const std = @import("std");
const vaxis = @import("vaxis");
const ghostty_vt = @import("ghostty-vt");
const Term = @import("term.zig");
const modal = @import("modal.zig");

extern "c" fn forkpty(
    amaster: *c_int,
    name: ?[*:0]u8,
    termp: ?*const anyopaque,
    winp: ?*const std.posix.winsize,
) c_int;

// execv, not execvp: no PATH search means no malloc in the child, so it's
// async-signal-safe and survives being forked from a multithreaded process.
extern "c" fn execv(
    path: [*:0]const u8,
    argv: [*:null]const ?[*:0]const u8,
) c_int;

extern "c" fn _exit(status: c_int) noreturn;
extern "c" fn chdir(path: [*:0]const u8) c_int;
extern "c" fn realpath(path: [*:0]const u8, resolved: [*]u8) ?[*:0]u8;

// basic file navigation
// basic shell io
// acme-style layout: columns split horizontally, windows stacked in each column.
// * /home/goblin/05-genizah/plan9port/src/cmd/acme
// right click and enter on lines to open files at specified lines

const PtyRead = struct { id: usize, bytes: []u8 };

pub const Command = struct {
    prev: ?*Command = null,
    pub var value: union(enum) {
        close,
        ping,
        nop,
        quit,
        tick,
        key_press: vaxis.Key,
        pty_read: PtyRead,
        pty_eof: usize,
        winsize: vaxis.Winsize,
        mouse: vaxis.Mouse,
    } = .nop;
};

// Modal yank register: gpa-owned, freed on overwrite / app exit. Helix-style
// `y` yanks the (line) selection into here; `p` pastes it after the cursor.
var yank_reg: ?[]u8 = null;
fn freeYank(gpa: std.mem.Allocator) void {
    if (yank_reg) |y| gpa.free(y);
    yank_reg = null;
}
fn setYank(gpa: std.mem.Allocator, text: []const u8) void {
    freeYank(gpa);
    yank_reg = gpa.dupe(u8, text) catch null;
}

const Loop = vaxis.Loop(@TypeOf(Command.value));

// layout constants
const GUTTER: u16 = 2; // left gutter: box (top) + scrollbar (below) per pane
const BOX_H: u16 = 1; // rows of the drag/rearrange box; scrollbar starts below it
const MINW: u16 = 10; // min pane width  (cells)
const MINH: u16 = 3; // min pane height (cells)
const MAX_TERMS: usize = 16;
const MAX_COLS: usize = 6;
const PREFIX_W: u16 = 5; // file-pane line-number prefix width: "%4d "

const Rect = struct { x: u16, y: u16, w: u16, h: u16 };

// acme model: columns own X (col_weight), windows own Y (Term.vweight). The
// per-column ordered list `col_terms[c][0..col_n[c]]` is the source of truth.
const Layout = struct {
    ncol: usize,
    col_weight: [MAX_COLS]f32,
    col_terms: [MAX_COLS][MAX_TERMS]usize,
    col_n: [MAX_COLS]usize,
};

const Drag = union(enum) {
    none,
    border_v: struct { left_col: usize, cur_x: u16 },
    border_h: struct { col: usize, top_idx: usize, cur_y: u16 },
    move: struct { id: usize, cur_x: u16, cur_y: u16 },
    select: struct { id: usize, button: Button },
};

// mouse button driving a selection; also indexes Term.sel.
const Button = enum { left, middle, right };

