pane.file.?.content);
+    try testing.expect(!pane_files.dirtyOf(pane));
+    try testing.expect(pane.file.?.history.undo_len > 0);
+
+    _ = wr(p, Node.of(serial, .name), try std.fmt.bufPrint(&name, "{s}/missing.txt\n", .{directory}));
+    _ = wr(p, Node.of(serial, .body), "unsaved\n");
+    try testing.expectEqual(E.NOENT, wr(p, ctl_node, "get\nexec Del\n").errno());
+    try testing.expect(p.paneBySerial(serial) != null);
+    try testing.expectEqualStrings("from disk\nunsaved\n", pane.file.?.content);
+    try testing.expect(pane_files.dirtyOf(pane));
+}
+
+test "exec Save writes the named file through the host" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "one\ntwo\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const pane = p.panes[0].?;
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .name), "/tmp/renamed.txt\n").reply.status);
+    try testing.expect(pane_files.dirtyOf(pane));
+    const saved = wr(p, Node.of(serial, .ctl), "exec Save\n");
+    try testing.expectEqual(Status.ok, saved.reply.status);
+    try testing.expect(saved.saved);
+    p.perform(.{ .save_file = .{ .pane = 0 } });
+    try testing.expect(!pane_files.dirtyOf(pane));
+    try testing.expectEqualStrings(pane.file.?.content, p.fallback.get("/tmp/renamed.txt").?);
+}
diff --git a/src/ninep/events.zig b/src/ninep/events.zig
new file mode 100644
index 00000000..fb49e41b
--- /dev/null
+++ b/src/ninep/events.zig
@@ -0,0 +1,558 @@
+//! Event streams: per-pane `event` records, the editor-wide `log`, and the
+//! bounded queues that park a read until something happens.
+const std = @import("std");
+const pardes = @import("../pardes.zig");
+const tree = @import("tree.zig");
+const pane_files = @import("pane.zig");
+
+const Pardes = pardes.Pardes;
+const Pane = pardes.Pane;
+const MAX_PANES = pardes.MAX_PANES;
+const Req = tree.Req;
+const Reply = tree.Reply;
+const E = tree.E;
+
+pub const queue_cap = 64 * 1024;
+
+/// Wall-clock seconds, or zero where the platform has no clock.
+pub fn now() u32 {
+    if (comptime !pardes.hosted) return 0;
+    var ts: std.c.timespec = undefined;
+    if (std.c.clock_gettime(.REALTIME, &ts) != 0) return 0;
+    return std.math.cast(u32, ts.sec) orelse 0;
+}
+
+pub const Queue = struct {
+    buf: std.ArrayList(u8) = .empty,
+    head: usize = 0,
+
+    pub fn deinit(q: *Queue, gpa: std.mem.Allocator) void {
+        q.buf.deinit(gpa);
+        q.head = 0;
+    }
+
+    pub fn push(q: *Queue, gpa: std.mem.Allocator, record: []const u8) void {
+        if (record.len > std.math.maxInt(u32)) return;
+        while (q.buf.items.len - q.head + record.len + 4 > queue_cap) {
+            if (q.peek() == null) return;
+            q.pop();
+        }
+        q.compact();
+        var head: [4]u8 = undefined;
+        std.mem.writeInt(u32, &head, @intCast(record.len), .little);
+        q.buf.appendSlice(gpa, &head) catch return;
+        q.buf.appendSlice(gpa, record) catch {
+            q.buf.shrinkRetainingCapacity(q.buf.items.len - 4);
+            return;
+        };
+    }
+
+    pub fn peek(q: *const Queue) ?[]const u8 {
+        const rest = q.buf.items[@min(q.head, q.buf.items.len)..];
+        if (rest.len < 4) return null;
+        const len = std.mem.readInt(u32, rest[0..4], .little);
+        if (rest.len < 4 + len) return null;
+        return rest[4 .. 4 + len];
+    }
+
+    pub fn pop(q: *Queue) void {
+        const record = q.peek() orelse return;
+        q.head += 4 + record.len;
+        if (q.head == q.buf.items.len) {
+            q.buf.clearRetainingCapacity();
+            q.head = 0;
+        }
+    }
+
+    pub fn popFront(q: *Queue, n: usize) void {
+        const record = q.peek() orelse return;
+        if (n >= record.len) return q.pop();
+        q.head += n;
+        std.mem.writeInt(u32, q.buf.items[q.head..][0..4], @intCast(record.len - n), .little);
+    }
+
+    fn compact(q: *Queue) void {
+        if (q.head == 0 or q.head * 2 < q.buf.items.len) return;
+        const rest = q.buf.items.len - q.head;
+        std.mem.copyForwards(u8, q.buf.items[0..rest], q.buf.items[q.head..]);
+        q.buf.shrinkRetainingCapacity(rest);
+        q.head = 0;
+    }
+
+    pub fn empty(q: *const Queue) bool {
+        return q.peek() == null;
+    }
+
+    pub fn clearAndFree(q: *Queue, gpa: std.mem.Allocator) void {
+        q.buf.clearAndFree(gpa);
+        q.head = 0;
+    }
+};
+
+/// One record per read; `.again` parks the read until a record arrives.
+pub fn readQueue(p: *Pardes, req: Req, q: *Queue) Reply {
+    const record = q.peek() orelse return .{ .tag = req.tag, .status = .again };
+    if (req.size < record.len) return Reply.fail(req.tag, E.INVAL);
+    const out = p.fs.stage(p.gpa);
+    out.appendSlice(p.gpa, record) catch return Reply.fail(req.tag, E.NOMEM);
+    q.pop();
+    return .{ .tag = req.tag, .payload = .{ .staged = @intCast(out.items.len) } };
+}
+
+// ---- the editor-wide log ----
+
+pub const LogKind = enum { new, del, rename, save };
+
+/// A pane installed now is announced by `announce` once the update that made
+/// it has finished, when its name is known.
+pub fn noteInstall(p: *Pardes, id: usize) void {
+    if (id < MAX_PANES) p.fs.panes[id].unannounced = true;
+}
+
+/// A pane leaving; one never announced leaves silently.
+pub fn noteRetire(p: *Pardes, id: usize, pane: *Pane) void {
+    if (id >= MAX_PANES) return;
+    if (p.fs.panes[id].unannounced) {
+        p.fs.panes[id].unannounced = false;
+        return;
+    }
+    noteLog(p, .del, pane);
+}
+
+/// Called at the end of every update: reports the panes installed by it.
+pub fn announce(p: *Pardes) void {
+    for (p.panes, 0..) |slot, id| {
+        const pf = &p.fs.panes[id];
+        if (!pf.unannounced) continue;
+        pf.unannounced = false;
+        if (slot) |pane| noteLog(p, .new, pane);
+    }
+}
+
+/// Records `<kind> <serial> <name>` while a reader holds /log open.
+pub fn noteLog(p: *Pardes, kind: LogKind, pane: *Pane) void {
+    if (p.fs.log_readers == 0) return;
+    var buf: [4096 + 64]u8 = undefined;
+    const name = pane_files.nameOf(pane);
+    const record = std.fmt.bufPrint(&buf, "{s} {d} {s}\n", .{ @tagName(kind), pane.serial, name[0..@min(name.len, 4096)] }) catch return;
+    p.fs.log.push(p.gpa, record);
+}
+
+// ---- per-pane event records ----
+
+pub const max_record_text = 256;
+
+pub const Action = enum(u8) {
+    body_delete = 'D',
+    tag_delete = 'd',
+    body_insert = 'I',
+    tag_insert = 'i',
+    body_look = 'L',
+    tag_look = 'l',
+    body_exec = 'X',
+    tag_exec = 'x',
+
+    pub fn char(a: Action) u8 {
+        return @intFromEnum(a);
+    }
+
+    pub fn fromChar(c: u8) ?Action {
+        return std.enums.fromInt(Action, c);
+    }
+
+    pub fn onTag(a: Action) bool {
+        return @intFromEnum(a) >= 'a';
+    }
+};
+
+pub const flag_builtin: u32 = 1;
+pub const flag_expansion: u32 = 2;
+pub const flag_filename: u32 = 4;
+pub const flag_chorded: u32 = 8;
+
+pub fn formatRecord(
+    buf: []u8,
+    origin: u8,
+    action: Action,
+    q0: u32,
+    q1: u32,
+    flag: u32,
+    text: []const u8,
+) []const u8 {
+    const sent = if (text.len >= max_record_text) text[0..0] else text;
+    return std.fmt.bufPrint(buf, "{c}{c}{d} {d} {d} {d} {s}\n", .{
+        origin,
+        action.char(),
+        q0,
+        q1,
+        flag,
+        sent.len,
+        sent,
+    }) catch buf[0..0];
+}
+
+pub const Span = struct { at: u32, removed: u32, inserted: u32 };
+
+pub fn diffSpan(old: []const u8, new: []const u8) Span {
+    const both = @min(old.len, new.len);
+    const stride = 64;
+    var head: usize = 0;
+    while (head + stride <= both and
+        std.mem.eql(u8, old[head..][0..stride], new[head..][0..stride])) head += stride;
+    while (head < both and old[head] == new[head]) head += 1;
+    var tail: usize = 0;
+    const rest = both - head;
+    while (tail + stride <= rest and std.mem.eql(
+        u8,
+        old[old.len - tail - stride ..][0..stride],
+        new[new.len - tail - stride ..][0..stride],
+    )) tail += stride;
+    while (tail < rest and old[old.len - 1 - tail] == new[new.len - 1 - tail]) tail += 1;
+    return .{
+        .at = @intCast(head),
+        .removed = @intCast(old.len - tail - head),
+        .inserted = @intCast(new.len - tail - head),
+    };
+}
+
+pub fn noteReplace(p: *Pardes, id: usize, on_tag: bool, old: []const u8, new: []const u8) void {
+    if (!p.fs.scripted(id)) return;
+    const span = diffSpan(old, new);
+    if (span.removed == 0 and span.inserted == 0) return;
+    if (span.removed > 0) _ = noteAction(
+        p,
+        id,
+        if (on_tag) .tag_delete else .body_delete,
+        span.at,
+        span.at + span.removed,
+        0,
+        "",
+    );
+    if (span.inserted > 0) _ = noteAction(
+        p,
+        id,
+        if (on_tag) .tag_insert else .body_insert,
+        span.at,
+        span.at + span.inserted,
+        0,
+        new[span.at..][0..span.inserted],
+    );
+}
+
+pub fn noteAction(
+    p: *Pardes,
+    id: usize,
+    action: Action,
+    q0: u32,
+    q1: u32,
+    flag: u32,
+    text: []const u8,
+) bool {
+    if (!p.fs.scripted(id)) return false;
+    var buf: [max_record_text + 64]u8 = undefined;
+    const record = formatRecord(&buf, p.fs.origin, action, q0, q1, flag, text);
+    p.fs.panes[id].events.push(p.gpa, record);
+    return true;
+}
+
+pub fn notePtyOutput(p: *Pardes, id: usize, bytes: []const u8) void {
+    if (id >= MAX_PANES or bytes.len == 0) return;
+    const pf = &p.fs.panes[id];
+    if (pf.pty_readers == 0) return;
+    var off: usize = 0;
+    while (off < bytes.len) {
+        const n = @min(bytes.len - off, queue_cap / 2);
+        pf.pty_out.push(p.gpa, bytes[off..][0..n]);
+        off += n;
+    }
+}
+
+const EventRecord = struct { action: Action, q0: u32, q1: u32 };
+
+const EventReader = struct {
+    data: []const u8,
+    i: usize = 0,
+
+    fn next(er: *EventReader) ?EventRecord {
+        if (er.i >= er.data.len) return null;
+        var i = er.i;
+        if (i + 2 > er.data.len) return null;
+        i += 1;
+        const action = Action.fromChar(er.data[i]) orelse return null;
+        i += 1;
+        const q0 = scanNumber(er.data, &i) orelse return null;
+        const q1 = scanNumber(er.data, &i) orelse return null;
+        while (i < er.data.len and er.data[i] == ' ') i += 1;
+        if (i >= er.data.len or er.data[i] != '\n') return null;
+        er.i = i + 1;
+        return .{ .action = action, .q0 = q0, .q1 = q1 };
+    }
+};
+
+fn scanNumber(data: []const u8, i: *usize) ?u32 {
+    while (i.* < data.len and data[i.*] == ' ') i.* += 1;
+    const s = i.*;
+    var n: u64 = 0;
+    while (i.* < data.len and data[i.*] >= '0' and data[i.*] <= '9') : (i.* += 1)
+        n = @min(n * 10 + (data[i.*] - '0'), std.math.maxInt(u32));
+    if (i.* == s) return null;
+    return @intCast(n);
+}
+
+/// Writing a Look or Exec record back performs the action it names.
+pub fn writeEvent(p: *Pardes, req: Req, id: usize) Reply {
+    const pane0 = p.panes[id] orelse return Reply.fail(req.tag, E.NOENT);
+    const serial = pane0.serial;
+    {
+        const body = pane_files.bodyOf(pane0);
+        const tag = pane_files.tagOf(p, pane0);
+        var check: EventReader = .{ .data = req.data };
+        while (check.next()) |r| {
+            switch (r.action) {
+                .body_look, .tag_look, .body_exec, .tag_exec => {},
+                else => return Reply.fail(req.tag, E.INVAL),
+            }
+            const n = if (r.action.onTag()) tag.len else body.len;
+            if (r.q0 > r.q1 or r.q1 > n) return Reply.fail(req.tag, E.INVAL);
+        }
+        if (check.i != req.data.len) return Reply.fail(req.tag, E.INVAL);
+    }
+    var run: EventReader = .{ .data = req.data };
+    while (run.next()) |r| {
+        const live = p.paneBySerial(serial) orelse break;
+        const pane = p.panes[live].?;
+        const whole = if (r.action.onTag()) pane_files.tagOf(p, pane) else pane_files.bodyOf(pane);
+        const lo = @min(@as(usize, r.q0), whole.len);
+        const hi = @max(lo, @min(@as(usize, r.q1), whole.len));
+        const text = p.scratch.allocator().dupe(u8, whole[lo..hi]) catch continue;
+        switch (r.action) {
+            .body_exec, .tag_exec => _ = p.execute(live, text),
+            .body_look, .tag_look => p.lookAt(live, text),
+            else => unreachable,
+        }
+    }
+    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
+}
+
+// ---- tests ----
+
+const testing = std.testing;
+const th = @import("testing.zig");
+const call = th.call;
+const rd = th.rd;
+const wr = th.wr;
+const withFile = th.withFile;
+const serialOf = th.serialOf;
+const Node = tree.Node;
+const Status = tree.Status;
+
+test "event records are acme's bytes, one per read, and .again when empty" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "Msg fs-ran\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const event = Node.of(serial, .event);
+
+    _ = noteAction(p, 0, .body_exec, 1, 4, flag_builtin, "sg ");
+    try testing.expect(p.fs.panes[0].events.empty());
+
+    const h = call(p, .{ .tag = 10, .op = .open, .node = event });
+    try testing.expect(h.reply.handle != 0);
+    try testing.expectEqual(@as(u16, 1), p.fs.listeners);
+
+    try testing.expectEqual(Status.again, rd(p, event, 0, 4096).reply.status);
+
+    p.fs.origin = 'M';
+    _ = noteAction(p, 0, .body_exec, 1, 4, flag_builtin, "ell");
+    _ = noteAction(p, 0, .body_delete, 0, 3, 0, "");
+    try testing.expectEqualStrings("MX1 4 1 3 ell\n", rd(p, event, 0, 4096).bytes);
+    try testing.expectEqualStrings("MD0 3 0 0 \n", rd(p, event, 0, 4096).bytes);
+    try testing.expectEqual(Status.again, rd(p, event, 0, 4096).reply.status);
+
+    _ = noteAction(p, 0, .body_look, 0, 3, flag_filename, "one");
+    try testing.expectEqual(E.INVAL, rd(p, event, 0, 4).errno());
+    try testing.expectEqualStrings("ML0 3 4 3 one\n", rd(p, event, 0, 4096).bytes);
+
+    const big = "z" ** max_record_text;
+    _ = noteAction(p, 0, .body_exec, 0, max_record_text, 0, big);
+    try testing.expectEqualStrings("MX0 256 0 0 \n", rd(p, event, 0, 4096).bytes);
+
+    _ = call(p, .{ .tag = 11, .op = .release, .node = event, .handle = h.reply.handle });
+    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
+}
+
+fn drainEvents(p: *Pardes, node: u64, store: []u8, out: [][]const u8) [][]const u8 {
+    var used: usize = 0;
+    var n: usize = 0;
+    while (n < out.len) {
+        const a = rd(p, node, 0, 4096);
+        if (a.reply.status != .ok) break;
+        @memcpy(store[used..][0..a.bytes.len], a.bytes);
+        out[n] = store[used..][0..a.bytes.len];
+        used += a.bytes.len;
+        n += 1;
+    }
+    return out[0..n];
+}
+
+test "a write through the filesystem is reported once, attributed to the file it came through" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "one\ntwo\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const event = Node.of(serial, .event);
+    _ = call(p, .{ .tag = 40, .op = .open, .node = event });
+    var store: [4096]u8 = undefined;
+    var slots: [16][]const u8 = undefined;
+    _ = drainEvents(p, event, &store, &slots);
+
+    _ = wr(p, Node.of(serial, .body), "three\n");
+    const body_recs = drainEvents(p, event, &store, &slots);
+    try testing.expect(body_recs.len >= 1);
+    try testing.expectEqualStrings("EI8 14 0 6 three\n\n", body_recs[0]);
+    for (body_recs[1..]) |r| {
+        try testing.expectEqual(@as(u8, 'E'), r[0]);
+        try testing.expect(Action.fromChar(r[1]).?.onTag());
+    }
+
+    _ = wr(p, Node.of(serial, .addr), "1");
+    _ = wr(p, Node.of(serial, .data), "ONE\n");
+    const data_recs = drainEvents(p, event, &store, &slots);
+    try testing.expectEqual(@as(usize, 2), data_recs.len);
+    try testing.expectEqualStrings("FD0 3 0 0 \n", data_recs[0]);
+    try testing.expectEqualStrings("FI0 3 0 3 ONE\n", data_recs[1]);
+    try testing.expectEqual(Status.again, rd(p, event, 0, 4096).reply.status);
+}
+
+test "two event readers each count once, and the second closing leaves the first" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "x\n");
+    defer p.deinit();
+    const event = Node.of(serialOf(p), .event);
+
+    _ = call(p, .{ .tag = 12, .op = .open, .node = event });
+    _ = call(p, .{ .tag = 13, .op = .open, .node = event });
+    try testing.expectEqual(@as(u16, 2), p.fs.panes[0].readers);
+    try testing.expectEqual(@as(u16, 2), p.fs.listeners);
+
+    _ = call(p, .{ .tag = 14, .op = .release, .node = event });
+    try testing.expectEqual(@as(u16, 1), p.fs.panes[0].readers);
+    try testing.expect(p.fs.scripted(0));
+
+    _ = call(p, .{ .tag = 15, .op = .release, .node = Node.of(serialOf(p), .body) });
+    try testing.expectEqual(@as(u16, 1), p.fs.listeners);
+
+    _ = call(p, .{ .tag = 16, .op = .release, .node = event });
+    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
+    try testing.expect(!p.fs.scripted(0));
+    _ = call(p, .{ .tag = 17, .op = .release, .node = event });
+    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
+}
+
+test "a pane deleted while its event file is open leaves no suppression behind" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "x\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const event = Node.of(serial, .event);
+    _ = try th.newPane(p);
+
+    const a = call(p, .{ .tag = 18, .op = .open, .node = event });
+    const b = call(p, .{ .tag = 19, .op = .open, .node = event });
+    try testing.expectEqual(@as(u16, 2), p.fs.listeners);
+
+    _ = wr(p, Node.of(serial, .ctl), "exec Del\n");
+    try testing.expect(p.paneBySerial(serial) == null);
+    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
+
+    _ = call(p, .{ .tag = 20, .op = .release, .node = event, .handle = a.reply.handle });
+    _ = call(p, .{ .tag = 21, .op = .release, .node = event, .handle = b.reply.handle });
+    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
+
+    try testing.expectEqual(E.NOENT, rd(p, event, 0, 64).errno());
+    try testing.expectEqual(E.NOENT, rd(p, Node.of(serial, .body), 0, 64).errno());
+    try testing.expectEqual(E.NOENT, wr(p, Node.of(serial, .ctl), "clean\n").errno());
+    try testing.expectEqual(E.NOENT, call(p, .{ .tag = 22, .op = .open, .node = event }).errno());
+}
+
+test "writing an event record back performs the action it names" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "Msg fs-ran\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const event = Node.of(serial, .event);
+    const pane = p.panes[0].?;
+
+    const w = wr(p, event, "FX0 10\n");
+    try testing.expectEqual(Status.ok, w.reply.status);
+    try testing.expectEqualStrings("fs-ran", pane.msg[0..pane.msg_len]);
+
+    pane.msg_len = 0;
+    try testing.expectEqual(Status.ok, wr(p, event, "FX0 10\nFX0 10\n").reply.status);
+    try testing.expectEqualStrings("fs-ran", pane.msg[0..pane.msg_len]);
+
+    pane.msg_len = 0;
+    for ([_][]const u8{
+        "FX0 10\nFQ0 1\n", // unknown type character
+        "FX0 999\n", // out of range
+        "FX0 10", // no newline
+        "FX5 1\n", // q0 > q1
+        "FD0 3\n", // a report, not a request
+        "F\n",
+    }) |bad| {
+        try testing.expectEqual(E.INVAL, wr(p, event, bad).errno());
+        try testing.expectEqual(@as(usize, 0), pane.msg_len);
+    }
+
+    _ = call(p, .{ .tag = 23, .op = .open, .node = event });
+    p.fs.origin = 'K';
+    _ = wr(p, event, "KX0 10\n");
+    try testing.expectEqual(@as(u8, 'F'), p.fs.origin);
+}
+
+test "the log parks until a pane is created, renamed, saved or deleted" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "logged\n");
+    defer p.deinit();
+    const log = @intFromEnum(tree.TopFile.log);
+    _ = try th.newPane(p);
+    try testing.expect(p.fs.log.empty());
+
+    const opened = call(p, .{ .tag = 1, .op = .open, .node = log });
+    try testing.expectEqual(Status.ok, opened.reply.status);
+    try testing.expectEqual(@as(u16, 1), p.fs.log_readers);
+    try testing.expectEqual(Status.again, rd(p, log, 0, 4096).reply.status);
+
+    const serial = try th.newPane(p);
+    const id = p.paneBySerial(serial).?;
+    var expected: [4200]u8 = undefined;
+    try testing.expectEqualStrings(
+        try std.fmt.bufPrint(&expected, "new {d} {s}\n", .{ serial, pane_files.nameOf(p.panes[id].?) }),
+        rd(p, log, 0, 4096).bytes,
+    );
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .name), "/tmp/logged.txt\n").reply.status);
+    try testing.expectEqualStrings(
+        try std.fmt.bufPrint(&expected, "rename {d} /tmp/logged.txt\n", .{serial}),
+        rd(p, log, 0, 4096).bytes,
+    );
+    const saving = wr(p, Node.of(serial, .ctl), "exec Save\n");
+    try testing.expectEqual(Status.ok, saving.reply.status);
+    try testing.expect(saving.saved);
+    p.perform(.{ .save_file = .{ .pane = @intCast(id) } });
+    try testing.expectEqualStrings(
+        try std.fmt.bufPrint(&expected, "save {d} /tmp/logged.txt\n", .{serial}),
+        rd(p, log, 0, 4096).bytes,
+    );
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "exec Del\n").reply.status);
+    try testing.expectEqualStrings(
+        try std.fmt.bufPrint(&expected, "del {d} /tmp/logged.txt\n", .{serial}),
+        rd(p, log, 0, 4096).bytes,
+    );
+    try testing.expectEqual(Status.again, rd(p, log, 0, 4096).reply.status);
+
+    _ = call(p, .{ .tag = 2, .op = .release, .node = log, .handle = opened.reply.handle });
+    try testing.expectEqual(@as(u16, 0), p.fs.log_readers);
+    _ = try th.newPane(p);
+    try testing.expect(p.fs.log.empty());
+    try testing.expectEqual(@as(usize, 0), p.fs.log.buf.capacity);
+}
diff --git a/src/ninep/pane.zig b/src/ninep/pane.zig
new file mode 100644
index 00000000..b37ae645
--- /dev/null
+++ b/src/ninep/pane.zig
@@ -0,0 +1,840 @@
+//! The files under /pane/<serial>/: name, body, tag, ctl, addr, data, xdata,
+//! sel, errors and event. Terminal panes add pty/ (pty.zig).