// monochrome, low-contrast palette (rgb grays).
const gray = struct {
    const bar = [3]u8{ 0x2a, 0x2a, 0x2a }; // dim bars (tag, scrollbar track, box idle)
    const dim = [3]u8{ 0x3c, 0x3c, 0x3c }; // separators
    const mid = [3]u8{ 0x52, 0x52, 0x52 }; // box active, scrollbar thumb, move preview
    const lit = [3]u8{ 0x6e, 0x6e, 0x6e }; // dashed resize preview
    const txt = [3]u8{ 0x96, 0x96, 0x96 }; // muted text
};
// the drag button: a distinctive red (brighter when the window is focused).
const red = [3]u8{ 0xcc, 0x33, 0x33 };
const red_dim = [3]u8{ 0x66, 0x2a, 0x2a };
// resize-border hover tint: a teal bar makes the (invisible) drag zones discoverable.
const hover_bg = [3]u8{ 0x3a, 0x70, 0x70 };
// char-mode inserted text (bright on a faint box so it reads as an overlay).
const edit_fg = [3]u8{ 0xea, 0xea, 0xea };
const edit_bg = [3]u8{ 0x3a, 0x3a, 0x3a };
// per-button selection highlight: distinct (muted) colors so left/middle/right
// selections are easy to tell apart against the otherwise-monochrome chrome.
const SEL_BG = [3][3]u8{ .{ 0x2c, 0x46, 0x74 }, .{ 0x2c, 0x60, 0x38 }, .{ 0x64, 0x2c, 0x58 } };
const sel_fg = [3]u8{ 0xd2, 0xd2, 0xd2 };
// modal line-selection (helix `x`) highlight: a warm tan so it reads as
// keyboard-driven, distinct from the mouse selections above.
const msel_bg = [3]u8{ 0x4a, 0x3a, 0x1e };
const msel_fg = [3]u8{ 0xd8, 0xc8, 0xa8 };

// Tag builtins. Middle-clicking the name in a window's tag runs the action;
// `builtins_str` is the space-joined name list, produced at comptime.
const SelectAction = enum { none, send, newcol, delcol, del, tutor };
const Builtin = struct { name: []const u8, action: SelectAction };
const builtins = [_]Builtin{
    .{ .name = "Newcol", .action = .newcol },
    .{ .name = "Delcol", .action = .delcol },
    .{ .name = "Del", .action = .del },
    .{ .name = "Tutor", .action = .tutor },
};
const builtins_str = blk: {
    var s: []const u8 = "";
    for (builtins, 0..) |b, i| s = s ++ (if (i == 0) "" else " ") ++ b.name;
    break :blk s;
};

// Path of the generated bash rc that adds OSC 133 semantic prompts (shell
// integration). bash --rcfile reads it for interactive shells; it sources the
// user's ~/.bashrc first, then wraps PS1 so each prompt emits 133;A..B. That
// marks the prompt rows so we can hide them in text mode. The A marker also
// carries cl=line, opting in to ghostty's promptClickMove (arrow-key
// repositioning of the shell cursor within the typed input) so enterTty can
// land the shell cursor where the modal cursor navigated (start vs end).
const rc_path = "/tmp/pardes-osc133.bash";
const bash_rc =
    \\[ -f "$HOME/.bashrc" ] && source "$HOME/.bashrc"
    \\PS1='\[\e]133;A;cl=line\a\]'"$PS1"'\[\e]133;B\a\]'
    \\PROMPT_COMMAND='printf "\e]133;D\a"'"${PROMPT_COMMAND:+;$PROMPT_COMMAND}"
    \\trap 'printf "\e]133;C\a"' DEBUG
    \\
;

fn writeFile(path: [*:0]const u8, data: []const u8) void {
    const l = std.os.linux;
    const rc = l.open(path, .{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true }, 0o644);
    const sfd: isize = @bitCast(rc);
    if (sfd < 0) return;
    const fd: i32 = @intCast(sfd);
    defer _ = l.close(fd);
    var off: usize = 0;
    while (off < data.len) {
        const w = l.write(fd, data[off..].ptr, data.len - off);
        const sw: isize = @bitCast(w);
        if (sw <= 0) break;
        off += @intCast(sw);
    }
}

fn fork_pty(argv: []const ?[*:0]const u8, cwd: ?[*:0]const u8, ws: std.posix.winsize) struct { file: std.Io.File, pid: std.posix.pid_t } {
    var master: c_int = undefined;
    const pid = forkpty(&master, null, null, &ws);
    if (pid == 0) {
        if (cwd) |c| _ = chdir(c); // start the shell in this directory
        var child_argv: [8]?[*:0]const u8 = @splat(null);
        for (argv, 0..) |a, i| child_argv[i] = a;
        _ = execv(argv[0].?, @ptrCast(&child_argv));
        _exit(127);
    }
    return .{ .file = .{ .handle = master, .flags = .{ .nonblocking = false } }, .pid = pid };
}

fn termWrite(t: *Term, bytes: []const u8) void {
    if (t.pty.handle < 0) return; // file pane: no pty
    var off: usize = 0;
    while (off < bytes.len) {
        const rc = std.os.linux.write(t.pty.handle, bytes[off..].ptr, bytes.len - off);
        // raw syscall: -errno, not the libc -1 sentinel. Check the sign.
        const n: isize = @bitCast(rc);
        if (n < 0) {
            if (n == -@as(isize, @intFromEnum(std.os.linux.E.INTR))) continue;
            break;
        }
        off += @intCast(n);
    }
}

fn fileLineCount(content: []const u8) usize {
    return std.mem.count(u8, content, "\n") + 1;
}

// the pane's current scroll offset (top visible row, in body coordinates). For a
// file it's `scroll`; for a terminal it's the emulator's scrollback offset.
fn paneScroll(t: *Term) i32 {
    if (t.file) |f| return @intCast(f.scroll);
    return @intCast(t.term.screens.active.pages.scrollbar().offset);
}

fn paneScrollBy(t: *Term, delta: i32) void {
    if (t.file) |*f| {
        const max: i64 = @intCast(fileLineCount(f.content) -| 1);
        const n = std.math.clamp(@as(i64, @intCast(f.scroll)) + delta, 0, max);
        f.scroll = @intCast(n);
    } else {
        t.term.screens.active.scroll(.{ .delta_row = @intCast(delta) });
    }
}

// Before running a composed command in a terminal, push the shell down so its
// output lands BELOW the last row the user typed overlay text on. The shell owns
// its cursor, so we move it the only way we can: emit newlines to the pty until
// the prompt sits at max(last-edit row, the shell's current prompt row). The empty
// prompts this creates are hidden in text mode (promptFlags), so the gap reads as
// blank padding and the output flows right under the composed text. No-op unless
// the shell is at a prompt (don't inject newlines into a running program).
// ponytail: pad caps at the grid height — you can't click/type past the pane.
fn padOutputBelowEdits(t: *Term) void {
    if (t.file != null or t.nedits == 0) return;
    if (!t.term.cursorIsAtPrompt()) return;
    var max_row: i32 = std.math.minInt(i32);
    for (t.edits[0..t.nedits]) |e| {
        if (e.len > 0 and e.row > max_row) max_row = e.row;
    }
    if (max_row == std.math.minInt(i32)) return;
    const cursor_abs: i32 = paneScroll(t) + @as(i32, @intCast(t.term.screens.active.cursor.y));
    const pad = std.math.clamp(max_row - cursor_abs, 0, @as(i32, t.rows));
    var i: i32 = 0;
    while (i < pad) : (i += 1) termWrite(t, "\r");
}

// The shell's current working directory (via /proc/<pid>/cwd). `buf` holds it.
// A raw linux syscall returns -errno; check the sign directly. (std.posix.errno
// is the LIBC convention here — `rc == -1` — and we link libc, so it misreads a
// raw -2/ENOENT as success and we'd slice buf[0..~2^64]. file panes (pid 0) and
// exited shells both hit this.)
fn termCwd(t: *Term, buf: []u8) []const u8 {
    if (t.file != null) return "?"; // file pane: no shell, no cwd
    var pathbuf: [64]u8 = undefined;
    const path = std.fmt.bufPrintZ(&pathbuf, "/proc/{d}/cwd", .{t.pid}) catch return "?";
    const rc = std.os.linux.readlinkat(std.os.linux.AT.FDCWD, path, buf.ptr, buf.len);
    const n: isize = @bitCast(rc);
    if (n < 0) return "?";
    return buf[0..@intCast(n)];
}

// True if `path` exists and is a directory. open(O_DIRECTORY) fails with ENOTDIR
// for anything else, so no stat struct needed.
fn isDir(path: [*:0]const u8) bool {
    const l = std.os.linux;
    const rc = l.open(path, .{ .ACCMODE = .RDONLY, .DIRECTORY = true, .CLOEXEC = true }, 0);
    const sfd: isize = @bitCast(rc);
    if (sfd < 0) return false;
    _ = l.close(@intCast(sfd));
    return true;
}