+const std = @import("std");
+const pardes = @import("../pardes.zig");
+const panes = @import("../panes.zig");
+const modal = @import("../modal.zig");
+const config = @import("../config.zig");
+const fs = @import("../fs.zig");
+const tree = @import("tree.zig");
+const events = @import("events.zig");
+const addressing = @import("addr.zig");
+const ctl = @import("ctl.zig");
+const pty = @import("pty.zig");
+const screen = @import("screen.zig");
+
+const Pardes = pardes.Pardes;
+const Pane = pardes.Pane;
+const Req = tree.Req;
+const Reply = tree.Reply;
+const E = tree.E;
+const PaneFile = tree.PaneFile;
+const Node = tree.Node;
+
+/// Filesystem state a pane carries beside its editor state.
+pub const State = struct {
+    addr: Range = .{},
+    limit: ?Range = null,
+    readers: u16 = 0,
+    events: events.Queue = .{},
+    nomark: bool = false,
+    noscroll: bool = false,
+    tag_snap: std.ArrayList(u8) = .empty,
+    pty_readers: u16 = 0,
+    pty_out: events.Queue = .{},
+    /// Installed during this update; /log hears about it once the update ends
+    /// and the pane has its file, name and place.
+    unannounced: bool = false,
+
+    pub const Range = struct { q0: u32 = 0, q1: u32 = 0 };
+
+    pub fn deinit(pf: *State, gpa: std.mem.Allocator) void {
+        pf.events.deinit(gpa);
+        pf.pty_out.deinit(gpa);
+        pf.tag_snap.deinit(gpa);
+        pf.* = .{};
+    }
+};
+
+// ---- what a pane is ----
+
+pub fn bodyOf(pane: *const Pane) []const u8 {
+    if (pane.file) |*f| return f.content;
+    return "";
+}
+
+pub fn fileOf(pane: *Pane) ?*panes.File.State {
+    return if (pane.file) |*f| f else null;
+}
+
+pub fn tagOf(p: *Pardes, pane: *Pane) []const u8 {
+    return p.tagText(p.scratch.allocator(), pane) catch "";
+}
+
+pub fn dirOf(pane: *Pane) []const u8 {
+    if (pane.file) |*f| return std.fs.path.dirname(f.path) orelse "/";
+    const cwd = pane.cwdSlice();
+    return if (cwd.len > 0) cwd else "/";
+}
+
+pub fn dirtyOf(pane: *const Pane) bool {
+    const f = if (pane.file) |*x| x else return false;
+    if (!panes.Output.fileTraits(f.output).saves) return false;
+    return f.revision != f.saved_revision;
+}
+
+/// The pane's name: its file, its document, or a terminal's directory.
+pub fn nameOf(pane: *const Pane) []const u8 {
+    if (pane.file) |*f| return f.path;
+    if (pane.pdfPath()) |path| return path;
+    if (pane.image) |state| return state.path;
+    const cwd = pane.cwdSlice();
+    return if (cwd.len > 0) cwd else "/";
+}
+
+pub fn kindOf(pane: *const Pane) []const u8 {
+    if (pane.file != null) return "text";
+    if (pane.pdfPath() != null) return "pdf";
+    if (pane.image != null) return "image";
+    return "term";
+}
+
+pub fn mtimeOf(p: *Pardes, pane: *const Pane) u32 {
+    if (pane.file) |*f| if (f.mtime != 0) return f.mtime;
+    return p.fs.started;
+}
+
+pub fn clip(n: usize) u32 {
+    return std.math.cast(u32, n) orelse std.math.maxInt(u32);
+}
+
+fn cellOf(row: i32, col: i32) modal.Cursor {
+    return .{ .row = @intCast(@max(0, row)), .col = @intCast(@max(0, col)) };
+}
+
+pub fn dotOf(pane: *Pane) State.Range {
+    const text = bodyOf(pane);
+    const head = modal.offsetAt(text, cellOf(pane.cur_row, pane.cur_col));
+    if (!pane.vsel.active) return .{ .q0 = clip(head), .q1 = clip(head) };
+    const anchor = modal.offsetAt(text, cellOf(pane.vsel.row, pane.vsel.col));
+    var hi = @max(head, anchor);
+    if (hi < text.len) hi = modal.nextGrapheme(text, hi);
+    return .{ .q0 = clip(@min(head, anchor)), .q1 = clip(hi) };
+}
+
+pub fn setDot(pane: *Pane, r: State.Range) void {
+    const text = bodyOf(pane);
+    const q0 = @min(@as(usize, r.q0), text.len);
+    const q1 = @max(q0, @min(@as(usize, r.q1), text.len));
+    const a = modal.positionAt(text, q0);
+    pane.vsel = .{ .active = q1 > q0, .row = @intCast(a.row), .col = @intCast(a.col), .explicit = true };
+    const h = modal.positionAt(text, if (q1 > q0) modal.prevGrapheme(text, q1) else q0);
+    pane.cur_row = @intCast(h.row);
+    pane.cur_col = @intCast(h.col);
+    pane.cur_pinned = true;
+    pane.sticky_col = -1;
+    pane.msel.active = false;
+    pane.ensureCursorVisible();
+}
+
+pub fn showOffset(pane: *Pane, off: usize) void {
+    const text = bodyOf(pane);
+    const c = modal.positionAt(text, @min(off, text.len));
+    pane.cur_row = @intCast(c.row);
+    pane.cur_col = @intCast(c.col);
+    pane.cur_pinned = true;
+    pane.sticky_col = -1;
+    pane.ensureCursorVisible();
+}
+
+pub fn clampAddr(pf: *State, len: usize) void {
+    const n = clip(len);
+    pf.addr.q0 = @min(pf.addr.q0, n);
+    pf.addr.q1 = @min(pf.addr.q1, n);
+    if (pf.limit) |*l| {
+        l.q0 = @min(l.q0, n);
+        l.q1 = @min(l.q1, n);
+    }
+}
+
+fn shiftBy(r: State.Range, at: u32, removed: u32, inserted: u32) State.Range {
+    return .{ .q0 = shiftOne(r.q0, at, removed, inserted), .q1 = shiftOne(r.q1, at, removed, inserted) };
+}
+
+fn shiftOne(v: u32, at: u32, removed: u32, inserted: u32) u32 {
+    if (v <= at) return v;
+    if (v <= at +| removed) return at +| inserted;
+    return v - removed +| inserted;
+}
+
+pub fn wholeUtf8(data: []const u8) usize {
+    var i = data.len;
+    var back: usize = 0;
+    while (i > 0 and back < 4) : (back += 1) {
+        i -= 1;
+        const c = data[i];
+        if (c < 0x80) return data.len;
+        if (c & 0xC0 == 0xC0) {
+            const need = std.unicode.utf8ByteSequenceLength(c) catch return data.len;
+            if (i + need <= data.len or i == 0) return data.len;
+            return i;
+        }
+    }
+    return data.len;
+}
+
+pub fn spliceBody(p: *Pardes, id: usize, pane: *Pane, q0: usize, q1: usize, bytes: []const u8) ?usize {
+    const f = fileOf(pane) orelse return null;
+    const take = if (bytes.len == 0) 0 else wholeUtf8(bytes);
+    const lo = @min(q0, f.content.len);
+    const hi = @max(lo, @min(q1, f.content.len));
+    const new = p.gpa.alloc(u8, f.content.len - (hi - lo) + take) catch return null;
+    @memcpy(new[0..lo], f.content[0..lo]);
+    @memcpy(new[lo..][0..take], bytes[0..take]);
+    @memcpy(new[lo + take ..], f.content[hi..]);
+    if (!p.fs.panes[id].nomark) panes.File.pushUndo(p, pane);
+    panes.File.setContent(p, f, new);
+    return take;
+}
+
+// ---- sizes reported by stat ----
+
+pub fn fileSize(p: *Pardes, id: usize, f: PaneFile) u64 {
+    const pane = p.panes[id] orelse return 0;
+    return switch (f) {
+        .body, .data, .xdata => bodyOf(pane).len,
+        .tag => tagOf(p, pane).len,
+        .name => nameOf(pane).len + 1,
+        .ctl => ctl: {
+            var buf: [ctl.pane_text_cap]u8 = undefined;
+            break :ctl ctl.paneText(p, pane, &buf).len;
+        },
+        .sel => sel: {
+            const d = dotOf(pane);
+            break :sel @max(d.q0, d.q1) - d.q0;
+        },
+        .dir, .addr, .errors, .event, .pty, .pty_ctl, .pty_status, .pty_data => 0,
+    };
+}
+
+// ---- the index ----
+
+pub fn stageIndexLine(p: *Pardes, out: *std.ArrayList(u8), pane: *Pane) void {
+    out.print(p.gpa, "{d} {s} {d} {s}\n", .{ pane.serial, kindOf(pane), @intFromBool(dirtyOf(pane)), nameOf(pane) }) catch {};
+}
+
+pub fn indexLen(p: *Pardes) u64 {
+    var n: u64 = 0;
+    var last: u32 = 0;
+    while (tree.nextSerialAfter(p, last)) |serial| {
+        last = serial;
+        const pane = p.panes[p.paneBySerial(serial).?].?;
+        var digits: [16]u8 = undefined;
+        n += (std.fmt.bufPrint(&digits, "{d}", .{serial}) catch unreachable).len;
+        n += 1 + kindOf(pane).len + 3 + nameOf(pane).len + 1;
+    }
+    return n;
+}
+
+// ---- reads ----
+
+pub fn read(p: *Pardes, req: Req, id: usize, pane: *Pane, file: PaneFile) Reply {
+    const pf = &p.fs.panes[id];
+    return switch (file) {
+        .name => name: {
+            const out = p.fs.stage(p.gpa);
+            out.appendSlice(p.gpa, nameOf(pane)) catch {};
+            out.append(p.gpa, '\n') catch {};
+            break :name tree.stagedReply(p, req);
+        },
+        .body => readBody(p, req, id, pane),
+        .tag => tag: {
+            const out = p.fs.stage(p.gpa);
+            out.appendSlice(p.gpa, tagOf(p, pane)) catch {};
+            break :tag tree.stagedReply(p, req);
+        },
+        .ctl => ctl.readPane(p, req, pane),
+        .addr => addr: {
+            clampAddr(pf, bodyOf(pane).len);
+            const out = p.fs.stage(p.gpa);
+            out.print(p.gpa, "{d:>11} {d:>11} ", .{ pf.addr.q0, pf.addr.q1 }) catch {};
+            break :addr tree.stagedReply(p, req);
+        },
+        .data => readData(req, id, pane, pf, false),
+        .xdata => readData(req, id, pane, pf, true),
+        .sel => readSel(req, id, pane),
+        .event => events.readQueue(p, req, &pf.events),
+        .pty_status => pty.readStatus(p, req, id, pane),
+        .pty_data => pty.readData(p, req, pf),
+        .dir, .errors, .pty, .pty_ctl => Reply.fail(req.tag, E.PERM),
+    };
+}
+
+fn readBody(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
+    if (pane.file != null) {
+        const text = bodyOf(pane);
+        const off = @min(req.off, text.len);
+        const n = @min(text.len - off, req.size);
+        return .{ .tag = req.tag, .payload = .{ .region = .{
+            .pane = @intCast(id),
+            .serial = pane.serial,
+            .off = clip(off),
+            .len = clip(n),
+        } } };
+    }
+    if (req.handle != 0 and pane.isTerminal()) return screen.readSnapshot(p, req, pane);
+    const text = panes.Terminal.screenTextAlloc(pane, p.gpa) catch
+        return Reply.fail(req.tag, E.NOMEM);
+    defer p.gpa.free(text);
+    const out = p.fs.stage(p.gpa);
+    out.appendSlice(p.gpa, text) catch return Reply.fail(req.tag, E.NOMEM);
+    return tree.stagedReply(p, req);
+}
+
+fn readData(req: Req, id: usize, pane: *Pane, pf: *State, stop_at_end: bool) Reply {
+    const text = bodyOf(pane);
+    clampAddr(pf, text.len);
+    const q0: usize = pf.addr.q0;
+    const hi: usize = if (stop_at_end) @max(q0, @as(usize, pf.addr.q1)) else text.len;
+    var end = @min(hi, q0 +| req.size);
+    end = @max(q0, modal.graphemeStart(text, end));
+    pf.addr.q0 = clip(end);
+    if (!stop_at_end) pf.addr.q1 = clip(end);
+    if (pane.file == null) return .{ .tag = req.tag };
+    return .{ .tag = req.tag, .payload = .{ .region = .{
+        .pane = @intCast(id),
+        .serial = pane.serial,
+        .off = clip(q0),
+        .len = clip(end - q0),
+    } } };
+}
+
+fn readSel(req: Req, id: usize, pane: *Pane) Reply {
+    if (pane.file == null) return .{ .tag = req.tag };
+    const text = bodyOf(pane);
+    const d = dotOf(pane);
+    const lo = @min(@as(usize, d.q0), text.len);
+    const hi = @max(lo, @min(@as(usize, d.q1), text.len));
+    const off = @min(req.off, hi - lo);
+    const n = @min(hi - lo - off, req.size);
+    return .{ .tag = req.tag, .payload = .{ .region = .{
+        .pane = @intCast(id),
+        .serial = pane.serial,
+        .off = clip(lo + off),
+        .len = clip(n),
+    } } };
+}
+
+// ---- writes ----
+
+pub fn write(p: *Pardes, req: Req, id: usize, pane: *Pane, file: PaneFile) Reply {
+    return switch (file) {
+        .name => writeName(p, req, id, pane),
+        .body => writeBody(p, req, id, pane),
+        .tag => writeTag(p, req, pane),
+        .ctl => ctl.writePane(p, req, pane.serial),
+        .addr => writeAddr(p, req, id, pane),
+        .data, .xdata => writeData(p, req, id, pane),
+        .sel => writeSel(p, req, id, pane),
+        .event => events.writeEvent(p, req, id),
+        .errors => if (appendErrors(p, id, req.data)) |took|
+            .{ .tag = req.tag, .written = @intCast(took) }
+        else
+            Reply.fail(req.tag, E.IO),
+        .pty_ctl => pty.writeCtl(p, req, id),
+        .pty_data => pty.writeData(p, req, id),
+        .dir, .pty, .pty_status => Reply.fail(req.tag, E.PERM),
+    };
+}
+
+fn writeBody(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
+    if (req.data.len == 0) return .{ .tag = req.tag, .written = 0 };
+    if (pane.file == null) {
+        const take = wholeUtf8(req.data);
+        p.emitWrite(id, req.data[0..take]);
+        return .{ .tag = req.tag, .written = @intCast(take) };
+    }
+    const at = bodyOf(pane).len;
+    const take = spliceBody(p, id, pane, at, at, req.data) orelse
+        return Reply.fail(req.tag, E.NOMEM);
+    if (!p.fs.panes[id].noscroll) showOffset(pane, at + take);
+    return .{ .tag = req.tag, .written = @intCast(take) };
+}
+
+fn writeTag(p: *Pardes, req: Req, pane: *Pane) Reply {
+    if (req.data.len == 0) return .{ .tag = req.tag, .written = 0 };
+    p.seedTail(pane);
+    const room = pane.tag_tail.len - pane.tag_tail_len;
+    if (room == 0) return Reply.fail(req.tag, E.NOSPC);
+    const take = wholeUtf8(req.data[0..@min(req.data.len, room)]);
+    @memcpy(pane.tag_tail[pane.tag_tail_len..][0..take], req.data[0..take]);
+    pane.tag_tail_len += take;
+    pane.tag_init = true;
+    return .{ .tag = req.tag, .written = @intCast(take) };
+}
+
+fn writeData(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
+    if (fileOf(pane) == null) return Reply.fail(req.tag, E.INVAL);
+    const pf = &p.fs.panes[id];
+    clampAddr(pf, bodyOf(pane).len);
+    const q0: usize = pf.addr.q0;
+    const q1: usize = @max(q0, @as(usize, pf.addr.q1));
+    const before = dotOf(pane);
+    const take = spliceBody(p, id, pane, q0, q1, req.data) orelse
+        return Reply.fail(req.tag, E.NOMEM);
+    setDot(pane, shiftBy(before, clip(q0), clip(q1 - q0), clip(take)));
+    pf.addr = .{ .q0 = clip(q0 + take), .q1 = clip(q0 + take) };
+    if (!pf.noscroll) showOffset(pane, q0 + take);
+    return .{ .tag = req.tag, .written = @intCast(take) };
+}
+
+fn writeSel(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
+    if (fileOf(pane) == null) return Reply.fail(req.tag, E.INVAL);
+    const d = dotOf(pane);
+    const q0: usize = d.q0;
+    const q1: usize = @max(q0, @as(usize, d.q1));
+    const take = spliceBody(p, id, pane, q0, q1, req.data) orelse
+        return Reply.fail(req.tag, E.NOMEM);
+    setDot(pane, .{ .q0 = clip(q0 + take), .q1 = clip(q0 + take) });
+    return .{ .tag = req.tag, .written = @intCast(take) };
+}
+
+fn writeAddr(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
+    const pf = &p.fs.panes[id];
+    const text = bodyOf(pane);
+    clampAddr(pf, text.len);
+    const expr = std.mem.trimEnd(u8, req.data, "\n");
+    var a: addressing.Addr = .{ .text = text, .lim = pf.limit, .expr = expr };
+    const r = a.address(pf.addr) orelse return Reply.fail(req.tag, E.INVAL);
+    if (a.i < expr.len) return Reply.fail(req.tag, E.INVAL);
+    pf.addr = r;
+    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
+}
+
+/// Writing `name` renames the buffer: a relative name resolves against the
+/// pane's directory, as the tag's name field does.