// The window tag: the shell's cwd followed by the builtin command names. Plain,
// selectable text (acme-style). Caller frees with `alloc`.
fn tagLine(alloc: std.mem.Allocator, t: *Term) ![]u8 {
    // a file pane's tag: mode + path (+ builtins); no cwd. Files never reach tty.
    if (t.file) |f| {
        const fmode = if (t.mode == .insert) "in" else "nm";
        return std.fmt.allocPrint(alloc, "{s} {s} {s}", .{ fmode, f.path, builtins_str });
    }
    var link: [1024]u8 = undefined;
    const cwd = termCwd(t, &link);
    // leading 2-char mode indicator: "nm" (normal), "in" (insert) or "sy" (sym/tty).
    const mode = switch (t.mode) {
        .normal => "nm",
        .insert => "in",
        .tty => "sy",
    };
    return std.fmt.allocPrint(alloc, "{s} {s} {s}", .{ mode, cwd, builtins_str });
}

// TIOCSWINSZ: set the pty size; the kernel raises SIGWINCH in the child so it
// (and anything running in it) reflows to the new grid.
fn pty_set_size(pty: std.Io.File, rows: u16, cols: u16) void {
    const ws: std.posix.winsize = .{ .row = rows, .col = cols, .xpixel = 0, .ypixel = 0 };
    _ = std.posix.system.ioctl(pty.handle, std.posix.T.IOCSWINSZ, @intFromPtr(&ws));
}

// One reader task per terminal. Tags each chunk with the terminal's slot id so
// the main loop knows which emulator to feed; posts pty_eof when the shell exits.
fn readPty(io: std.Io, gpa: std.mem.Allocator, pty: std.Io.File, id: usize, loop: *Loop) anyerror!void {
    var read_buf: [0x1000]u8 = undefined;
    var reader = pty.readerStreaming(io, &read_buf);
    while (true) {
        var buf: [0x1000]u8 = undefined;
        var vec = [_][]u8{&buf};
        const n = reader.interface.readVec(&vec) catch break;
        if (n == 0) break;
        const bytes = try gpa.dupe(u8, buf[0..n]);
        loop.postEvent(.{ .pty_read = .{ .id = id, .bytes = bytes } }) catch {
            gpa.free(bytes);
            break;
        };
    }
    loop.postEvent(.{ .pty_eof = id }) catch {};
}

// fork the pty + boot the emulator. Forking is the dangerous part in a
// multithreaded process, so this does NOT start the reader task (no new
// threads) — call startReader once the term is wired in.
fn forkTerm(gpa: std.mem.Allocator, init_cols: u16, init_rows: u16, cwd: ?[*:0]const u8) !*Term {
    const argv = [_]?[*:0]const u8{ "/usr/bin/bash", "--rcfile", rc_path };
    const child = fork_pty(&argv, cwd, .{ .row = init_rows, .col = init_cols, .xpixel = 0, .ypixel = 0 });
    const t = try gpa.create(Term);
    errdefer gpa.destroy(t);
    t.* = .{
        .pty = child.file,
        .pid = child.pid,
        .term = try ghostty_vt.Terminal.init(gpa, .{ .cols = init_cols, .rows = init_rows, .max_scrollback = 16 * 1024 * 1024 }),
        .stream = undefined,
        .reader = .{ .any_future = null, .result = {} }, // no-op until startReader
        .cols = init_cols,
        .rows = init_rows,
    };
    t.stream = t.term.vtStream();
    return t;
}

// peel a trailing :NNN (or :NNN:) line suffix: main.zig:100 -> {main.zig, 100}.
fn parsePathLine(tok: []const u8) struct { path: []const u8, line: usize } {
    var sep: usize = 0;
    while (sep < tok.len) : (sep += 1) {
        if (tok[sep] != ':') continue;
        var j = sep + 1;
        var line: usize = 0;
        while (j < tok.len and std.ascii.isDigit(tok[j])) : (j += 1) line = line * 10 + (tok[j] - '0');
        if (j == sep + 1) continue; // no digits after ':'
        if (j < tok.len and tok[j] != ':') continue; // junk after the number
        return .{ .path = tok[0..sep], .line = line };
    }
    return .{ .path = tok, .line = 0 };
}

// slurp a whole file via linux syscalls (kept flat; no std.fs). Caller frees.
fn readFileAlloc(gpa: std.mem.Allocator, path: [*:0]const u8) ![]u8 {
    const l = std.os.linux;
    const rc = l.open(path, .{ .ACCMODE = .RDONLY, .CLOEXEC = true }, 0);
    const sfd: isize = @bitCast(rc);
    if (sfd < 0) return error.OpenFailed;
    const fd: i32 = @intCast(sfd);
    defer _ = l.close(fd);
    var buf: std.ArrayList(u8) = .empty;
    errdefer buf.deinit(gpa);
    var chunk: [64 * 1024]u8 = undefined;
    while (true) {
        const rrc = l.read(fd, &chunk, chunk.len);
        // raw syscall: -errno, not the libc -1 sentinel. Check the sign.
        const n: isize = @bitCast(rrc);
        if (n < 0) {
            if (n == -@as(isize, @intFromEnum(l.E.INTR))) continue;
            return error.ReadFailed;
        }
        if (n == 0) break;
        try buf.appendSlice(gpa, chunk[0..@intCast(n)]);
    }
    return buf.toOwnedSlice(gpa);
}

// Open a read/editable FILE pane: a Term with `file` set, sharing all the
// layout/selection/edit machinery. No pty (handle -1), no reader, a stub 1x1
// emulator (only its allocator-owned bits are touched). `line` (1-based, 0=none)
// scrolls the target near the top. Caller wires it into the layout + frees later.
fn openFileView(gpa: std.mem.Allocator, rp: [*:0]const u8, line: usize, cols: u16, rows: u16) !*Term {
    const content = try readFileAlloc(gpa, rp);
    errdefer gpa.free(content);
    const path = try gpa.dupe(u8, std.mem.span(rp));
    errdefer gpa.free(path);
    const t = try gpa.create(Term);
    errdefer gpa.destroy(t);
    const total = fileLineCount(content);
    const scroll: usize = if (line > 0 and line <= total) line - 1 else 0;
    t.* = .{
        .pty = .{ .handle = -1, .flags = .{ .nonblocking = false } },
        .pid = 0,
        .term = try ghostty_vt.Terminal.init(gpa, .{ .cols = 1, .rows = 1 }),
        .stream = undefined,
        .reader = .{ .any_future = null, .result = {} },
        .file = .{ .path = path, .content = content, .scroll = scroll },
        .cols = cols,
        .rows = rows,
        .cur_pinned = true,
        .cur_row = @intCast(scroll),
    };
    t.stream = t.term.vtStream();
    return t;
}

// Open the built-in Tutor as a read/editable FILE pane: same machinery as
// openFileView, but the content is the tutor text embedded at comptime (so the
// binary is self-contained — no external file needed) and the path is a fixed
// label. Middle-clicking "Tutor" in any tag spawns this. Caller wires it into
// the layout + frees later.
const tutor_text = @embedFile("tutor.txt");
fn openTutorView(gpa: std.mem.Allocator, cols: u16, rows: u16) !*Term {
    const content = try gpa.dupe(u8, tutor_text);
    errdefer gpa.free(content);
    const path = try gpa.dupe(u8, "/Tutor");
    errdefer gpa.free(path);
    const t = try gpa.create(Term);
    errdefer gpa.destroy(t);
    t.* = .{
        .pty = .{ .handle = -1, .flags = .{ .nonblocking = false } },
        .pid = 0,
        .term = try ghostty_vt.Terminal.init(gpa, .{ .cols = 1, .rows = 1 }),
        .stream = undefined,
        .reader = .{ .any_future = null, .result = {} },
        .file = .{ .path = path, .content = content, .scroll = 0 },
        .cols = cols,
        .rows = rows,
        .cur_pinned = true,
        .cur_row = 0,
    };
    t.stream = t.term.vtStream();
    return t;
}

fn startReader(io: std.Io, gpa: std.mem.Allocator, loop: *Loop, t: *Term, id: usize) !void {
    t.reader = try io.concurrent(readPty, .{ io, gpa, t.pty, id, loop });
}