+fn writeName(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
+    const name = std.mem.trim(u8, req.data, " \t\r\n");
+    if (name.len == 0) return Reply.fail(req.tag, E.INVAL);
+    for (name) |c| if (c < ' ') return Reply.fail(req.tag, E.INVAL);
+    if (fileOf(pane) == null) return Reply.fail(req.tag, E.PERM);
+    const full = std.fs.path.resolvePosix(p.scratch.allocator(), &.{ pardes.Pardes.paneDir(pane), name }) catch
+        return Reply.fail(req.tag, E.NOMEM);
+    if (!std.fs.path.isAbsolute(full) or full.len >= 4096) return Reply.fail(req.tag, E.INVAL);
+    nameBuffer(p, id, full, false) catch |err| return Reply.fail(req.tag, switch (err) {
+        error.OutOfMemory => E.NOMEM,
+        else => E.INVAL,
+    });
+    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
+}
+
+/// Change a buffer's absolute name without writing or renaming a disk file.
+/// Tag editing preserves user commands; the filesystem's rename resets its tail.
+pub fn nameBuffer(p: *Pardes, id: usize, full: []const u8, preserve_tag: bool) !void {
+    const pane = p.panes[id] orelse return error.NoPane;
+    const f = fileOf(pane) orelse return error.NotFile;
+    if (!std.fs.path.isAbsolute(full) or full.len == 0 or full.len >= 4096)
+        return error.InvalidName;
+    for (full) |c| if (c < ' ') return error.InvalidName;
+    if (std.mem.eql(u8, f.path, full)) return;
+    const copy = try p.gpa.dupe(u8, full);
+    p.gpa.free(f.path);
+    f.path = copy;
+    if (panes.Output.fileTraits(f.output).saves) {
+        f.output = null;
+        pane.clearCwd();
+        if (!preserve_tag) {
+            pane.tag_init = false;
+            pane.tag_tail_len = 0;
+        }
+        f.saved_revision = f.revision -% 1;
+        f.watch_after_save = fs.localPath(full) != null;
+    }
+    if (f.highlights.len > 0) p.tree_sitter_gpa.free(f.highlights);
+    f.highlights = &.{};
+    f.highlight_start = 0;
+    f.syntax_dirty = true;
+    f.context_revision = null;
+    pane.context_rows = 0;
+    pane.wrap_n = 0;
+    p.invalidateLookHover(id);
+    p.emit(.{ .watch = .{ .pane = @intCast(id), .on = false } });
+    events.noteLog(p, .rename, pane);
+}
+
+/// Diagnostics go to the +Errors buffer of the pane's directory.
+pub fn appendErrors(p: *Pardes, id: usize, text: []const u8) ?usize {
+    if (text.len == 0) return 0;
+    const pane = p.panes[id] orelse return null;
+    const dir = dirOf(pane);
+    for (p.panes, 0..) |slot, i| {
+        const q = slot orelse continue;
+        const qf = fileOf(q) orelse continue;
+        const o = qf.output orelse continue;
+        if (std.meta.activeTag(o.from) != .errors) continue;
+        if (!std.mem.eql(u8, std.fs.path.dirname(qf.path) orelse "", dir)) continue;
+        return spliceBody(p, i, q, qf.content.len, qf.content.len, text);
+    }
+    const free = p.freeSlot() orelse return null;
+    const content = p.gpa.dupe(u8, text) catch return null;
+    const np = panes.Output.open(p, free, dir, .errors, "", content) catch {
+        p.gpa.free(content);
+        return null;
+    };
+    p.placeDoc(id, free, np);
+    return text.len;
+}
+
+/// Truncation empties a body; every other setattr just answers fresh attributes.
+pub fn truncate(p: *Pardes, id: usize, pane: *Pane, file: PaneFile) tree.Status {
+    switch (file) {
+        .body, .data, .xdata => if (fileOf(pane) != null) {
+            _ = spliceBody(p, id, pane, 0, bodyOf(pane).len, "") orelse return .err;
+            p.fs.panes[id].addr = .{};
+            setDot(pane, .{});
+        },
+        else => {},
+    }
+    return .ok;
+}
+
+// ---- tests ----
+
+const testing = std.testing;
+const th = @import("testing.zig");
+const call = th.call;
+const rd = th.rd;
+const wr = th.wr;
+const withFile = th.withFile;
+const serialOf = th.serialOf;
+const Status = tree.Status;
+
+test "index lists serial kind dirty flag and name and stat reports its length" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "hello\nthere\n");
+    defer p.deinit();
+    const pane = p.panes[0].?;
+    const index = @intFromEnum(tree.TopFile.index);
+
+    const a = rd(p, index, 0, 4096);
+    try testing.expectEqual(Status.ok, a.reply.status);
+    var want: [64]u8 = undefined;
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&want, "{d} text 0 /test.txt\n", .{pane.serial}), a.bytes);
+    const stat = call(p, .{ .tag = 1, .op = .getattr, .node = index });
+    try testing.expectEqual(@as(u64, a.bytes.len), stat.reply.attr.size);
+    try testing.expectEqual(@as(u16, 0o444), stat.reply.attr.mode);
+
+    pane.file.?.saved_revision = pane.file.?.revision -% 1;
+    try testing.expectEqualStrings(try std.fmt.bufPrint(&want, "{d} text 1 /test.txt\n", .{pane.serial}), rd(p, index, 0, 4096).bytes);
+    const mid = rd(p, index, 2, 4);
+    try testing.expectEqualStrings("text", mid.bytes);
+
+    const serial = try th.newPane(p);
+    const both = rd(p, index, 0, 4096);
+    try testing.expectEqual(@as(u64, both.bytes.len), indexLen(p));
+    var expected: [4200]u8 = undefined;
+    const made = p.panes[p.paneBySerial(serial).?].?;
+    try testing.expect(std.mem.endsWith(u8, both.bytes, try std.fmt.bufPrint(&expected, "\n{d} text 0 {s}\n", .{ serial, nameOf(made) })));
+}
+
+test "a terminal is listed as term with its directory as name" {
+    const p = try th.withTerm(testing.allocator);
+    defer p.deinit();
+    const pane = p.panes[0].?;
+    p.setCwd(0, "/work/dir");
+    var want: [64]u8 = undefined;
+    try testing.expectEqualStrings(
+        try std.fmt.bufPrint(&want, "{d} term 0 /work/dir\n", .{pane.serial}),
+        rd(p, @intFromEnum(tree.TopFile.index), 0, 4096).bytes,
+    );
+    try testing.expectEqualStrings("/work/dir\n", rd(p, Node.of(pane.serial, .name), 0, 4096).bytes);
+    try testing.expectEqual(E.PERM, wr(p, Node.of(pane.serial, .name), "/elsewhere\n").errno());
+}
+
+test "body reads at any offset and writes append" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "one\ntwo\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const body = Node.of(serial, .body);
+
+    try testing.expectEqualStrings("one\ntwo\n", rd(p, body, 0, 100).bytes);
+    try testing.expectEqualStrings("two\n", rd(p, body, 4, 100).bytes);
+    try testing.expectEqualStrings("wo", rd(p, body, 5, 2).bytes);
+    try testing.expectEqualStrings("", rd(p, body, 999, 2).bytes);
+    try testing.expect(rd(p, body, 0, 100).bytes.ptr == p.panes[0].?.file.?.content.ptr);
+
+    const w = call(p, .{ .tag = 5, .op = .write, .node = body, .off = 0, .data = "three\n" });
+    try testing.expectEqual(@as(u32, 6), w.reply.written);
+    try testing.expectEqualStrings("one\ntwo\nthree\n", p.panes[0].?.file.?.content);
+
+    const short = wr(p, body, "a\xC3");
+    try testing.expectEqual(@as(u32, 1), short.reply.written);
+    try testing.expectEqualStrings("one\ntwo\nthree\na", p.panes[0].?.file.?.content);
+
+    const stat = call(p, .{ .tag = 6, .op = .getattr, .node = body });
+    try testing.expectEqual(@as(u64, "one\ntwo\nthree\na".len), stat.reply.attr.size);
+    try testing.expectEqual(@as(u16, 0o666), stat.reply.attr.mode);
+    try testing.expect(stat.reply.attr.mtime >= p.fs.started);
+}
+
+test "a body write to a terminal pane types at its shell" {
+    const gpa = testing.allocator;
+    const p = try Pardes.init(gpa, .{ .tty_only = true, .cols = 40, .rows = 10 });
+    defer p.deinit();
+    while (p.nextEffect()) |_| {}
+    const pane = p.panes[0].?;
+    try testing.expect(pane.isTerminal());
+
+    const a = wr(p, Node.of(pane.serial, .body), "ls -l\r");
+    try testing.expectEqual(@as(u32, 6), a.reply.written);
+    try testing.expectEqualStrings("ls -l\r", a.pty());
+
+    const r = rd(p, Node.of(pane.serial, .body), 0, 64);
+    try testing.expectEqual(Status.ok, r.reply.status);
+}
+
+test "tag reads the whole tag and writes append to the editable tail" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "x\n");
+    defer p.deinit();
+    const pane = p.panes[0].?;
+    const node = Node.of(pane.serial, .tag);
+
+    const whole = rd(p, node, 0, 4096);
+    try testing.expect(std.mem.startsWith(u8, whole.bytes, "/test.txt"));
+    try testing.expect(std.mem.indexOf(u8, whole.bytes, "Del") != null);
+    try testing.expectEqual(@as(u64, whole.bytes.len), call(p, .{ .tag = 1, .op = .getattr, .node = node }).reply.attr.size);
+
+    const before = rd(p, node, 0, 4096).bytes.len;
+    const w = wr(p, node, " Mine");
+    try testing.expectEqual(@as(u32, 5), w.reply.written);
+    try testing.expect(std.mem.endsWith(u8, pane.tag_tail[0..pane.tag_tail_len], " Mine"));
+    const after = rd(p, node, 0, 4096);
+    try testing.expectEqual(before + 5, after.bytes.len);
+    try testing.expect(std.mem.endsWith(u8, after.bytes, " Mine"));
+
+    pane.tag_tail_len = pane.tag_tail.len;
+    try testing.expectEqual(E.NOSPC, wr(p, node, "x").errno());
+}
+
+test "data and xdata read from addr, move it, and write through it" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "one\ntwo\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const addr = Node.of(serial, .addr);
+    const data = Node.of(serial, .data);
+    const xdata = Node.of(serial, .xdata);
+
+    _ = wr(p, addr, "#0");
+    try testing.expectEqualStrings("one", rd(p, data, 0, 3).bytes);
+    try testing.expectEqual(@as(u32, 3), p.fs.panes[0].addr.q0);
+    try testing.expectEqual(@as(u32, 3), p.fs.panes[0].addr.q1);
+
+    _ = wr(p, addr, "1");
+    try testing.expectEqualStrings("one\n", rd(p, xdata, 0, 100).bytes);
+    _ = wr(p, addr, "1");
+    try testing.expectEqualStrings("one\ntwo\n", rd(p, data, 0, 100).bytes);
+
+    _ = wr(p, addr, "1");
+    const w = wr(p, data, "ONE\n");
+    try testing.expectEqual(@as(u32, 4), w.reply.written);
+    try testing.expectEqualStrings("ONE\ntwo\n", p.panes[0].?.file.?.content);
+    try testing.expectEqual(@as(u32, 4), p.fs.panes[0].addr.q0);
+}
+
+test "data never splits a grapheme, in either direction" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "\u{00e9}x\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    _ = wr(p, Node.of(serial, .addr), "#0");
+    try testing.expectEqualStrings("", rd(p, Node.of(serial, .data), 0, 1).bytes);
+    _ = wr(p, Node.of(serial, .addr), "#0");
+    try testing.expectEqualStrings("\u{00e9}", rd(p, Node.of(serial, .data), 0, 2).bytes);
+
+    _ = wr(p, Node.of(serial, .addr), "#0");
+    try testing.expectEqual(@as(u32, 1), wr(p, Node.of(serial, .data), "a\xC3").reply.written);
+}
+
+test "sel reads the selection and writing sel replaces it" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "one\ntwo\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const ctl_node = Node.of(serial, .ctl);
+    const sel = Node.of(serial, .sel);
+
+    _ = wr(p, Node.of(serial, .addr), "#0,#3");
+    try testing.expectEqual(Status.ok, wr(p, ctl_node, "dot=addr\n").reply.status);
+    try testing.expectEqualStrings("one", rd(p, sel, 0, 100).bytes);
+    try testing.expectEqual(@as(u64, 3), call(p, .{ .tag = 1, .op = .getattr, .node = sel }).reply.attr.size);
+
+    _ = wr(p, ctl_node, "addr=dot\n");
+    try testing.expectEqual(@as(u32, 0), p.fs.panes[0].addr.q0);
+    try testing.expectEqual(@as(u32, 3), p.fs.panes[0].addr.q1);
+
+    try testing.expectEqual(Status.ok, wr(p, sel, "ONE").reply.status);
+    try testing.expectEqualStrings("ONE\ntwo\n", p.panes[0].?.file.?.content);
+    _ = wr(p, sel, "!");
+    try testing.expectEqualStrings("ONE!\ntwo\n", p.panes[0].?.file.?.content);
+    try testing.expectEqual(@as(u64, 0), call(p, .{ .tag = 2, .op = .getattr, .node = sel }).reply.attr.size);
+}
+
+test "name reads the file name and writing it promotes a scratch without touching its body" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "opener\n");
+    defer p.deinit();
+    const serial = try th.newPane(p);
+    const pane = p.panes[p.paneBySerial(serial).?].?;
+    const name = Node.of(serial, .name);
+    const body = Node.of(serial, .body);
+    var scratch_name: [4200]u8 = undefined;
+    const scratch_line = try std.fmt.bufPrint(&scratch_name, "{s}\n", .{nameOf(pane)});
+    try testing.expectEqualStrings(scratch_line, rd(p, name, 0, 4096).bytes);
+    try testing.expectEqual(@as(u64, scratch_line.len), call(p, .{ .tag = 1, .op = .getattr, .node = name }).reply.attr.size);
+    _ = wr(p, body, "scratch ");
+    _ = wr(p, body, "work\n");
+    const undo_len = pane.file.?.history.undo_len;
+    var tmp = testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.writeFile(testing.io, .{ .sub_path = "renamed.zig", .data = "existing target\n" });
+    var directory_buf: [4096]u8 = undefined;
+    const directory = directory_buf[0..try tmp.dir.realPath(testing.io, &directory_buf)];
+    var path_buffer: [4096]u8 = undefined;
+    const path = try std.fmt.bufPrint(&path_buffer, "{s}/renamed.zig", .{directory});
+    var line: [4104]u8 = undefined;
+    const renamed = wr(p, name, try std.fmt.bufPrint(&line, "{s}\n", .{path}));
+    try testing.expectEqual(Status.ok, renamed.reply.status);
+    try testing.expectEqual(false, renamed.watch.?);
+    try testing.expectEqualStrings("scratch work\n", pane.file.?.content);
+    try testing.expectEqual(undo_len, pane.file.?.history.undo_len);
+    try testing.expect(pane.file.?.output == null and dirtyOf(pane));
+    try testing.expect(pane.file.?.watch_after_save);
+    try testing.expectEqualStrings(path, std.mem.trimEnd(u8, rd(p, name, 0, 4096).bytes, "\n"));
+    const target = try fs.readFile(gpa, path);
+    defer gpa.free(target);
+    try testing.expectEqualStrings("existing target\n", target);
+    for ([_][]const u8{ "", "\n", "bad\x01name\n" }) |bad|
+        try testing.expectEqual(E.INVAL, wr(p, name, bad).errno());
+    try testing.expectEqualStrings(path, pane.file.?.path);
+    try testing.expectEqual(Status.ok, wr(p, name, "two words.zig\n").reply.status);
+    const spaced = try std.fmt.bufPrint(&path_buffer, "{s}/two words.zig", .{directory});
+    try testing.expectEqualStrings(spaced, pane.file.?.path);
+}
+
+test "name refreshes cached syntax for unchanged contents" {
+    if (!pardes.syntax.enabled) return error.SkipZigTest;
+    pardes.syntax.start(testing.allocator);
+    defer pardes.syntax.stop();
+    const p = try withFile(testing.allocator, "fn check() void {}\n");
+    defer p.deinit();
+    const pane = p.panes[0].?;
+    panes.File.refreshHighlights(p);
+    try testing.expectEqual(@as(usize, 0), pane.file.?.highlights.len);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(pane.serial, .name), "renamed.zig\n").reply.status);
+    panes.File.refreshHighlights(p);
+    try testing.expect(pane.file.?.highlights.len >= 2);
+    try testing.expectEqual(@intFromEnum(pardes.syntax.Syn.keyword), pane.file.?.highlights[0]);
+    try testing.expectEqual(@intFromEnum(pardes.syntax.Syn.keyword), pane.file.?.highlights[1]);
+    try testing.expectEqualStrings("fn check() void {}\n", pane.file.?.content);
+}
+
+test "relative names use the file directory and can name a new target" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "retained body\n");
+    defer p.deinit();
+    var tmp = testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.writeFile(testing.io, .{ .sub_path = "note.txt", .data = "from pane directory\n" });
+    var directory_buf: [4096]u8 = undefined;
+    const directory = directory_buf[0..try tmp.dir.realPath(testing.io, &directory_buf)];
+    const pane = p.panes[0].?;
+    const name = Node.of(pane.serial, .name);
+    const ctl_node = Node.of(pane.serial, .ctl);
+    var line: [4140]u8 = undefined;
+    try testing.expectEqual(Status.ok, wr(p, name, try std.fmt.bufPrint(&line, "{s}/old.txt\n", .{directory})).reply.status);
+    try testing.expectEqual(Status.ok, wr(p, name, "child/../note.txt\n").reply.status);
+    try testing.expectEqual(Status.ok, wr(p, ctl_node, "get\n").reply.status);
+    try testing.expectEqualStrings("from pane directory\n", pane.file.?.content);
+    try testing.expectEqualStrings(directory, Pardes.paneDir(pane));
+    try testing.expectEqual(Status.ok, wr(p, name, "./created.txt\n").reply.status);
+    var expected_buf: [4096]u8 = undefined;
+    const expected = try std.fmt.bufPrint(&expected_buf, "{s}/created.txt", .{directory});
+    try testing.expectEqualStrings(expected, pane.file.?.path);
+    try testing.expect(dirtyOf(pane));
+}
+
+test "relative names follow inherited scratch and virtual directories" {
+    const p = try withFile(testing.allocator, "source body\n");
+    defer p.deinit();
+    const source = p.panes[0].?;
+    try testing.expectEqual(Status.ok, wr(p, Node.of(source.serial, .name), "/project/src/source.zig\n").reply.status);
+    const serial = try th.newPane(p);
+    const scratch = p.panes[p.paneBySerial(serial).?].?;
+    const name = Node.of(serial, .name);
+    try testing.expectEqualStrings("/project/src", Pardes.paneDir(scratch));
+    try testing.expectEqual(Status.ok, wr(p, name, "../out/./notes.txt\n").reply.status);
+    try testing.expectEqualStrings("/project/out/notes.txt", scratch.file.?.path);
+    try testing.expectEqualStrings("/project/out", Pardes.paneDir(scratch));
+    var line: [256]u8 = undefined;
+    try testing.expectEqual(Status.ok, wr(p, name, try std.fmt.bufPrint(&line, "/virtual/pane/{d}/./body\n", .{source.serial})).reply.status);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "get\n").reply.status);
+    try testing.expectEqualStrings(source.file.?.content, scratch.file.?.content);
+    const unchanged = wr(p, name, "./body\n");
+    try testing.expectEqual(Status.ok, unchanged.reply.status);
+    try testing.expect(unchanged.watch == null);
+    try testing.expect(!dirtyOf(scratch));
+    try testing.expect(!scratch.file.?.watch_after_save);
+}
+
+test "owned cwd name promotion releases the former directory" {
+    const p = try th.withTerm(testing.allocator);
+    defer p.deinit();
+    p.newScratchBelow(0);
+    const id = p.active;
+    const pane = p.panes[id].?;
+    try pane.setOwnedCwd("/old/directory");
+    const result = wr(p, Node.of(pane.serial, .name), "saved.txt\n");
+    try testing.expectEqual(Status.ok, result.reply.status);
+    try testing.expect(pane.cwd == .none);
+    try testing.expect(pane.file.?.output == null);
+    try testing.expectEqualStrings("/old/directory/saved.txt", pane.file.?.path);
+    try testing.expectEqualStrings("/old/directory", Pardes.paneDir(pane));
+}
+
+test "errors append to one +Errors buffer per directory" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "x\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+
+    const live = for (p.panes) |slot| {
+        if (slot) |q| if (q.file) |f| if (f.output) |o| if (std.meta.activeTag(o.from) == .errors) break q;
+    } else null;
+    try testing.expect(live == null);
+
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .errors), "boom\n").reply.status);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .errors), "again\n").reply.status);
+
+    var found: usize = 0;
+    for (p.panes) |slot| {
+        const q = slot orelse continue;
+        const f = q.file orelse continue;
+        const o = f.output orelse continue;
+        if (std.meta.activeTag(o.from) != .errors) continue;
+        found += 1;
+        try testing.expectEqualStrings("boom\nagain\n", f.content);
+        try testing.expectEqualStrings("/+Errors", f.path);
+    }
+    try testing.expectEqual(@as(usize, 1), found);
+}
+
+test "setattr truncation empties the body and answers fresh attributes" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "one\ntwo\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+
+    const a = call(p, .{ .tag = 7, .op = .setattr, .node = Node.of(serial, .body), .truncate = true });
+    try testing.expectEqual(Status.ok, a.reply.status);
+    try testing.expectEqual(@as(u64, 0), a.reply.attr.size);
+    try testing.expectEqualStrings("", p.panes[0].?.file.?.content);
+
+    _ = wr(p, Node.of(serial, .body), "new text\n");
+    try testing.expectEqualStrings("new text\n", p.panes[0].?.file.?.content);
+
+    const noop = call(p, .{ .tag = 8, .op = .setattr, .node = Node.of(serial, .body) });
+    try testing.expectEqual(@as(u64, 9), noop.reply.attr.size);
+}
diff --git a/src/ninep/pty.zig b/src/ninep/pty.zig
new file mode 100644
index 00000000..e6696b85
--- /dev/null
+++ b/src/ninep/pty.zig
@@ -0,0 +1,388 @@
+//! A terminal pane's pty/: ctl (winsize, sig, exec), status and the raw data stream.
+const std = @import("std");
+const pardes = @import("../pardes.zig");
+const tree = @import("tree.zig");
+const pane_files = @import("pane.zig");
+
+const Pardes = pardes.Pardes;
+const Pane = pardes.Pane;
+const Req = tree.Req;
+const Reply = tree.Reply;
+const E = tree.E;
+const PaneFile = tree.PaneFile;
+
+pub fn fileNamed(name: []const u8) ?PaneFile {
+    if (std.mem.eql(u8, name, "ctl")) return .pty_ctl;
+    if (std.mem.eql(u8, name, "status")) return .pty_status;
+    if (std.mem.eql(u8, name, "data")) return .pty_data;
+    return null;
+}
+
+const Verb = enum { winsize, sig, exec };
+
+fn dimension(word: []const u8) ?u16 {
+    if (word.len == 0 or word.len > 5) return null;
+    for (word) |c| if (c < '0' or c > '9') return null;
+    const n = std.fmt.parseInt(u16, word, 10) catch return null;
+    return if (n == 0) null else n;
+}
+
+fn signalNamed(word: []const u8) ?pardes.PtySignal {
+    if (std.mem.eql(u8, word, "INT")) return .int;
+    if (std.mem.eql(u8, word, "TERM")) return .term;
+    if (std.mem.eql(u8, word, "HUP")) return .hup;
+    if (std.mem.eql(u8, word, "QUIT")) return .quit;
+    if (std.mem.eql(u8, word, "KILL")) return .kill;
+    return null;
+}
+
+pub fn writeCtl(p: *Pardes, req: Req, id: usize) Reply {
+    for ([2]bool{ false, true }) |apply| {
+        var it = std.mem.splitScalar(u8, req.data, '\n');
+        while (it.next()) |raw| {
+            const line = std.mem.trim(u8, raw, " \t\r");
+            if (line.len == 0) continue;
+            if (!verb(p, id, line, apply)) return Reply.fail(req.tag, E.INVAL);
+        }
+    }
+    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
+}
+
+fn verb(p: *Pardes, id: usize, line: []const u8, apply: bool) bool {
+    const pane = p.panes[id] orelse return false;
+    var words = std.mem.tokenizeAny(u8, line, " \t");
+    const v = std.meta.stringToEnum(Verb, words.next() orelse return false) orelse return false;
+    switch (v) {
+        .winsize => {
+            const cols = dimension(words.next() orelse return false) orelse return false;
+            const rows = dimension(words.next() orelse return false) orelse return false;
+            if (words.next() != null) return false;
+            if (!apply) return true;
+            p.emit(.{ .resize_pty = .{ .pane = @intCast(id), .cols = cols, .rows = rows } });
+        },
+        .sig => {
+            const which = signalNamed(words.next() orelse return false) orelse return false;
+            if (words.next() != null) return false;
+            if (!apply) return true;
+            p.emit(.{ .signal_pty = .{ .pane = @intCast(id), .sig = which } });
+        },
+        .exec => {
+            if (words.next() != null) return false;
+            if (pane.cwdSlice().len > pardes.effect_path_cap) return false;
+            if (!apply) return true;
+            p.emit(.{ .spawn = .{ .pane = @intCast(id), .cwd = .from(pane.cwdSlice()) } });
+        },
+    }
+    return true;
+}
+
+pub fn readStatus(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
+    const out = p.fs.stage(p.gpa);
+    out.print(p.gpa, "{d:>11} {d:>11} {d:>11} ", .{
+        pane.cols,
+        pane.rows,
+        @intFromBool(p.hostTtyTaken(id)),
+    }) catch {};
+    return tree.stagedReply(p, req);
+}
+
+pub fn readData(p: *Pardes, req: Req, pf: *pane_files.State) Reply {
+    if (pf.pty_out.empty()) return .{ .tag = req.tag, .status = .again };
+    const out = p.fs.stage(p.gpa);
+    while (out.items.len < req.size) {
+        const chunk = pf.pty_out.peek() orelse break;
+        const n = @min(chunk.len, req.size - out.items.len);
+        out.appendSlice(p.gpa, chunk[0..n]) catch break;
+        pf.pty_out.popFront(n);
+    }
+    return .{ .tag = req.tag, .payload = .{ .staged = @intCast(out.items.len) } };
+}
+
+pub fn writeData(p: *Pardes, req: Req, id: usize) Reply {
+    if (req.data.len == 0) return .{ .tag = req.tag, .written = 0 };
+    const take = pane_files.wholeUtf8(req.data);
+    p.emitWrite(id, req.data[0..take]);
+    return .{ .tag = req.tag, .written = @intCast(take) };
+}
+
+// ---- tests ----
+
+const testing = std.testing;
+const th = @import("testing.zig");
+const events = @import("events.zig");
+const call = th.call;
+const rd = th.rd;
+const wr = th.wr;
+const rdir = th.rdir;
+const look_up = th.look_up;
+const withFile = th.withFile;
+const withTerm = th.withTerm;
+const serialOf = th.serialOf;
+const Node = tree.Node;
+const Status = tree.Status;
+
+test "a pane that is not a terminal has no pty/ at all" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "hello\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const dir = Node.of(serial, .dir);
+
+    try testing.expectEqual(E.NOENT, look_up(p, dir, "pty").errno());
+    try testing.expectEqual(E.NOENT, call(p, .{
+        .tag = 1,
+        .op = .getattr,
+        .node = Node.of(serial, .pty),
+    }).errno());
+    try testing.expectEqual(E.NOENT, rd(p, Node.of(serial, .pty_status), 0, 256).errno());
+    try testing.expectEqual(E.NOENT, wr(p, Node.of(serial, .pty_ctl), "winsize 80 24\n").errno());
+    try testing.expectEqual(E.NOENT, rdir(p, Node.of(serial, .pty), 0).errno());
+    try testing.expectEqual(E.NOENT, call(p, .{
+        .tag = 2,
+        .op = .open,
+        .node = Node.of(serial, .pty_data),
+    }).errno());
+    try testing.expectEqual(@as(u16, 0), p.fs.panes[0].pty_readers);
+
+    var buf: [32]th.Dirent = undefined;
+    const files = th.dirents(rdir(p, dir, 0).bytes, &buf);
+    try testing.expectEqual(@as(usize, 10), files.len);
+    try testing.expect(th.nameAt(files, "pty") == null);
+
+    try testing.expectEqual(E.NOENT, look_up(p, dir, "pty_ctl").errno());
+    try testing.expectEqual(E.NOENT, look_up(p, dir, "status").errno());
+}
+
+test "a terminal pane's pty/ holds exactly ctl, status and data" {
+    const gpa = testing.allocator;
+    const p = try withTerm(gpa);
+    defer p.deinit();
+    const serial = serialOf(p);
+    const dir = Node.of(serial, .dir);
+
+    const pty = look_up(p, dir, "pty");
+    try testing.expectEqual(Node.of(serial, .pty), pty.reply.attr.node);
+    try testing.expect(pty.reply.attr.dir);
+    try testing.expectEqual(@as(u16, 0o755), pty.reply.attr.mode);
+
+    var buf: [32]th.Dirent = undefined;
+    const files = th.dirents(rdir(p, dir, 0).bytes, &buf);
+    try testing.expectEqual(@as(usize, 11), files.len);
+    try testing.expect(th.nameAt(files, "pty").?.dir);
+
+    const inside = th.dirents(rdir(p, Node.of(serial, .pty), 0).bytes, &buf);
+    try testing.expectEqual(@as(usize, 3), inside.len);
+    try testing.expectEqualStrings("ctl", inside[0].name);
+    try testing.expectEqualStrings("status", inside[1].name);
+    try testing.expectEqualStrings("data", inside[2].name);
+    for (inside) |d| try testing.expect(!d.dir);
+    try testing.expectEqual(Node.of(serial, .pty_data), inside[2].node);
+
+    const ctl = look_up(p, Node.of(serial, .pty), "ctl");
+    try testing.expectEqual(Node.of(serial, .pty_ctl), ctl.reply.attr.node);
+    try testing.expectEqual(@as(u16, 0o222), ctl.reply.attr.mode);
+    try testing.expectEqual(@as(u16, 0o444), look_up(p, Node.of(serial, .pty), "status").reply.attr.mode);
+    try testing.expectEqual(E.NOENT, look_up(p, Node.of(serial, .pty), "body").errno());
+    try testing.expectEqual(E.NOENT, look_up(p, Node.of(serial, .pty), "pty").errno());
+
+    try testing.expectEqual(E.NOTDIR, look_up(p, Node.of(serial, .pty_ctl), "x").errno());
+    try testing.expectEqual(E.NOTDIR, rdir(p, Node.of(serial, .pty_ctl), 0).errno());
+    try testing.expectEqual(E.PERM, rd(p, Node.of(serial, .pty), 0, 16).errno());
+    try testing.expectEqual(E.PERM, rd(p, Node.of(serial, .pty_ctl), 0, 16).errno());
+    try testing.expectEqual(E.PERM, wr(p, Node.of(serial, .pty_status), "x").errno());
+}
+
+test "every pty/ctl verb, and every refusal" {
+    const gpa = testing.allocator;
+    const p = try withTerm(gpa);
+    defer p.deinit();
+    const ctl = Node.of(serialOf(p), .pty_ctl);
+
+    const pane = p.panes[0].?;
+    const cols = pane.cols;
+    const rows = pane.rows;
+    const ws = wr(p, ctl, "winsize 132 44\n");
+    try testing.expectEqual(@as(u32, "winsize 132 44\n".len), ws.reply.written);
+    try testing.expectEqual(@as(u16, 132), ws.winsize.?.cols);
+    try testing.expectEqual(@as(u16, 44), ws.winsize.?.rows);
+    try testing.expectEqual(cols, pane.cols);
+    try testing.expectEqual(rows, pane.rows);
+
+    for ([_]struct { line: []const u8, want: pardes.PtySignal }{
+        .{ .line = "sig INT", .want = .int },
+        .{ .line = "sig TERM", .want = .term },
+        .{ .line = "sig HUP", .want = .hup },
+        .{ .line = "sig QUIT", .want = .quit },
+        .{ .line = "sig KILL", .want = .kill },
+    }) |c| {
+        const a = wr(p, ctl, c.line);
+        try testing.expectEqual(Status.ok, a.reply.status);
+        try testing.expectEqual(c.want, a.signal.?);
+    }
+
+    const ex = wr(p, ctl, "exec\n");
+    try testing.expectEqual(Status.ok, ex.reply.status);
+    try testing.expect(ex.spawned);
+
+    const both = wr(p, ctl, "winsize 100 30\nsig TERM");
+    try testing.expectEqual(@as(u16, 100), both.winsize.?.cols);
+    try testing.expectEqual(pardes.PtySignal.term, both.signal.?);
+
+    for ([_][]const u8{
+        "winsize", // no arguments
+        "winsize 80", // one argument
+        "winsize 80 24 extra", // three
+        "winsize 0 24", // zero is "unknown", never a width
+        "winsize 80 0",
+        "winsize -1 24", // not a decimal
+        "winsize 999999 24", // wider than a u16
+        "sig", // no name
+        "sig INT TERM", // two
+        "sig SIGINT",
+        "sig int", // lower case
+        "sig 9", // a number is one platform's number
+        "sig USR1", // a real signal, deliberately not offered
+        "exec /bin/sh", // the effect carries no argv; refused, never ignored
+        "raw",
+        "cooked",
+        "winsize 80 24\nbogus", // a good verb beside a bad one
+        "bogus\nwinsize 80 24",
+        "look x", // a `ctl` verb; the two files share no vocabulary
+        "exec Del",
+    }) |bad| {
+        const a = wr(p, ctl, bad);
+        try testing.expectEqual(E.INVAL, a.errno());
+        try testing.expect(a.winsize == null);
+        try testing.expect(a.signal == null);
+        try testing.expect(!a.spawned);
+    }
+
+    const spaced = wr(p, ctl, "\n  winsize 90 20  \n\n");
+    try testing.expectEqual(Status.ok, spaced.reply.status);
+    try testing.expectEqual(@as(u16, 90), spaced.winsize.?.cols);
+    try testing.expectEqual(Status.ok, wr(p, ctl, "").reply.status);
+}
+
+test "owned cwd pty exec rejects long paths before applying its batch" {
+    const p = try withTerm(testing.allocator);
+    defer p.deinit();
+    const pane = p.panes[0].?;
+    const ctl = Node.of(pane.serial, .pty_ctl);
+    var path: [1025]u8 = @splat('d');
+    path[0] = '/';
+    p.setCwd(0, &path);
+    for ([_][]const u8{ "exec\n", "winsize 100 30\nexec\n", "sig TERM\nexec\n" }) |command| {
+        const result = wr(p, ctl, command);
+        try testing.expectEqual(E.INVAL, result.errno());
+        try testing.expect(!result.spawned);
+        try testing.expect(result.signal == null and result.winsize == null);
+    }
+    try testing.expectEqualStrings(&path, pane.cwdSlice());
+    p.setCwd(0, path[0..pardes.effect_path_cap]);
+    const accepted = wr(p, ctl, "exec\n");
+    try testing.expectEqual(Status.ok, accepted.reply.status);
+    try testing.expect(accepted.spawned);
+}
+
+test "pty/status reports the grid and who holds the tty" {
+    const gpa = testing.allocator;
+    const p = try withTerm(gpa);
+    defer p.deinit();
+    const pane = p.panes[0].?;
+    const status = Node.of(pane.serial, .pty_status);
+
+    const a = rd(p, status, 0, 256);
+    try testing.expectEqual(Status.ok, a.reply.status);
+    var want: [64]u8 = undefined;
+    const whole = try std.fmt.bufPrint(&want, "{d:>11} {d:>11} {d:>11} ", .{ pane.cols, pane.rows, 0 });
+    try testing.expectEqualStrings(whole, a.bytes);
+    try testing.expectEqual(@as(usize, 3 * 12), a.bytes.len);
+    try testing.expectEqualStrings(whole[12..], rd(p, status, 12, 256).bytes);
+
+    var probe: th.FakeTty = .{ .taken = true };
+    p.host = .{ .ctx = &probe, .vtable = &th.FakeTty.vtable };
+    const held = rd(p, status, 0, 256);
+    try testing.expectEqualStrings(
+        try std.fmt.bufPrint(&want, "{d:>11} {d:>11} {d:>11} ", .{ pane.cols, pane.rows, 1 }),
+        held.bytes,
+    );
+}
+
+test "pty/data writes at the shell and reads the raw stream" {