// ---- layout surgery over the per-column ordered lists ----

fn layoutFindTerm(l: *const Layout, id: usize) ?struct { col: usize, idx: usize } {
    var c: usize = 0;
    while (c < l.ncol) : (c += 1) {
        var k: usize = 0;
        while (k < l.col_n[c]) : (k += 1) {
            if (l.col_terms[c][k] == id) return .{ .col = c, .idx = k };
        }
    }
    return null;
}

fn layoutRemove(l: *Layout, id: usize) void {
    const f = layoutFindTerm(l, id) orelse return;
    const c = f.col;
    var k = f.idx;
    while (k + 1 < l.col_n[c]) : (k += 1) l.col_terms[c][k] = l.col_terms[c][k + 1];
    l.col_n[c] -= 1;
    if (l.col_n[c] == 0) {
        var j = c;
        while (j + 1 < l.ncol) : (j += 1) {
            l.col_terms[j] = l.col_terms[j + 1];
            l.col_n[j] = l.col_n[j + 1];
            l.col_weight[j] = l.col_weight[j + 1];
        }
        l.ncol -= 1;
    }
}

fn layoutInsert(l: *Layout, c: usize, idx: usize, id: usize) void {
    var k = l.col_n[c];
    while (k > idx) : (k -= 1) l.col_terms[c][k] = l.col_terms[c][k - 1];
    l.col_terms[c][idx] = id;
    l.col_n[c] += 1;
}

// Before deleting `id`, hand its vertical weight to a sibling (the pane above, or
// below if it's the first) so the freed space goes to ONE neighbor and the rest of
// the column keeps its sizes — the deletion mirror of splitBelow. Call BEFORE
// layoutRemove/deinit, while `id` is still in the layout and not yet freed. No-op
// when `id` is alone in its column (the column just closes).
fn absorbVWeight(l: *const Layout, terms: *const [MAX_TERMS]?*Term, id: usize) void {
    const f = layoutFindTerm(l, id) orelse return;
    if (l.col_n[f.col] <= 1) return;
    const gone = terms[id] orelse return;
    const sib = if (f.idx > 0) l.col_terms[f.col][f.idx - 1] else l.col_terms[f.col][f.idx + 1];
    if (terms[sib]) |s| s.vweight += gone.vweight;
}

fn layoutAppendColumn(l: *Layout, id: usize) void {
    if (l.ncol >= MAX_COLS) return;
    const c = l.ncol;
    l.col_weight[c] = 1;
    l.col_terms[c][0] = id;
    l.col_n[c] = 1;
    l.ncol += 1;
}

// Open a window BELOW `src` (acme-style) without rebalancing the whole column:
// shrink ONLY `src` to the height of its content so far — and hand the freed
// rows to the new window `nw`. `src`+`nw` together fill `src`'s old slot, so the
// other panes in the column keep their proportions. `src_h` is `src`'s height.
//
// `keep` INCLUDES the cursor row (`cursor.y + 1`): shrinking to exactly the
// content height means ghostty only drops blank rows *below* the cursor, so the
// source's visible text doesn't scroll. Cutting at `cursor.y` would evict the
// cursor's row and force a 1-row scroll to keep it visible.
fn splitBelow(src: *Term, nw: *Term, src_h: u16) void {
    const body: u16 = if (src_h > BOX_H) src_h - BOX_H else 1;
    const cur: u16 = if (src.file != null) body / 2 else @as(u16, @intCast(src.term.screens.active.cursor.y)) + 1;
    const keep = std.math.clamp(cur, 1, @max(1, body -| 1)); // both panes get >=1 row
    const w = src.vweight;
    src.vweight = w * @as(f32, @floatFromInt(BOX_H + keep)) / @as(f32, @floatFromInt(src_h));
    nw.vweight = w - src.vweight;
}