+    const gpa = testing.allocator;
+    const p = try withTerm(gpa);
+    defer p.deinit();
+    const serial = serialOf(p);
+    const data = Node.of(serial, .pty_data);
+
+    const w = call(p, .{ .tag = 2, .op = .write, .node = data, .off = 999, .data = "ls -l\r" });
+    try testing.expectEqual(@as(u32, 6), w.reply.written);
+    try testing.expectEqualStrings("ls -l\r", w.pty());
+    try testing.expectEqual(@as(u32, 1), wr(p, data, "a\xC3").reply.written);
+    try testing.expectEqual(@as(u32, 0), wr(p, data, "").reply.written);
+
+    try testing.expectEqual(Status.again, rd(p, data, 0, 64).reply.status);
+
+    p.update(.{ .output = .{ .pane = 0, .bytes = "unwatched" } });
+    while (p.nextEffect()) |_| {}
+    try testing.expectEqual(@as(usize, 0), p.fs.panes[0].pty_out.buf.items.len);
+    try testing.expectEqual(Status.again, rd(p, data, 0, 64).reply.status);
+
+    _ = call(p, .{ .tag = 5, .op = .open, .node = data });
+    try testing.expectEqual(@as(u16, 1), p.fs.panes[0].pty_readers);
+    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
+    try testing.expect(!p.fs.scripted(0));
+
+    p.update(.{ .output = .{ .pane = 0, .bytes = "hello" } });
+    while (p.nextEffect()) |_| {}
+    try testing.expectEqualStrings("hello", rd(p, data, 0, 64).bytes);
+    try testing.expectEqual(Status.again, rd(p, data, 0, 64).reply.status);
+
+    p.update(.{ .output = .{ .pane = 0, .bytes = "abcdef" } });
+    while (p.nextEffect()) |_| {}
+    try testing.expectEqualStrings("ab", rd(p, data, 0, 2).bytes);
+    try testing.expectEqualStrings("cd", rd(p, data, 0, 2).bytes);
+    p.update(.{ .output = .{ .pane = 0, .bytes = "ghi" } });
+    while (p.nextEffect()) |_| {}
+    try testing.expectEqualStrings("efghi", rd(p, data, 0, 64).bytes);
+
+    p.update(.{ .output = .{ .pane = 0, .bytes = "orphan" } });
+    while (p.nextEffect()) |_| {}
+    _ = call(p, .{ .tag = 6, .op = .release, .node = data });
+    try testing.expectEqual(@as(u16, 0), p.fs.panes[0].pty_readers);
+    try testing.expectEqual(@as(usize, 0), p.fs.panes[0].pty_out.buf.capacity);
+    try testing.expectEqual(Status.again, rd(p, data, 0, 64).reply.status);
+
+    _ = call(p, .{ .tag = 7, .op = .open, .node = data });
+    _ = call(p, .{ .tag = 8, .op = .open, .node = data });
+    _ = call(p, .{ .tag = 9, .op = .release, .node = data });
+    try testing.expectEqual(@as(u16, 1), p.fs.panes[0].pty_readers);
+    p.update(.{ .output = .{ .pane = 0, .bytes = "still" } });
+    while (p.nextEffect()) |_| {}
+    try testing.expectEqualStrings("still", rd(p, data, 0, 64).bytes);
+}
+
+test "the pty queue drops the oldest at its cap" {
+    const gpa = testing.allocator;
+    const p = try withTerm(gpa);
+    defer p.deinit();
+    const data = Node.of(serialOf(p), .pty_data);
+    _ = call(p, .{ .tag = 5, .op = .open, .node = data });
+
+    const oldest: [4096]u8 = @splat('A');
+    const rest: [4096]u8 = @splat('B');
+    events.notePtyOutput(p, 0, &oldest);
+    for (0..events.queue_cap / rest.len + 4) |_| events.notePtyOutput(p, 0, &rest);
+    const q = &p.fs.panes[0].pty_out;
+    try testing.expect(q.buf.items.len - q.head <= events.queue_cap);
+
+    var seen: usize = 0;
+    while (true) {
+        const a = rd(p, data, 0, 1 << 16);
+        if (a.reply.status == .again) break;
+        try testing.expect(std.mem.indexOfScalar(u8, a.bytes, 'A') == null);
+        if (a.bytes.len == 0) break;
+        seen += a.bytes.len;
+    }
+    try testing.expect(seen > 0 and seen <= events.queue_cap);
+}
diff --git a/src/ninep/screen.zig b/src/ninep/screen.zig
new file mode 100644
index 00000000..5b68b34c
--- /dev/null
+++ b/src/ninep/screen.zig
@@ -0,0 +1,351 @@
+//! /screen, /listeners and the snapshot handles that freeze a screen or a
+//! terminal history between open and close.
+const std = @import("std");
+const pardes = @import("../pardes.zig");
+const panes = @import("../panes.zig");
+const tree = @import("tree.zig");
+
+const Pardes = pardes.Pardes;
+const Pane = pardes.Pane;
+const Req = tree.Req;
+const Reply = tree.Reply;
+const E = tree.E;
+
+pub const Snapshot = struct { node: u64 = 0, bytes: ?[]const u8 = null };
+pub const snapshot_slots = 32;
+
+pub fn render(p: *Pardes) ![]u8 {
+    var arena: std.heap.ArenaAllocator = .init(p.gpa);
+    defer arena.deinit();
+    const surface = try p.render(arena.allocator());
+    var styles: std.ArrayList(pardes.CellStyle) = .empty;
+    var indices: std.ArrayList(usize) = .empty;
+    for (surface.cells) |cell| {
+        const style: pardes.CellStyle = if (cell.default) .{} else cell.style;
+        const index = for (styles.items, 0..) |previous, i| {
+            if (std.meta.eql(style, previous)) break i;
+        } else new: {
+            try styles.append(arena.allocator(), style);
+            break :new styles.items.len - 1;
+        };
+        try indices.append(arena.allocator(), index);
+    }
+    var out: std.Io.Writer.Allocating = .init(p.gpa);
+    errdefer out.deinit();
+    const writer = &out.writer;
+    try writer.print("{{\"cols\":{d},\"rows\":{d},\"cursor\":", .{ surface.cols, surface.rows });
+    try std.json.Stringify.value(surface.cursor, .{}, writer);
+    try writer.writeAll(",\"styles\":");
+    try std.json.Stringify.value(styles.items, .{ .emit_strings_as_arrays = true }, writer);
+    try writer.writeAll(",\"cells\":[");
+    for (surface.cells, indices.items, 0..) |cell, index, i| {
+        if (i != 0) try writer.writeByte(',');
+        try std.json.Stringify.value(.{ if (cell.default) " " else cell.grapheme(), index }, .{}, writer);
+    }
+    try writer.writeAll("]}\n");
+    return out.toOwnedSlice();
+}
+
+/// A screen freezes at open; a terminal body freezes at its first read.
+pub fn openSnapshot(p: *Pardes, req: Req, of_screen: bool) Reply {
+    for (&p.fs.snapshots, 0..) |*slot, i| {
+        if (slot.node != 0) continue;
+        const bytes = if (of_screen) render(p) catch return Reply.fail(req.tag, E.NOMEM) else null;
+        slot.* = .{ .node = req.node, .bytes = bytes };
+        return .{ .tag = req.tag, .handle = @intCast(i + 1) };
+    }
+    return Reply.fail(req.tag, E.NFILE);
+}
+
+pub fn readSnapshot(p: *Pardes, req: Req, pane: ?*Pane) Reply {
+    if (req.handle == 0 or req.handle > p.fs.snapshots.len) return Reply.fail(req.tag, E.INVAL);
+    const snapshot = &p.fs.snapshots[req.handle - 1];
+    if (snapshot.node == 0 or snapshot.node != req.node) return Reply.fail(req.tag, E.INVAL);
+    if (snapshot.bytes == null) {
+        const terminal = pane orelse return Reply.fail(req.tag, E.INVAL);
+        snapshot.bytes = panes.Terminal.screenTextAlloc(terminal, p.gpa) catch return Reply.fail(req.tag, E.NOMEM);
+    }
+    const bytes = snapshot.bytes.?;
+    const off = @min(req.off, bytes.len);
+    const len = @min(bytes.len - off, req.size);
+    const out = p.fs.stage(p.gpa);
+    out.appendSlice(p.gpa, bytes[off..][0..len]) catch return Reply.fail(req.tag, E.NOMEM);
+    return .{ .tag = req.tag, .payload = .{ .staged = @intCast(len) } };
+}
+
+/// True when the handle named a snapshot, which is now released.
+pub fn releaseSnapshot(p: *Pardes, req: Req) bool {
+    if (req.handle == 0 or req.handle > p.fs.snapshots.len) return false;
+    const snapshot = &p.fs.snapshots[req.handle - 1];
+    if (snapshot.node != req.node) return false;
+    if (snapshot.bytes) |bytes| p.gpa.free(bytes);
+    snapshot.* = .{};
+    return true;
+}
+
+pub fn readListeners(p: *Pardes, req: Req) Reply {
+    var buf: [512]u8 = undefined;
+    var text = std.Io.Writer.fixed(&buf);
+    if (p.fs.socket_path.len != 0)
+        text.print("unix!{s}\n", .{p.fs.socket_path}) catch return Reply.fail(req.tag, E.IO);
+    for ([_]?std.Io.net.IpAddress{ p.fs.tcp_address, p.fs.quic_address }, [_][]const u8{ "tcp", "quic" }) |maybe, transport| {
+        const address = maybe orelse continue;
+        switch (address) {
+            .ip4 => |ip| text.print("{s}!{d}.{d}.{d}.{d}!{d}\n", .{ transport, ip.bytes[0], ip.bytes[1], ip.bytes[2], ip.bytes[3], ip.port }) catch
+                return Reply.fail(req.tag, E.IO),
+            .ip6 => |ip| text.print("{s}!{f}!{d}\n", .{ transport, std.Io.net.Ip6Address.Unresolved{ .bytes = ip.bytes, .interface_name = null }, ip.port }) catch
+                return Reply.fail(req.tag, E.IO),
+        }
+    }
+    const out = p.fs.stage(p.gpa);
+    out.appendSlice(p.gpa, text.buffered()) catch return Reply.fail(req.tag, E.NOMEM);
+    return tree.stagedReply(p, req);
+}
+
+// ---- tests ----
+
+const testing = std.testing;
+const th = @import("testing.zig");
+const fs = @import("../fs.zig");
+const call = th.call;
+const rd = th.rd;
+const wr = th.wr;
+const withFile = th.withFile;
+const withTerm = th.withTerm;
+const serialOf = th.serialOf;
+const Node = tree.Node;
+const Status = tree.Status;
+const screen_node = @intFromEnum(tree.TopFile.screen);
+
+test "listener addresses are readable canonical dials with bounded partial reads" {
+    const gpa = testing.allocator;
+    const p = try Pardes.init(gpa, .{ .tty_only = true });
+    defer p.deinit();
+    p.fs.socket_path = "/tmp/a socket.sock";
+    p.fs.tcp_address = .{ .ip4 = .loopback(5640) };
+    p.fs.quic_address = .{ .ip6 = .loopback(5641) };
+    const expected = "unix!/tmp/a socket.sock\ntcp!127.0.0.1!5640\nquic!::1!5641\n";
+    for ([_][]const u8{ "/virtual/listeners", "/n/self/listeners" }) |path| {
+        const bytes = try fs.read(p, path);
+        defer gpa.free(bytes);
+        try testing.expectEqualStrings(expected, bytes);
+        try testing.expectError(error.ReadOnlyFilesystem, fs.write(p, path, ""));
+    }
+    const node = @intFromEnum(tree.TopFile.listeners);
+    var off: usize = 0;
+    while (off < expected.len) {
+        const part = rd(p, node, off, 3);
+        try testing.expectEqual(Status.ok, part.reply.status);
+        try testing.expectEqualStrings(expected[off..][0..@min(3, expected.len - off)], part.bytes);
+        off += part.bytes.len;
+    }
+    try testing.expectEqual(@as(usize, 0), rd(p, node, off, 3).bytes.len);
+}
+
+test "terminal body handles keep one history snapshot across fragmented reads" {
+    const gpa = testing.allocator;
+    const p = try withTerm(gpa);
+    defer p.deinit();
+    const node = Node.of(serialOf(p), .body);
+    p.update(.{ .output = .{ .pane = 0, .bytes = "old caf\xc3\xa9\r\nold tail" } });
+    while (p.nextEffect()) |_| {}
+    const original = try panes.Terminal.screenTextAlloc(p.panes[0].?, gpa);
+    defer gpa.free(original);
+    const opened = call(p, .{ .tag = 1, .op = .open, .node = node });
+    try testing.expectEqual(Status.ok, opened.reply.status);
+    const first = call(p, .{ .tag = 2, .op = .read, .node = node, .handle = opened.reply.handle, .size = 3 });
+    try testing.expectEqualStrings(original[0..3], first.bytes);
+    p.update(.{ .output = .{ .pane = 0, .bytes = "\x1b[3J\x1b[2J\x1b[Hnew output" } });
+    while (p.nextEffect()) |_| {}
+    var offset: usize = 3;
+    while (offset < original.len) {
+        const part = call(p, .{ .tag = 3, .op = .read, .node = node, .handle = opened.reply.handle, .off = offset, .size = 3 });
+        try testing.expectEqualStrings(original[offset..][0..@min(3, original.len - offset)], part.bytes);
+        offset += part.bytes.len;
+    }
+    try testing.expectEqualStrings("", call(p, .{ .tag = 4, .op = .read, .node = node, .handle = opened.reply.handle, .off = original.len, .size = 3 }).bytes);
+    _ = call(p, .{ .tag = 5, .op = .release, .node = node, .handle = opened.reply.handle });
+    const newer = call(p, .{ .tag = 6, .op = .open, .node = node });
+    try testing.expectEqualStrings("new output", call(p, .{ .tag = 7, .op = .read, .node = node, .handle = newer.reply.handle, .size = 32 }).bytes);
+    _ = call(p, .{ .tag = 8, .op = .release, .node = node, .handle = newer.reply.handle });
+}
+
+test "an empty terminal body snapshot stays empty while output continues" {
+    const p = try withTerm(std.testing.allocator);
+    defer p.deinit();
+    const node = Node.of(serialOf(p), .body);
+    const opened = call(p, .{ .tag = 1, .op = .open, .node = node });
+    try testing.expectEqualStrings("", call(p, .{ .tag = 2, .op = .read, .node = node, .handle = opened.reply.handle, .size = 32 }).bytes);
+    try testing.expect(p.fs.snapshots[opened.reply.handle - 1].bytes != null);
+    p.update(.{ .output = .{ .pane = 0, .bytes = "new output" } });
+    while (p.nextEffect()) |_| {}
+    try testing.expectEqualStrings("", call(p, .{ .tag = 3, .op = .read, .node = node, .handle = opened.reply.handle, .size = 32 }).bytes);
+    _ = call(p, .{ .tag = 4, .op = .release, .node = node, .handle = opened.reply.handle });
+    const newer = call(p, .{ .tag = 5, .op = .open, .node = node });
+    try testing.expectEqualStrings("new output", call(p, .{ .tag = 6, .op = .read, .node = node, .handle = newer.reply.handle, .size = 32 }).bytes);
+    _ = call(p, .{ .tag = 7, .op = .release, .node = node, .handle = newer.reply.handle });
+}
+
+test "terminal body snapshots are lazy bounded and released after the pane closes" {
+    const gpa = testing.allocator;
+    const p = try withTerm(gpa);
+    defer p.deinit();
+    const node = Node.of(serialOf(p), .body);
+    var handles: [snapshot_slots]u32 = undefined;
+    for (&handles) |*opened_handle| {
+        const opened = call(p, .{ .tag = 1, .op = .open, .node = node });
+        try testing.expectEqual(Status.ok, opened.reply.status);
+        opened_handle.* = opened.reply.handle;
+    }
+    for (p.fs.snapshots) |snapshot| {
+        try testing.expectEqual(node, snapshot.node);
+        try testing.expect(snapshot.bytes == null);
+    }
+    try testing.expectEqual(E.NFILE, call(p, .{ .tag = 2, .op = .open, .node = node }).errno());
+    const scratch = try th.newPane(p);
+    const scratch_id = p.paneBySerial(scratch).?;
+    try testing.expect(p.panes[scratch_id].?.file != null);
+    p.update(.{ .output = .{ .pane = 0, .bytes = "latest output" } });
+    while (p.nextEffect()) |_| {}
+    var failing = std.testing.FailingAllocator.init(gpa, .{ .fail_index = 0 });
+    p.gpa = failing.allocator();
+    const failed = call(p, .{ .tag = 3, .op = .read, .node = node, .handle = handles[0], .size = 32 });
+    p.gpa = gpa;
+    try testing.expectEqual(E.NOMEM, failed.errno());
+    try testing.expect(p.fs.snapshots[handles[0] - 1].bytes == null);
+    const first = call(p, .{ .tag = 4, .op = .read, .node = node, .handle = handles[0], .size = 32 });
+    try testing.expectEqualStrings("latest output", first.bytes);
+    const saved = p.fs.snapshots[handles[0] - 1].bytes.?;
+    failing = std.testing.FailingAllocator.init(gpa, .{ .fail_index = 0 });
+    p.gpa = failing.allocator();
+    const second = call(p, .{ .tag = 5, .op = .read, .node = node, .handle = handles[0], .off = 7, .size = 32 });
+    p.gpa = gpa;
+    try testing.expectEqualStrings("output", second.bytes);
+    try testing.expect(!failing.has_induced_failure);
+    try testing.expectEqual(saved.ptr, p.fs.snapshots[handles[0] - 1].bytes.?.ptr);
+    try testing.expectEqual(E.INVAL, call(p, .{ .tag = 6, .op = .read, .node = screen_node, .handle = handles[0], .size = 32 }).errno());
+    _ = call(p, .{ .tag = 7, .op = .release, .node = screen_node, .handle = handles[0] });
+    try testing.expectEqual(node, p.fs.snapshots[handles[0] - 1].node);
+    try p.removePane(0);
+    for (handles) |opened_handle| _ = call(p, .{ .tag = 8, .op = .release, .node = node, .handle = opened_handle });
+    for (p.fs.snapshots) |snapshot| {
+        try testing.expectEqual(@as(u64, 0), snapshot.node);
+        try testing.expect(snapshot.bytes == null);
+    }
+}
+
+test "screen snapshots preserve rendered cells and styles until their handle is released" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "const value = 1;\n");
+    defer p.deinit();
+    const opened = call(p, .{ .tag = 1, .op = .open, .node = screen_node });
+    try testing.expectEqual(Status.ok, opened.reply.status);
+    const snapshot = p.fs.snapshots[opened.reply.handle - 1].bytes.?;
+    const original = try gpa.dupe(u8, snapshot);
+    defer gpa.free(original);
+    const parsed = try std.json.parseFromSlice(std.json.Value, gpa, original, .{});
+    defer parsed.deinit();
+    const data = parsed.value.object;
+    try testing.expectEqual(@as(i64, p.screen_w), data.get("cols").?.integer);
+    try testing.expectEqual(@as(i64, p.screen_h), data.get("rows").?.integer);
+    try testing.expectEqual(@as(usize, p.screen_w) * p.screen_h, data.get("cells").?.array.items.len);
+    try testing.expect(data.get("styles").?.array.items.len > 0);
+    _ = wr(p, Node.of(serialOf(p), .body), "changed\n");
+    const newer = call(p, .{ .tag = 2, .op = .open, .node = screen_node });
+    try testing.expectEqual(Status.ok, newer.reply.status);
+    try testing.expect(!std.mem.eql(u8, original, p.fs.snapshots[newer.reply.handle - 1].bytes.?));
+    var off: usize = 0;
+    while (off < original.len) {
+        const result = call(p, .{ .tag = 3, .op = .read, .node = screen_node, .handle = opened.reply.handle, .off = off, .size = 13 });
+        try testing.expectEqual(Status.ok, result.reply.status);
+        try testing.expectEqualSlices(u8, original[off..][0..@min(13, original.len - off)], result.bytes);
+        off += result.bytes.len;
+    }
+    _ = call(p, .{ .tag = 4, .op = .release, .node = screen_node, .handle = opened.reply.handle });
+    _ = call(p, .{ .tag = 5, .op = .release, .node = screen_node, .handle = newer.reply.handle });
+    for (p.fs.snapshots) |slot| try testing.expect(slot.node == 0);
+}
+
+test "screen inspection preserves acknowledged presentation and the next real frame" {
+    const gpa = testing.allocator;
+    for (std.enums.values(pardes.layout.Transition)) |transition| {
+        errdefer std.debug.print("screen inspection during {s}\n", .{@tagName(transition)});
+        const control = try withFile(gpa, "const value = 1;\n");
+        defer control.deinit();
+        const inspected = try withFile(gpa, "const value = 1;\n");
+        defer inspected.deinit();
+        var arena: std.heap.ArenaAllocator = .init(gpa);
+        defer arena.deinit();
+        for ([_]*Pardes{ control, inspected }) |p| {
+            p.settings.panel_transition = .off;
+            p.update(.tick);
+            p.acknowledgePanelPresentation((try p.render(arena.allocator())).panelTracks());
+            p.settings.panel_transition = transition;
+        }
+        for (0..4) |phase| {
+            for ([_]*Pardes{ control, inspected }) |p| switch (phase) {
+                0 => {},
+                1 => p.update(.{ .command = "New" }),
+                2 => {
+                    p.update(.tick);
+                    try testing.expectEqual(Status.ok, wr(p, Node.of(p.panes[p.active].?.serial, .body), "changed cells\n").reply.status);
+                },
+                3 => {
+                    for (0..transition.frames() + 1) |_| p.update(.tick);
+                    p.acknowledgePanelPresentation((try p.render(arena.allocator())).panelTracks());
+                    p.update(.{ .command = "Del" });
+                },
+                else => unreachable,
+            };
+            for (0..2) |_| {
+                const opened = call(inspected, .{ .tag = 1, .op = .open, .node = screen_node });
+                try testing.expectEqual(Status.ok, opened.reply.status);
+                const captured = call(inspected, .{ .tag = 2, .op = .read, .node = screen_node, .handle = opened.reply.handle, .size = 32 });
+                try testing.expectEqual(Status.ok, captured.reply.status);
+                try testing.expect(captured.bytes.len > 0);
+                try testing.expectEqual(Status.ok, call(inspected, .{
+                    .tag = 3,
+                    .op = .release,
+                    .node = screen_node,
+                    .handle = opened.reply.handle,
+                }).reply.status);
+            }
+            const a = &control.presentation;
+            const b = &inspected.presentation;
+            try testing.expectEqualDeep(a.shown, b.shown);
+            try testing.expectEqualDeep(a.shown_tracks, b.shown_tracks);
+            try testing.expectEqualDeep(a.shown_closing[0..a.shown_closing_len], b.shown_closing[0..b.shown_closing_len]);
+            try testing.expectEqual(a.pending, b.pending);
+            try testing.expectEqual(a.acknowledged, b.acknowledged);
+            try testing.expectEqual(a.previous_valid, b.previous_valid);
+            try testing.expectEqual(a.previous_cols, b.previous_cols);
+            try testing.expectEqual(a.previous_rows, b.previous_rows);
+            try testing.expectEqualDeep(a.previous_layout, b.previous_layout);
+            try testing.expectEqual(a.previous_cells.len, b.previous_cells.len);
+            for (a.previous_cells, b.previous_cells) |*expected, *actual|
+                try testing.expect(expected.visuallyEqual(actual));
+
+            const expected = try control.render(arena.allocator());
+            const actual = try inspected.render(arena.allocator());
+            try testing.expectEqual(expected.cols, actual.cols);
+            try testing.expectEqual(expected.rows, actual.rows);
+            try testing.expectEqualDeep(expected.cursor, actual.cursor);
+            try testing.expectEqualDeep(expected.panelTracks(), actual.panelTracks());
+            try testing.expectEqualDeep(expected.cell_diffs, actual.cell_diffs);
+            try testing.expectEqual(expected.previous_cells.len, actual.previous_cells.len);
+            try testing.expectEqual(expected.cells.len, actual.cells.len);
+            try testing.expectEqual(@as(usize, 0), expected.nimages);
+            try testing.expectEqual(@as(usize, 0), actual.nimages);
+            for (expected.cells, actual.cells) |*left, *right| try testing.expect(left.visuallyEqual(right));
+            for (expected.previous_cells, actual.previous_cells) |*left, *right| try testing.expect(left.visuallyEqual(right));
+            if (phase == 1 and transition != .off) try testing.expect(expected.panelTracks().len > 0);
+            if (phase == 1 and transition.needsPreviousGrid()) try testing.expect(expected.hasPanelDiff());
+            if (phase == 3 and transition == .vertical) try testing.expect(a.closing_len > 0);
+            control.acknowledgePanelPresentation(expected.panelTracks());
+            inspected.acknowledgePanelPresentation(actual.panelTracks());
+            try testing.expectEqualDeep(a.shown, b.shown);
+            try testing.expectEqualDeep(a.previous_layout, b.previous_layout);
+            _ = arena.reset(.retain_capacity);
+        }
+    }
+}
diff --git a/src/ninep/sources.zig b/src/ninep/sources.zig
new file mode 100644
index 00000000..b0408594
--- /dev/null
+++ b/src/ninep/sources.zig
@@ -0,0 +1,152 @@
+//! The editor's embedded sources, served read-only from the root when the
+//! build carries them (-Dembed-sources=true).
+const std = @import("std");
+const pardes = @import("../pardes.zig");
+const fs = @import("../fs.zig");
+const tree = @import("tree.zig");
+
+const Pardes = pardes.Pardes;
+const Req = tree.Req;
+const Reply = tree.Reply;
+const E = tree.E;
+const sources = fs.sources;
+
+pub const archive_node: u64 = 1 << 61;
+
+/// The node of an archive path, or null when nothing embedded lives there.
+/// The empty path is the archive root, which is never listed by itself.
+pub fn node(path: []const u8) ?u64 {
+    if (path.len == 0) return archive_node;
+    if (path.len >= 4096) return null;
+    for (sources, 0..) |source, i| {
+        if (std.mem.eql(u8, source.path, path) or
+            (source.path.len > path.len and source.path[path.len] == '/' and std.mem.startsWith(u8, source.path, path)))
+            return archive_node | (@as(u64, i) << 12) | path.len;
+    }
+    return null;
+}
+
+fn info(n: u64) ?struct { path: []const u8, contents: []const u8, dir: bool } {
+    if (n == archive_node) return .{ .path = "", .contents = "", .dir = true };
+    const index = (n & ~archive_node) >> 12;
+    if (index >= sources.len) return null;
+    const source = sources[@intCast(index)];
+    const len: usize = @intCast(n & 4095);
+    if (len > source.path.len) return null;
+    return .{ .path = source.path[0..len], .contents = source.contents, .dir = len < source.path.len };
+}
+
+/// Stages the children of one archive directory; "" lists the top-level
+/// directories the served root shows.
+pub fn stage(p: *Pardes, out: *std.ArrayList(u8), path: []const u8, skip: *u64) void {
+    var seen: [sources.len][]const u8 = undefined;
+    var count: usize = 0;
+    for (sources, 0..) |source, i| {
+        if (path.len != 0 and
+            (source.path.len <= path.len or source.path[path.len] != '/' or !std.mem.startsWith(u8, source.path, path))) continue;
+        const start = if (path.len == 0) 0 else path.len + 1;
+        const rest = source.path[start..];
+        const len = std.mem.indexOfScalar(u8, rest, '/') orelse rest.len;
+        const name = rest[0..len];
+        var duplicate = false;
+        for (seen[0..count]) |previous| if (std.mem.eql(u8, name, previous)) {
+            duplicate = true;
+            break;
+        };
+        if (duplicate) continue;
+        seen[count] = name;
+        count += 1;
+        if (skip.* > 0) {
+            skip.* -= 1;
+            continue;
+        }
+        const child = archive_node | (@as(u64, i) << 12) | (start + len);
+        tree.stageDirent(out, p.gpa, child, len < rest.len, name);
+    }
+}
+
+pub fn handle(p: *Pardes, req: Req) Reply {
+    const found = info(req.node) orelse return Reply.fail(req.tag, E.NOENT);
+    const attr: Reply.Attr = .{
+        .node = req.node,
+        .name = std.fs.path.basename(found.path),
+        .dir = found.dir,
+        .size = if (found.dir) 0 else found.contents.len,
+        .mode = if (found.dir) 0o555 else 0o444,
+        .mtime = p.fs.started,
+    };
+    switch (req.op) {
+        .getattr => return .{ .tag = req.tag, .attr = attr },
+        .open => return .{ .tag = req.tag, .handle = 1 },
+        .release => return .{ .tag = req.tag },
+        .lookup => {
+            if (!found.dir) return Reply.fail(req.tag, E.NOTDIR);
+            if (std.mem.eql(u8, req.data, "..")) {
+                const parent = std.fs.path.dirname(found.path) orelse "";
+                if (parent.len == 0) return tree.handle(p, .{ .tag = req.tag, .op = .getattr, .node = tree.root });
+                const parent_node = node(parent) orelse return Reply.fail(req.tag, E.NOENT);
+                return handle(p, .{ .tag = req.tag, .op = .getattr, .node = parent_node });
+            }
+            if (req.data.len == 0 or std.mem.indexOfAny(u8, req.data, "/\x00") != null) return Reply.fail(req.tag, E.NOENT);
+            var buf: [4096]u8 = undefined;
+            const path = if (found.path.len == 0) req.data else std.fmt.bufPrint(&buf, "{s}/{s}", .{ found.path, req.data }) catch return Reply.fail(req.tag, E.NOENT);
+            const child = node(path) orelse return Reply.fail(req.tag, E.NOENT);
+            return handle(p, .{ .tag = req.tag, .op = .getattr, .node = child });
+        },
+        .readdir => {
+            if (!found.dir) return Reply.fail(req.tag, E.NOTDIR);
+            const out = p.fs.stage(p.gpa);
+            var skip = req.off;
+            stage(p, out, found.path, &skip);
+            return .{ .tag = req.tag, .payload = .{ .staged = @intCast(out.items.len) } };
+        },
+        .read => {
+            if (found.dir) return Reply.fail(req.tag, E.PERM);
+            const off: usize = @intCast(@min(req.off, found.contents.len));
+            const bytes = found.contents[off..][0..@min(req.size, found.contents.len - off)];
+            const out = p.fs.stage(p.gpa);
+            out.appendSlice(p.gpa, bytes) catch return Reply.fail(req.tag, E.NOMEM);
+            return .{ .tag = req.tag, .payload = .{ .staged = @intCast(bytes.len) } };
+        },
+        else => return Reply.fail(req.tag, E.PERM),
+    }
+}
+
+const testing = std.testing;
+const th = @import("testing.zig");
+const limits = @import("../memory.zig").limits;
+
+test "embedded sources appear under the root only when the build carries them" {
+    const p = try th.withFile(testing.allocator, "x\n");
+    defer p.deinit();
+    var buf: [64]th.Dirent = undefined;
+    const top = th.dirents(th.rdir(p, tree.root, 0).bytes, &buf);
+    const listed = th.nameAt(top, "src");
+    try testing.expectEqual(limits.embedded_sources, listed != null);
+    const looked = th.look_up(p, tree.root, "src");
+    try testing.expectEqual(limits.embedded_sources, looked.reply.status == .ok);
+    try testing.expect(tree.resolveSelf(p, "shaders/../src/../") == tree.root);
+    if (!limits.embedded_sources) {
+        try testing.expect(tree.resolveSelf(p, "src/pardes.zig") == null);
+        try testing.expect(tree.resolveSelf(p, "src") == null);
+        try testing.expect(th.nameAt(top, "build.zig") == null);
+        return;
+    }
+    try testing.expect(looked.reply.attr.dir);
+    try testing.expectEqual(@as(u16, 0o555), looked.reply.attr.mode);
+    try testing.expectEqual(listed.?.node, looked.reply.attr.node);
+    try testing.expect(th.nameAt(top, "build.zig") == null);
+    try testing.expect(th.nameAt(top, "build.zig.zon") == null);
+    const file = th.look_up(p, looked.reply.attr.node, "pardes.zig");
+    try testing.expectEqual(tree.Status.ok, file.reply.status);
+    try testing.expectEqual(@as(u16, 0o444), file.reply.attr.mode);
+    try testing.expectEqual(@as(u64, fs.sourceBytes("src/pardes.zig").?.len), file.reply.attr.size);
+    try testing.expectEqual(file.reply.attr.node, tree.resolveSelf(p, "src/pardes.zig").?);
+    const opened = th.call(p, .{ .tag = 1, .op = .open, .node = file.reply.attr.node });
+    const head = th.call(p, .{ .tag = 2, .op = .read, .node = file.reply.attr.node, .handle = opened.reply.handle, .size = 16 });
+    try testing.expectEqualStrings(fs.sourceBytes("src/pardes.zig").?[0..16], head.bytes);
+    const parent = th.look_up(p, looked.reply.attr.node, "..");
+    try testing.expectEqual(tree.root, parent.reply.attr.node);
+    try testing.expectEqual(E.NOENT, th.look_up(p, tree.root, "build.zig").errno());
+    try testing.expectEqual(E.NOENT, th.look_up(p, looked.reply.attr.node, "missing.zig").errno());
+}
diff --git a/src/ninep/testing.zig b/src/ninep/testing.zig
new file mode 100644
index 00000000..ba26cb38
--- /dev/null
+++ b/src/ninep/testing.zig
@@ -0,0 +1,135 @@
+//! Test helpers shared by the control-tree modules: drive one request
+//! through the core and collect the reply and the effects it raised.
+const std = @import("std");
+const pardes = @import("../pardes.zig");
+const tree = @import("tree.zig");
+
+const Pardes = pardes.Pardes;
+pub const Req = tree.Req;
+pub const Reply = tree.Reply;
+pub const E = tree.E;
+
+pub const Answer = struct {
+    reply: Reply = .{ .tag = 0, .status = .err, .errno = E.IO },
+    bytes: []const u8 = "",
+    saved: bool = false,
+    watch: ?bool = null,
+    pty_buf: [256]u8 = undefined,
+    pty_len: usize = 0,
+    winsize: ?struct { cols: u16, rows: u16 } = null,
+    signal: ?pardes.PtySignal = null,
+    spawned: bool = false,
+
+    pub fn pty(a: *const Answer) []const u8 {
+        return a.pty_buf[0..a.pty_len];
+    }
+
+    pub fn errno(a: Answer) u16 {
+        return if (a.reply.status == .err) a.reply.errno else 0;
+    }
+};
+
+pub fn call(p: *Pardes, req: Req) Answer {
+    p.update(.{ .fs_req = req });
+    var ans: Answer = .{};
+    while (p.nextEffect()) |e| switch (e) {
+        .fs_reply => |r| {
+            ans.reply = r;
+            ans.bytes = p.fsPayload(r);
+        },
+        .save_file, .save_text => ans.saved = true,
+        .watch => |w| ans.watch = w.on,
+        .write => |w| {
+            const b = w.bytes.slice();
+            const n = @min(b.len, ans.pty_buf.len - ans.pty_len);
+            @memcpy(ans.pty_buf[ans.pty_len..][0..n], b[0..n]);
+            ans.pty_len += n;
+        },
+        .resize_pty => |r| ans.winsize = .{ .cols = r.cols, .rows = r.rows },
+        .signal_pty => |s| ans.signal = s.sig,
+        .spawn => ans.spawned = true,
+        else => {},
+    };
+    return ans;
+}
+
+pub fn rd(p: *Pardes, node: u64, off: u64, size: u32) Answer {
+    return call(p, .{ .tag = 1, .op = .read, .node = node, .off = off, .size = size });
+}
+
+pub fn wr(p: *Pardes, node: u64, data: []const u8) Answer {
+    return call(p, .{ .tag = 2, .op = .write, .node = node, .data = data });
+}
+
+pub fn rdir(p: *Pardes, node: u64, skip: u64) Answer {
+    return call(p, .{ .tag = 4, .op = .readdir, .node = node, .off = skip, .size = 4096 });
+}
+
+pub fn look_up(p: *Pardes, dir: u64, name: []const u8) Answer {
+    return call(p, .{ .tag = 3, .op = .lookup, .node = dir, .data = name });
+}
+
+/// Opens /new once, as a client would, and returns the pane it made.