// A pristine terminal: a shell nobody has touched — no char-mode typing, the
// cursor still on its first prompt line, and no scrollback. Used to recognize a
// throwaway placeholder shell that a file should replace. (A file pane is never
// pristine — it isn't a terminal.)
fn isPristineTty(t: *Term) bool {
    if (t.file != null) return false;
    if (t.nedits != 0) return false;
    if (t.term.screens.active.cursor.y != 0) return false;
    return t.term.screens.active.pages.scrollbar().total <= t.rows;
}

// When a file lands in `col` (the files column), a leftover UNUSED shell there is
// just clutter: if the column holds exactly one terminal and it's pristine, delete
// it. Its weight flows to a neighbor via absorbVWeight (typically the file), so the
// file takes its place. `keep_id` is the file, used to refocus if the shell was active.
fn evictLonePristineTty(l: *Layout, terms: *[MAX_TERMS]?*Term, io: std.Io, gpa: std.mem.Allocator, col: usize, keep_id: usize, active: *usize) void {
    var n_tty: usize = 0;
    var tty_id: usize = 0;
    var k: usize = 0;
    while (k < l.col_n[col]) : (k += 1) {
        const cid = l.col_terms[col][k];
        if (terms[cid]) |ct| if (ct.file == null) {
            n_tty += 1;
            tty_id = cid;
        };
    }
    if (n_tty != 1) return;
    const tt = terms[tty_id] orelse return;
    if (!isPristineTty(tt)) return;
    absorbVWeight(l, terms, tty_id);
    layoutRemove(l, tty_id);
    tt.deinit(io, gpa);
    gpa.destroy(tt);
    terms[tty_id] = null;
    if (active.* == tty_id) active.* = keep_id;
}

// Find a terminal whose shell cwd is `dir`, else fork a fresh shell there and
// stack it at the bottom of the rightmost column. Returns the terminal to drive,
// or null if no slot is free / the fork failed. Backs middle-click "send" from a
// file pane: the selected line runs in a shell in the file's dir. Does NOT change
// focus — middle/execute keeps you where you were editing (right/look focuses).
// ponytail: rightmost column; with a single column the tty stacks under the file.
fn ttyForDir(
    io: std.Io,
    gpa: std.mem.Allocator,
    loop: *Loop,
    terms: *[MAX_TERMS]?*Term,
    layout: *Layout,
    sw: u16,
    sh: u16,
    dir: []const u8,
) ?*Term {
    for (terms.*) |slot| if (slot) |tt| {
        if (tt.file != null) continue;
        var cb: [1024]u8 = undefined;
        if (std.mem.eql(u8, termCwd(tt, &cb), dir)) return tt;
    };
    const free: usize = for (terms.*, 0..) |slot, i| {
        if (slot == null) break i;
    } else return null;
    var dirbuf: [1024]u8 = undefined;
    const dirz = std.fmt.bufPrintZ(&dirbuf, "{s}", .{dir}) catch return null;
    const nt = forkTerm(gpa, sw, sh, dirz.ptr) catch return null;
    terms[free] = nt;
    startReader(io, gpa, loop, nt, free) catch {};
    const rc = if (layout.ncol > 0) layout.ncol - 1 else 0;
    layoutInsert(layout, rc, layout.col_n[rc], free);
    return nt;
}

// Tile the screen: columns by col_weight, then each column's windows by vweight.
// Last column/window absorbs rounding so the tiling always fills the screen.
// `out_*` written for every live term and live column.
fn computeGeom(
    l: *const Layout,
    terms: *const [MAX_TERMS]?*Term,
    sw: u16,
    sh: u16,
    rects: *[MAX_TERMS]Rect,
    col_x: *[MAX_COLS]u16,
    col_w: *[MAX_COLS]u16,
) void {
    if (l.ncol == 0) return;
    var wsum: f32 = 0;
    for (0..l.ncol) |c| wsum += l.col_weight[c];
    if (wsum <= 0) wsum = 1;
    const sep_cols: u16 = @intCast(l.ncol - 1);
    const avail_w: u16 = sw -| sep_cols;

    var x: u16 = 0;
    for (0..l.ncol) |c| {
        const last = (c + 1 == l.ncol);
        const cw: u16 = if (last) (sw -| x) else @max(1, @as(u16, @intFromFloat(@round(@as(f32, @floatFromInt(avail_w)) * l.col_weight[c] / wsum))));
        col_x[c] = x;
        col_w[c] = cw;