+pub fn newPane(p: *Pardes) !u32 {
+    const new = @intFromEnum(tree.TopFile.new);
+    const opened = call(p, .{ .tag = 9, .op = .open, .node = new });
+    if (opened.reply.status != .ok or opened.reply.handle == 0) return error.NoPane;
+    _ = call(p, .{ .tag = 9, .op = .release, .node = new, .handle = opened.reply.handle });
+    return opened.reply.handle;
+}
+
+pub fn withFile(gpa: std.mem.Allocator, text: []const u8) !*Pardes {
+    const p = try Pardes.init(gpa, .{ .tty_only = true, .cols = 80, .rows = 24 });
+    errdefer p.deinit();
+    while (p.nextEffect()) |_| {}
+    _ = try p.setTestFile(text);
+    while (p.nextEffect()) |_| {}
+    return p;
+}
+
+pub fn withTerm(gpa: std.mem.Allocator) !*Pardes {
+    const p = try Pardes.init(gpa, .{ .tty_only = true, .cols = 80, .rows = 24 });
+    errdefer p.deinit();
+    while (p.nextEffect()) |_| {}
+    std.debug.assert(p.panes[0].?.isTerminal());
+    return p;
+}
+
+pub fn serialOf(p: *Pardes) u32 {
+    return p.panes[0].?.serial;
+}
+
+pub const Dirent = struct { node: u64, dir: bool, name: []const u8 };
+
+pub fn dirents(bytes: []const u8, out: []Dirent) []Dirent {
+    var n: usize = 0;
+    var i: usize = 0;
+    while (i + 10 <= bytes.len and n < out.len) {
+        const node = std.mem.readInt(u64, bytes[i..][0..8], .little);
+        const kind = bytes[i + 8];
+        const len = bytes[i + 9];
+        i += 10;
+        if (i + len > bytes.len) break;
+        out[n] = .{ .node = node, .dir = kind == 1, .name = bytes[i .. i + len] };
+        i += len;
+        n += 1;
+    }
+    return out[0..n];
+}
+
+pub fn nameAt(list: []const Dirent, want: []const u8) ?Dirent {
+    for (list) |d| if (std.mem.eql(u8, d.name, want)) return d;
+    return null;
+}
+
+pub const FakeTty = struct {
+    taken: bool,
+
+    pub const vtable: pardes.Host.VTable = .{ .tty_taken = answer };
+
+    fn answer(ctx: ?*anyopaque, pane: u8) bool {
+        _ = pane;
+        const f: *FakeTty = @ptrCast(@alignCast(ctx.?));
+        return f.taken;
+    }
+};
diff --git a/src/ninep/tree.zig b/src/ninep/tree.zig
new file mode 100644
index 00000000..f716bf5f
--- /dev/null
+++ b/src/ninep/tree.zig
@@ -0,0 +1,850 @@
+//! The control tree served over 9P: its nodes, lookup, readdir and the
+//! dispatch that answers the engine in src/9p.zig. The backend contract
+//! (Req, Reply, Op, Status, E) lives here; fs.zig keeps host access and mounts.
+//!
+//!   /README /index /ctl /new /log /screen /listeners
+//!   /pane/<serial>/{name,body,tag,ctl,addr,data,xdata,sel,errors,event,pty/}
+//!   /os/...  the host filesystem      /src/...  embedded sources (opt-in)
+const std = @import("std");
+const pardes = @import("../pardes.zig");
+const fs = @import("../fs.zig");
+pub const events = @import("events.zig");
+pub const pane = @import("pane.zig");
+pub const addr = @import("addr.zig");
+pub const ctl = @import("ctl.zig");
+pub const pty = @import("pty.zig");
+pub const sources = @import("sources.zig");
+pub const screen = @import("screen.zig");
+
+const Pardes = pardes.Pardes;
+const Pane = pardes.Pane;
+const MAX_PANES = pardes.MAX_PANES;
+
+pub const name_capacity: usize = 255;
+
+// ---- the backend contract ----
+
+pub const Op = enum(u8) {
+    lookup,
+    getattr,
+    setattr,
+    open,
+    read,
+    write,
+    release,
+    readdir,
+};
+
+pub const Status = enum(u8) {
+    ok,
+    again,
+    err,
+};
+
+pub const Req = struct {
+    tag: u64,
+    op: Op,
+    node: u64,
+    handle: u32 = 0,
+    off: u64 = 0,
+    size: u32 = 0,
+    data: []const u8 = &.{},
+    truncate: bool = false,
+
+    pub fn changesPane(req: Req) bool {
+        return switch (req.op) {
+            .write, .setattr => true,
+            .open => req.node == @intFromEnum(TopFile.new),
+            .lookup, .getattr, .read, .release, .readdir => false,
+        };
+    }
+};
+
+pub const Reply = struct {
+    tag: u64,
+    status: Status = .ok,
+    errno: u16 = 0,
+    attr: Attr = .{},
+    handle: u32 = 0,
+    payload: Payload = .none,
+    written: u32 = 0,
+
+    pub const Attr = struct {
+        name: []const u8 = "",
+        node: u64 = 0,
+        dir: bool = false,
+        size: u64 = 0,
+        mode: u16 = 0o644,
+        mtime: u32 = 0,
+    };
+
+    pub const Payload = union(enum) {
+        none,
+        staged: u32,
+        region: struct { pane: u8, serial: u32, off: u32, len: u32 },
+    };
+
+    pub fn fail(tag: u64, e: u16) Reply {
+        return .{ .tag = tag, .status = .err, .errno = e };
+    }
+};
+
+pub const E = struct {
+    pub const PERM: u16 = 1;
+    pub const NOENT: u16 = 2;
+    pub const IO: u16 = 5;
+    pub const NOMEM: u16 = 12;
+    pub const NOTDIR: u16 = 20;
+    pub const INVAL: u16 = 22;
+    pub const NFILE: u16 = 23;
+    pub const NOSPC: u16 = 28;
+    pub const NOSYS: u16 = 38;
+};
+
+pub const out_reserve = 4 * 1024;
+
+// ---- nodes ----
+
+pub const TopFile = enum(u4) {
+    root = 1,
+    README,
+    index,
+    ctl,
+    new,
+    log,
+    screen,
+    listeners,
+    pane,
+
+    pub fn fileName(f: TopFile) []const u8 {
+        return if (f == .root) "/" else @tagName(f);
+    }
+
+    pub fn mode(f: TopFile) u16 {
+        return switch (f) {
+            .root, .pane => 0o755,
+            .ctl => 0o666,
+            .README, .index, .new, .log, .screen, .listeners => 0o444,
+        };
+    }
+
+    pub fn dir(f: TopFile) bool {
+        return f == .root or f == .pane;
+    }
+};
+
+pub const root: u64 = @intFromEnum(TopFile.root);
+
+pub const PaneFile = enum(u4) {
+    dir = 0,
+    name,
+    body,
+    tag,
+    ctl,
+    addr,
+    data,
+    xdata,
+    sel,
+    errors,
+    event,
+    pty,
+    pty_ctl,
+    pty_status,
+    pty_data,
+
+    pub fn fileName(f: PaneFile) []const u8 {
+        return switch (f) {
+            .dir => ".",
+            .pty_ctl => "ctl",
+            .pty_status => "status",
+            .pty_data => "data",
+            else => @tagName(f),
+        };
+    }
+
+    pub fn mode(f: PaneFile) u16 {
+        return switch (f) {
+            .dir, .pty => 0o755,
+            .errors, .pty_ctl => 0o222,
+            .pty_status => 0o444,
+            else => 0o666,
+        };
+    }
+
+    pub fn isDir(f: PaneFile) bool {
+        return f == .dir or f == .pty;
+    }
+
+    pub fn inPty(f: PaneFile) bool {
+        return switch (f) {
+            .pty, .pty_ctl, .pty_status, .pty_data => true,
+            else => false,
+        };
+    }
+};
+
+pub const Node = packed struct(u64) {
+    file: u4 = 0,
+    serial: u60 = 0,
+
+    pub fn of(serial: u32, file: PaneFile) u64 {
+        std.debug.assert(serial != 0);
+        return @bitCast(Node{ .file = @intFromEnum(file), .serial = serial });
+    }
+
+    pub fn target(node: u64) ?Target {
+        const n: Node = @bitCast(node);
+        if (n.serial == 0) {
+            return .{ .top = std.enums.fromInt(TopFile, n.file) orelse return null };
+        }
+        return .{ .pane = .{
+            .serial = std.math.cast(u32, n.serial) orelse return null,
+            .file = std.enums.fromInt(PaneFile, n.file) orelse return null,
+        } };
+    }
+};
+
+pub const Target = union(enum) {
+    top: TopFile,
+    pane: struct { serial: u32, file: PaneFile },
+};
+
+fn paneFileNamed(name: []const u8) ?PaneFile {
+    const f = std.meta.stringToEnum(PaneFile, name) orelse return null;
+    if (f == .dir) return null;
+    return if (f.inPty() and f != .pty) null else f;
+}
+
+fn topFileNamed(name: []const u8) ?TopFile {
+    const f = std.meta.stringToEnum(TopFile, name) orelse return null;
+    return if (f == .root) null else f;
+}
+
+fn serialNamed(name: []const u8) ?u32 {
+    if (name.len == 0 or name.len > 10) return null;
+    for (name) |c| if (c < '0' or c > '9') return null;
+    const serial = std.fmt.parseInt(u32, name, 10) catch return null;
+    return if (serial == 0) null else serial;
+}
+
+pub fn nextSerialAfter(p: *Pardes, after: u32) ?u32 {
+    var best: ?u32 = null;
+    for (p.panes) |slot| {
+        const pn = slot orelse continue;
+        if (pn.serial <= after) continue;
+        if (best == null or pn.serial < best.?) best = pn.serial;
+    }
+    return best;
+}
+
+/// An editor path below the root (`pane/3/body`, `index`, `src/...`) to its node.
+pub fn resolveSelf(p: *Pardes, path: []const u8) ?u64 {
+    var path_buf: [4096]u8 = undefined;
+    const normalized = fs.normalizeVirtualPath(path, &path_buf) orelse return null;
+    var parts = std.mem.tokenizeScalar(u8, normalized, '/');
+    const first = parts.next() orelse return root;
+    if (!std.mem.eql(u8, first, "pane")) {
+        const top = topFileNamed(first) orelse return sources.node(normalized);
+        if (top == .pane or parts.next() != null) return null;
+        return @intFromEnum(top);
+    }
+    const serial = serialNamed(parts.next() orelse return @intFromEnum(TopFile.pane)) orelse return null;
+    const id = p.paneBySerial(serial) orelse return null;
+    const file = paneFileNamed(parts.next() orelse return Node.of(serial, .dir)) orelse return null;
+    if (file.inPty() and !p.panes[id].?.isTerminal()) return null;
+    if (parts.next()) |name| {
+        if (file != .pty) return null;
+        const child = pty.fileNamed(name) orelse return null;
+        if (parts.next() != null) return null;
+        return Node.of(serial, child);
+    }
+    return Node.of(serial, file);
+}
+
+// ---- staging ----
+
+pub fn stageDirent(out: *std.ArrayList(u8), gpa: std.mem.Allocator, node: u64, dir: bool, name: []const u8) void {
+    if (name.len == 0 or name.len > 255) return;
+    var head: [10]u8 = undefined;
+    std.mem.writeInt(u64, head[0..8], node, .little);
+    head[8] = @intFromBool(dir);
+    head[9] = @intCast(name.len);
+    out.appendSlice(gpa, &head) catch return;
+    out.appendSlice(gpa, name) catch return;
+}
+
+/// Answers a read from whatever the handler staged, windowed by offset and size.
+pub fn stagedReply(p: *Pardes, req: Req) Reply {
+    const out = &p.fs.out;
+    const off = @min(req.off, out.items.len);
+    const n = @min(out.items.len - off, req.size);
+    if (off > 0) std.mem.copyForwards(u8, out.items[0..n], out.items[off..][0..n]);
+    out.shrinkRetainingCapacity(n);
+    return .{ .tag = req.tag, .payload = .{ .staged = @intCast(n) } };
+}
+
+// ---- dispatch ----
+
+pub fn handle(p: *Pardes, req: Req) Reply {
+    if (req.node == fs.os_root or req.node & fs.os_node != 0) return fs.osHandle(p, req);
+    if (req.node & sources.archive_node != 0) return sources.handle(p, req);
+    const target = Node.target(req.node) orelse return Reply.fail(req.tag, E.NOENT);
+    if (req.op == .write or req.op == .setattr) p.fs.origin = switch (target) {
+        .pane => |t| @as(u8, if (t.file == .body or t.file == .tag) 'E' else 'F'),
+        .top => 'F',
+    };
+    return switch (req.op) {
+        .lookup => lookup(p, req, target),
+        .getattr => attrReply(p, req.tag, target),
+        .setattr => setattr(p, req, target),
+        .open => open(p, req, target),
+        .release => release(p, req),
+        .readdir => readdir(p, req, target),
+        .read => read(p, req, target),
+        .write => write(p, req, target),
+    };
+}
+
+fn attrOf(p: *Pardes, target: Target) ?Reply.Attr {
+    switch (target) {
+        .top => |f| return .{
+            .name = f.fileName(),
+            .node = @intFromEnum(f),
+            .dir = f.dir(),
+            .mode = f.mode(),
+            .size = topSize(p, f),
+            .mtime = p.fs.started,
+        },
+        .pane => |t| {
+            const id = p.paneBySerial(t.serial) orelse return null;
+            const pn = p.panes[id].?;
+            if (t.file.inPty() and !pn.isTerminal()) return null;
+            return .{
+                .name = if (t.file == .dir) (std.fmt.bufPrint(&p.fs.node_name, "{d}", .{t.serial}) catch unreachable) else t.file.fileName(),
+                .node = Node.of(t.serial, t.file),
+                .dir = t.file.isDir(),
+                .mode = t.file.mode(),
+                .size = pane.fileSize(p, id, t.file),
+                .mtime = pane.mtimeOf(p, pn),
+            };
+        },
+    }
+}
+
+fn attrReply(p: *Pardes, tag: u64, target: Target) Reply {
+    const a = attrOf(p, target) orelse return Reply.fail(tag, E.NOENT);
+    return .{ .tag = tag, .attr = a };
+}
+
+fn topSize(p: *Pardes, f: TopFile) u64 {
+    return switch (f) {
+        .root, .pane, .new, .log, .screen, .listeners => 0,
+        .index => pane.indexLen(p),
+        .README => fs.help.len,
+        .ctl => ctl: {
+            var buf: [ctl.root_text_cap]u8 = undefined;
+            break :ctl ctl.rootText(p, &buf).len;
+        },
+    };
+}
+
+fn lookup(p: *Pardes, req: Req, target: Target) Reply {
+    const name = req.data;
+    if (std.mem.eql(u8, name, "..")) {
+        const parent: u64 = switch (target) {
+            .top => |top| if (top.dir()) root else return Reply.fail(req.tag, E.NOTDIR),
+            .pane => |t| switch (t.file) {
+                .dir => @intFromEnum(TopFile.pane),
+                .pty => Node.of(t.serial, .dir),
+                else => return Reply.fail(req.tag, E.NOTDIR),
+            },
+        };
+        return handle(p, .{ .tag = req.tag, .op = .getattr, .node = parent });
+    }
+    if (name.len == 0 or std.mem.indexOfAny(u8, name, "/\x00") != null) return Reply.fail(req.tag, E.NOENT);
+    const node: u64 = switch (target) {
+        .top => |f| switch (f) {
+            .root => root: {
+                if (std.mem.eql(u8, name, "os")) return fs.osHandle(p, .{ .tag = req.tag, .op = .getattr, .node = fs.os_root });
+                if (topFileNamed(name)) |t| break :root @intFromEnum(t);
+                if (sources.node(name)) |n| return sources.handle(p, .{ .tag = req.tag, .op = .getattr, .node = n });
+                return Reply.fail(req.tag, E.NOENT);
+            },
+            .pane => pane: {
+                const serial = serialNamed(name) orelse return Reply.fail(req.tag, E.NOENT);
+                break :pane Node.of(serial, .dir);
+            },
+            else => return Reply.fail(req.tag, E.NOTDIR),
+        },
+        .pane => |t| pane: {
+            _ = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
+            const f = switch (t.file) {
+                .dir => paneFileNamed(name),
+                .pty => pty.fileNamed(name),
+                else => return Reply.fail(req.tag, E.NOTDIR),
+            } orelse return Reply.fail(req.tag, E.NOENT);
+            break :pane Node.of(t.serial, f);
+        },
+    };
+    return attrReply(p, req.tag, Node.target(node) orelse return Reply.fail(req.tag, E.NOENT));
+}
+
+fn stagePaneFiles(p: *Pardes, out: *std.ArrayList(u8), serial: u32, terminal: bool, skip: *u64) void {
+    inline for (comptime std.enums.values(PaneFile)) |f| {
+        if (comptime f == .dir or (f.inPty() and f != .pty)) continue;
+        const present = f != .pty or terminal;
+        if (present) {
+            if (skip.* > 0) skip.* -= 1 else stageDirent(out, p.gpa, Node.of(serial, f), f.isDir(), f.fileName());
+        }
+    }
+}
+
+fn stagePtyFiles(p: *Pardes, out: *std.ArrayList(u8), serial: u32, skip: *u64) void {
+    inline for (comptime std.enums.values(PaneFile)) |f| {
+        if (comptime !f.inPty() or f == .pty) continue;
+        if (skip.* > 0) skip.* -= 1 else stageDirent(out, p.gpa, Node.of(serial, f), false, f.fileName());
+    }
+}
+
+fn readdir(p: *Pardes, req: Req, target: Target) Reply {
+    const out = p.fs.stage(p.gpa);
+    var skip = req.off;
+    switch (target) {
+        .top => |f| switch (f) {
+            .root => {
+                inline for (.{ TopFile.README, TopFile.index, TopFile.ctl, TopFile.new, TopFile.log, TopFile.screen, TopFile.listeners, TopFile.pane }) |t| {
+                    if (skip > 0) skip -= 1 else stageDirent(out, p.gpa, @intFromEnum(t), t.dir(), t.fileName());
+                }
+                if (skip > 0) skip -= 1 else stageDirent(out, p.gpa, fs.os_root, true, "os");
+                sources.stage(p, out, "", &skip);
+            },
+            .pane => {
+                var last: u32 = 0;
+                while (nextSerialAfter(p, last)) |serial| {
+                    last = serial;
+                    if (skip > 0) {
+                        skip -= 1;
+                        continue;
+                    }
+                    var buf: [16]u8 = undefined;
+                    const name = std.fmt.bufPrint(&buf, "{d}", .{serial}) catch unreachable;
+                    stageDirent(out, p.gpa, Node.of(serial, .dir), true, name);
+                }
+            },
+            else => return Reply.fail(req.tag, E.NOTDIR),
+        },
+        .pane => |t| {
+            const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
+            const terminal = p.panes[id].?.isTerminal();
+            switch (t.file) {
+                .dir => stagePaneFiles(p, out, t.serial, terminal, &skip),
+                .pty => {
+                    if (!terminal) return Reply.fail(req.tag, E.NOENT);
+                    stagePtyFiles(p, out, t.serial, &skip);
+                },
+                else => return Reply.fail(req.tag, E.NOTDIR),
+            }
+        },
+    }
+    return .{ .tag = req.tag, .payload = .{ .staged = @intCast(out.items.len) } };
+}
+
+fn newPane(p: *Pardes) ?u32 {
+    const slot = p.freeSlot() orelse return null;
+    p.newScratchBelow(p.active);
+    const pn = p.panes[slot] orelse return null;
+    return pn.serial;
+}
+
+fn open(p: *Pardes, req: Req, target: Target) Reply {
+    switch (target) {
+        .top => |f| switch (f) {
+            // The clone idiom: each open makes one empty pane; reads answer its serial.
+            .new => {
+                const serial = newPane(p) orelse return Reply.fail(req.tag, E.NFILE);
+                return .{ .tag = req.tag, .handle = serial };
+            },
+            .screen => return screen.openSnapshot(p, req, true),
+            .log => p.fs.log_readers +|= 1,
+            else => {},
+        },
+        .pane => |t| {
+            const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
+            const pn = p.panes[id].?;
+            const pf = &p.fs.panes[id];
+            if (t.file.inPty() and !pn.isTerminal()) return Reply.fail(req.tag, E.NOENT);
+            switch (t.file) {
+                .body => if (pn.isTerminal()) return screen.openSnapshot(p, req, false),
+                .ctl => pf.limit = null,
+                .addr => {
+                    pf.addr = .{};
+                    pf.limit = null;
+                },
+                .event => {
+                    pf.readers +|= 1;
+                    p.fs.listeners +|= 1;
+                },
+                .pty_data => pf.pty_readers +|= 1,
+                else => {},
+            }
+        },
+    }
+    return .{ .tag = req.tag, .handle = 1 };
+}
+
+fn release(p: *Pardes, req: Req) Reply {
+    if (screen.releaseSnapshot(p, req)) return .{ .tag = req.tag };
+    const target = Node.target(req.node) orelse return .{ .tag = req.tag };
+    switch (target) {
+        .top => |f| if (f == .log and p.fs.log_readers > 0) {
+            p.fs.log_readers -= 1;
+            if (p.fs.log_readers == 0) p.fs.log.clearAndFree(p.gpa);
+        },
+        .pane => |t| {
+            if (t.file != .event and t.file != .pty_data) return .{ .tag = req.tag };
+            const id = p.paneBySerial(t.serial) orelse return .{ .tag = req.tag };
+            const pf = &p.fs.panes[id];
+            if (t.file == .pty_data) {
+                if (pf.pty_readers == 0) return .{ .tag = req.tag };
+                pf.pty_readers -= 1;
+                if (pf.pty_readers == 0) pf.pty_out.clearAndFree(p.gpa);
+                return .{ .tag = req.tag };
+            }
+            if (pf.readers == 0) return .{ .tag = req.tag };
+            pf.readers -= 1;
+            p.fs.listeners -|= 1;
+            if (pf.readers == 0) pf.tag_snap.clearAndFree(p.gpa);
+        },
+    }
+    return .{ .tag = req.tag };
+}
+
+fn setattr(p: *Pardes, req: Req, target: Target) Reply {
+    if (req.truncate) switch (target) {
+        .pane => |t| {
+            const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
+            if (pane.truncate(p, id, p.panes[id].?, t.file) != .ok) return Reply.fail(req.tag, E.NOMEM);
+        },
+        .top => {},
+    };
+    return attrReply(p, req.tag, target);
+}
+
+fn read(p: *Pardes, req: Req, target: Target) Reply {
+    switch (target) {
+        .top => |f| return switch (f) {
+            .README => help: {
+                const off = @min(req.off, fs.help.len);
+                const bytes = fs.help[off..][0..@min(req.size, fs.help.len - off)];
+                p.fs.stage(p.gpa).appendSlice(p.gpa, bytes) catch break :help Reply.fail(req.tag, E.NOMEM);
+                break :help .{ .tag = req.tag, .payload = .{ .staged = @intCast(bytes.len) } };
+            },
+            .index => index: {
+                const out = p.fs.stage(p.gpa);
+                var last: u32 = 0;
+                while (nextSerialAfter(p, last)) |serial| {
+                    last = serial;
+                    pane.stageIndexLine(p, out, p.panes[p.paneBySerial(serial).?].?);
+                }
+                break :index stagedReply(p, req);
+            },
+            .ctl => ctl.readRoot(p, req),
+            .new => new: {
+                if (req.handle == 0) break :new Reply.fail(req.tag, E.INVAL);
+                p.fs.stage(p.gpa).print(p.gpa, "{d}\n", .{req.handle}) catch break :new Reply.fail(req.tag, E.NOMEM);
+                break :new stagedReply(p, req);
+            },
+            .log => events.readQueue(p, req, &p.fs.log),
+            .screen => screen.readSnapshot(p, req, null),
+            .listeners => screen.readListeners(p, req),
+            .root, .pane => Reply.fail(req.tag, E.PERM),
+        },
+        .pane => |t| {
+            const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
+            const pn = p.panes[id].?;
+            if (t.file.inPty() and !pn.isTerminal()) return Reply.fail(req.tag, E.NOENT);
+            return pane.read(p, req, id, pn, t.file);
+        },
+    }
+}
+
+fn write(p: *Pardes, req: Req, target: Target) Reply {
+    switch (target) {
+        .top => |f| return switch (f) {
+            .ctl => ctl.writeRoot(p, req),
+            else => Reply.fail(req.tag, E.PERM),
+        },
+        .pane => |t| {
+            const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
+            const pn = p.panes[id].?;
+            if (t.file.inPty() and !pn.isTerminal()) return Reply.fail(req.tag, E.NOENT);
+            return pane.write(p, req, id, pn, t.file);
+        },
+    }
+}
+
+// ---- tests ----
+
+test {
+    _ = events;
+    _ = pane;
+    _ = addr;
+    _ = ctl;
+    _ = pty;
+    _ = sources;
+    _ = screen;
+}
+
+const testing = std.testing;
+const th = @import("testing.zig");
+const config = @import("../config.zig");
+const call = th.call;
+const rd = th.rd;
+const wr = th.wr;
+const rdir = th.rdir;
+const look_up = th.look_up;
+const withFile = th.withFile;
+const serialOf = th.serialOf;
+
+test "filesystem inspection preserves pending and displayed Look hover" {
+    const delay = config.look_preview_delay_frames orelse return;
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "alpha beta gamma\n");
+    defer p.deinit();
+    var frame: std.heap.ArenaAllocator = .init(gpa);
+    defer frame.deinit();
+    _ = try p.render(frame.allocator());
+    const rect = p.rects[0];
+    p.update(.{ .mouse = .{
+        .button = .none,
+        .kind = .motion,
+        .col = rect.x + config.GUTTER + config.PREFIX_W + 7,
+        .row = if (p.settings.tag_bottom) rect.y else rect.y + pardes.BOX_H,
+    } });
+    try testing.expect(p.look_hover_wait != null);
+    const body = Node.of(serialOf(p), .body);
+    const screen_node = @intFromEnum(TopFile.screen);
+    for (0..2) |phase| {
+        if (phase == 1) {
+            for (0..delay) |_| p.update(.tick);
+            try testing.expect(p.look_hover_preview != null);
+        }
+        const waiting = p.look_hover_wait;
+        const preview = p.look_hover_preview;
+        const requests = [_]Req{
+            .{ .tag = 1, .op = .lookup, .node = root, .data = "screen" },
+            .{ .tag = 1, .op = .lookup, .node = root, .data = "new" },
+            .{ .tag = 1, .op = .getattr, .node = @intFromEnum(TopFile.new) },
+            .{ .tag = 2, .op = .getattr, .node = body },
+            .{ .tag = 3, .op = .open, .node = body },
+            .{ .tag = 4, .op = .read, .node = body, .size = 5 },
+            .{ .tag = 5, .op = .release, .node = body, .handle = 1 },
+            .{ .tag = 6, .op = .readdir, .node = root, .size = 4096 },
+            .{ .tag = 7, .op = .open, .node = screen_node },
+            .{ .tag = 8, .op = .read, .node = screen_node, .handle = 1, .size = 32 },
+            .{ .tag = 9, .op = .release, .node = screen_node, .handle = 1 },
+        };
+        for (requests) |req| {
+            const answer = call(p, req);
+            try testing.expectEqual(Status.ok, answer.reply.status);
+            try testing.expect(std.meta.eql(waiting, p.look_hover_wait));
+            try testing.expect(std.meta.eql(preview, p.look_hover_preview));
+            try testing.expect(p.raw_hover_intent);
+        }
+    }
+    try testing.expectEqual(Status.ok, wr(p, body, "changed").reply.status);
+    try testing.expect(p.look_hover_wait == null);
+    try testing.expect(p.look_hover_preview == null);
+    try testing.expect(!p.raw_hover_intent);
+}
+
+test "filesystem pane creation and truncation cancel Look hover" {
+    const requests = [_]Req{
+        .{ .tag = 2, .op = .open, .node = @intFromEnum(TopFile.new) },
+        .{ .tag = 3, .op = .setattr, .node = 0, .truncate = true },
+    };
+    for (requests) |request| {
+        const p = try withFile(testing.allocator, "word\n");
+        defer p.deinit();
+        p.look_hover_wait = .{ .col = 1, .row = 1, .pane = 0, .serial = serialOf(p) };
+        p.raw_hover_intent = true;
+        var req = request;
+        if (req.op == .setattr) req.node = Node.of(serialOf(p), .body);
+        try testing.expectEqual(Status.ok, call(p, req).reply.status);
+        try testing.expect(p.look_hover_wait == null);
+        try testing.expect(p.look_hover_preview == null);
+        try testing.expect(!p.raw_hover_intent);
+    }
+}
+
+test "readdir lists the root and a pane directory without creating anything" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "hello\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    var buf: [32]th.Dirent = undefined;
+
+    const before = p.next_serial;
+    const top_reply = rdir(p, root, 0);
+    try testing.expectEqual(Status.ok, top_reply.reply.status);
+    var top_bytes: [4096]u8 = undefined;
+    @memcpy(top_bytes[0..top_reply.bytes.len], top_reply.bytes);
+    const top = th.dirents(top_bytes[0..top_reply.bytes.len], &buf);
+    try testing.expect(top.len >= 9);
+    for ([_][]const u8{ "README", "index", "ctl", "new", "log", "screen", "listeners", "pane", "os" }, 0..) |name, i| {
+        try testing.expectEqualStrings(name, top[i].name);
+        try testing.expectEqual(i >= 7, top[i].dir);
+    }
+    try testing.expectEqual(@intFromEnum(TopFile.pane), top[7].node);
+    try testing.expectEqual(fs.os_root, top[8].node);
+    var idbuf: [16]u8 = undefined;
+    try testing.expect(th.nameAt(top, try std.fmt.bufPrint(&idbuf, "{d}", .{serial})) == null);
+    try testing.expect(th.nameAt(top, "self") == null);
+    try testing.expect(th.nameAt(top, "cons") == null);
+
+    var rest_buf: [32]th.Dirent = undefined;
+    const rest = rdir(p, root, 3);
+    try testing.expectEqual(top.len - 3, th.dirents(rest.bytes, &rest_buf).len);
+    const eof = rdir(p, root, 99);
+    try testing.expectEqual(Status.ok, eof.reply.status);
+    try testing.expectEqual(@as(usize, 0), eof.bytes.len);
+
+    // Walk, stat and open-for-read every top entry a browser would, then every pane file.
+    for (top) |entry| {
+        if (std.mem.eql(u8, entry.name, "os")) continue;
+        const inspected = look_up(p, root, entry.name);
+        try testing.expectEqual(Status.ok, inspected.reply.status);
+        try testing.expectEqual(entry.node, inspected.reply.attr.node);
+        try testing.expectEqual(Status.ok, call(p, .{ .tag = 1, .op = .getattr, .node = entry.node }).reply.status);
+        try testing.expectEqual(before, p.next_serial);
+    }
+    const panes_dir = rdir(p, @intFromEnum(TopFile.pane), 0);
+    const listed = th.dirents(panes_dir.bytes, &buf);
+    try testing.expectEqual(@as(usize, 1), listed.len);
+    try testing.expectEqual(Node.of(serial, .dir), listed[0].node);
+
+    const dir = rdir(p, Node.of(serial, .dir), 0);
+    const files = th.dirents(dir.bytes, &buf);
+    try testing.expectEqual(@as(usize, 10), files.len);
+    for ([_][]const u8{ "name", "body", "tag", "ctl", "addr", "data", "xdata", "sel", "errors", "event" }, 0..) |name, i|
+        try testing.expectEqualStrings(name, files[i].name);
+    try testing.expect(th.nameAt(files, ".") == null);
+    try testing.expect(th.nameAt(files, "rdsel") == null);
+    try testing.expectEqual(Node.of(serial, .body), th.nameAt(files, "body").?.node);
+    for (files) |entry| {
+        const stat = call(p, .{ .tag = 1, .op = .getattr, .node = entry.node });
+        try testing.expectEqual(Status.ok, stat.reply.status);
+        try testing.expectEqual(p.fs.started, stat.reply.attr.mtime);
+    }
+    try testing.expectEqual(before, p.next_serial);
+
+    try testing.expectEqual(E.NOTDIR, rdir(p, Node.of(serial, .body), 0).errno());
+    try testing.expectEqual(E.NOTDIR, rdir(p, @intFromEnum(TopFile.index), 0).errno());
+}
+
+test "lookup resolves top files, pane serials and pane files" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "hello\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+
+    try testing.expectEqual(@as(u64, @intFromEnum(TopFile.index)), look_up(p, root, "index").reply.attr.node);
+    try testing.expect(!look_up(p, root, "new").reply.attr.dir);
+    try testing.expectEqual(@as(u16, 0o444), look_up(p, root, "new").reply.attr.mode);
+    try testing.expect(look_up(p, root, "pane").reply.attr.dir);
+    try testing.expect(look_up(p, root, "os").reply.attr.dir);
+    try testing.expectEqual(E.NOENT, look_up(p, root, "nosuchthing").errno());
+    try testing.expectEqual(E.NOENT, look_up(p, root, "self").errno());
+    try testing.expectEqual(E.NOENT, look_up(p, root, "cons").errno());
+    try testing.expectEqual(E.NOTDIR, look_up(p, @intFromEnum(TopFile.index), "x").errno());
+
+    var idbuf: [16]u8 = undefined;
+    const serial_name = try std.fmt.bufPrint(&idbuf, "{d}", .{serial});
+    try testing.expectEqual(E.NOENT, look_up(p, root, serial_name).errno());
+    const dir = look_up(p, @intFromEnum(TopFile.pane), serial_name);
+    try testing.expectEqual(Node.of(serial, .dir), dir.reply.attr.node);
+    try testing.expect(dir.reply.attr.dir);
+    try testing.expectEqualStrings(serial_name, dir.reply.attr.name);
+    try testing.expectEqual(E.NOENT, look_up(p, @intFromEnum(TopFile.pane), "99999").errno());
+    try testing.expectEqual(E.NOENT, look_up(p, @intFromEnum(TopFile.pane), "0").errno());
+
+    const body = look_up(p, Node.of(serial, .dir), "body");
+    try testing.expectEqual(Node.of(serial, .body), body.reply.attr.node);
+    const stat = call(p, .{ .tag = 4, .op = .getattr, .node = Node.of(serial, .body) });
+    try testing.expectEqual(body.reply.attr.size, stat.reply.attr.size);
+    try testing.expectEqual(@as(u64, "hello\n".len), stat.reply.attr.size);
+    try testing.expectEqual(E.NOENT, look_up(p, Node.of(serial, .dir), "editout").errno());
+    try testing.expectEqual(E.NOENT, look_up(p, Node.of(serial, .dir), "rdsel").errno());
+    try testing.expectEqual(E.NOTDIR, look_up(p, Node.of(serial, .body), "x").errno());
+}
+
+test "each open of new creates exactly one pane whose serial the read answers" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "first\n");
+    defer p.deinit();
+    const before = p.next_serial;
+    const new = @intFromEnum(TopFile.new);
+
+    try testing.expectEqual(Status.ok, look_up(p, root, "new").reply.status);
+    try testing.expectEqual(Status.ok, call(p, .{ .tag = 1, .op = .getattr, .node = new }).reply.status);
+    try testing.expectEqual(before, p.next_serial);
+
+    const opened = call(p, .{ .tag = 1, .op = .open, .node = new });
+    try testing.expectEqual(Status.ok, opened.reply.status);
+    try testing.expectEqual(before + 1, p.next_serial);
+    try testing.expectEqual(before + 1, opened.reply.handle);
+    try testing.expectEqual(@as(u64, 0), opened.reply.attr.node);
+    var expected: [16]u8 = undefined;
+    const line = try std.fmt.bufPrint(&expected, "{d}\n", .{opened.reply.handle});
+    try testing.expectEqualStrings(line, call(p, .{ .tag = 2, .op = .read, .node = new, .handle = opened.reply.handle, .size = 64 }).bytes);
+    try testing.expectEqualStrings(line[1..], call(p, .{ .tag = 2, .op = .read, .node = new, .handle = opened.reply.handle, .off = 1, .size = 64 }).bytes);
+    try testing.expectEqualStrings("", call(p, .{ .tag = 2, .op = .read, .node = new, .handle = opened.reply.handle, .off = line.len, .size = 64 }).bytes);
+    try testing.expectEqual(before + 1, p.next_serial);
+    _ = call(p, .{ .tag = 3, .op = .release, .node = new, .handle = opened.reply.handle });
+
+    const id = p.paneBySerial(opened.reply.handle).?;
+    try testing.expectEqualStrings("", p.panes[id].?.file.?.content);
+    _ = wr(p, Node.of(opened.reply.handle, .body), "hi");
+    const second = call(p, .{ .tag = 4, .op = .open, .node = new });
+    try testing.expectEqual(Status.ok, second.reply.status);
+    try testing.expect(second.reply.handle != opened.reply.handle);
+    try testing.expectEqualStrings("hi", p.panes[id].?.file.?.content);
+    try testing.expectEqual(E.INVAL, call(p, .{ .tag = 5, .op = .read, .node = new, .size = 64 }).errno());
+}
+
+test "directories walk to their parents" {
+    const p = try withFile(testing.allocator, "contents\n");
+    defer p.deinit();
+    for ([_]struct { node: u64, parent: u64 }{
+        .{ .node = root, .parent = root },
+        .{ .node = @intFromEnum(TopFile.pane), .parent = root },
+        .{ .node = Node.of(serialOf(p), .dir), .parent = @intFromEnum(TopFile.pane) },
+        .{ .node = fs.os_root, .parent = root },
+    }) |case| {
+        const result = look_up(p, case.node, "..");
+        try testing.expectEqual(Status.ok, result.reply.status);
+        try testing.expectEqual(case.parent, result.reply.attr.node);
+    }
+    try testing.expectEqual(E.NOTDIR, look_up(p, @intFromEnum(TopFile.index), "..").errno());
+    try testing.expectEqual(E.NOTDIR, look_up(p, Node.of(serialOf(p), .body), "..").errno());
+}
+
+test "editor paths resolve to the same nodes the wire serves" {
+    const p = try withFile(testing.allocator, "contents\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    var path: [64]u8 = undefined;
+    try testing.expectEqual(root, resolveSelf(p, "").?);
+    try testing.expectEqual(root, resolveSelf(p, "/./pane/../").?);
+    try testing.expectEqual(@intFromEnum(TopFile.index), resolveSelf(p, "index").?);
+    try testing.expectEqual(@intFromEnum(TopFile.new), resolveSelf(p, "new").?);
+    try testing.expectEqual(@intFromEnum(TopFile.pane), resolveSelf(p, "pane").?);
+    try testing.expectEqual(Node.of(serial, .dir), resolveSelf(p, try std.fmt.bufPrint(&path, "pane/{d}", .{serial})).?);
+    try testing.expectEqual(Node.of(serial, .sel), resolveSelf(p, try std.fmt.bufPrint(&path, "pane/{d}/sel", .{serial})).?);
+    try testing.expect(resolveSelf(p, try std.fmt.bufPrint(&path, "pane/{d}/pty/ctl", .{serial})) == null);
+    try testing.expect(resolveSelf(p, try std.fmt.bufPrint(&path, "pane/{d}/rdsel", .{serial})) == null);
+    try testing.expect(resolveSelf(p, "self/index") == null);
+    try testing.expect(resolveSelf(p, "new/ctl") == null);
+    try testing.expect(resolveSelf(p, "cons") == null);
+    try testing.expect(resolveSelf(p, "pane/0") == null);
+}