        const n = l.col_n[c];
        var vsum: f32 = 0;
        for (0..n) |k| {
            if (terms[l.col_terms[c][k]]) |t| vsum += t.vweight;
        }
        if (vsum <= 0) vsum = 1;
        const sep_rows: u16 = if (n > 0) @intCast(n - 1) else 0;
        const avail_h: u16 = sh -| sep_rows;

        var y: u16 = 0;
        for (0..n) |k| {
            const id = l.col_terms[c][k];
            const t = terms[id] orelse continue;
            const lastk = (k + 1 == n);
            const ch: u16 = if (lastk) (sh -| y) else @max(1, @as(u16, @intFromFloat(@round(@as(f32, @floatFromInt(avail_h)) * t.vweight / vsum))));
            rects[id] = .{ .x = x, .y = y, .w = cw, .h = ch };
            y +|= ch + 1;
        }
        x +|= cw + 1;
    }
}

// `alloc` must be a per-frame arena, NOT freed before vx.render(): vaxis is a
// retained renderer and stores each cell's grapheme as a *slice* into the text
// we hand it, so plainString has to outlive the render call.
fn renderPane(root: vaxis.Window, alloc: std.mem.Allocator, t: *Term, r: Rect, active: bool) !void {
    if (r.w <= GUTTER or r.h == 0) return;

    // text area: tag line on row 0 (same row as the box), terminal body below.
    const text = root.child(.{ .x_off = r.x + GUTTER, .y_off = r.y, .width = r.w - GUTTER, .height = r.h });
    text.clear();

    // tag: cwd + builtins, drawn as a dim bar; plain selectable text.
    const tag = try tagLine(alloc, t);
    const tagw = text.child(.{ .height = BOX_H });
    tagw.fill(.{ .char = .{ .grapheme = " " }, .style = .{ .bg = .{ .rgb = gray.bar } } });
    _ = tagw.printSegment(.{ .text = tag, .style = .{ .fg = .{ .rgb = gray.txt }, .bg = .{ .rgb = gray.bar } } }, .{ .wrap = .none });

    // body = tty rows with insertion runs spliced in (text shifts, not overdrawn).
    // In text mode bodyText also hides OSC 133 prompt rows (see promptFlags); the
    // shells emit OSC 133 via the rc we drop at startup (writeFile/bash_rc).
    const off: i32 = paneScroll(t);
    const body = try bodyText(alloc, t);
    var it = std.mem.splitAny(u8, body, "\n");
    var i: u16 = 0;
    while (it.next()) |line| : (i += 1) {
        _ = text.printSegment(.{ .text = line }, .{ .row_offset = BOX_H + i, .wrap = .none });
    }

    // recolor the inserted cells so typed text reads distinctly from tty output.
    // edit.row is absolute; its visual pane row is (row - off) + BOX_H.
    for (t.edits[0..t.nedits]) |*e| {
        if (e.len == 0) continue;
        const prow = e.row - off + @as(i32, BOX_H);
        if (prow < 0 or prow >= @as(i32, r.h)) continue;
        const start = renderedCol(t, e.row, e.col, 0);
        var k: u16 = 0;
        while (k < e.len) : (k += 1) {
            const cc = start + @as(i32, k);
            if (cc < 0 or cc >= @as(i32, r.w - GUTTER)) continue;
            if (text.readCell(@intCast(cc), @intCast(prow))) |cell| {
                var nc = cell;
                nc.default = false;
                nc.style.fg = .{ .rgb = edit_fg };
                nc.style.bg = .{ .rgb = edit_bg };
                text.writeCell(@intCast(cc), @intCast(prow), nc);
            }
        }
    }

    // recolor selected cells (pane-local visual coords), one pass per button —
    // later buttons win on overlap; only while actively dragging.
    for (t.sel, 0..) |sl, b| {
        if (sl.state != .dragging) continue;
        const r0 = @min(sl.r0, sl.r1);
        const r1 = @max(sl.r0, sl.r1);
        const c0 = @max(0, @min(sl.c0, sl.c1));
        const c1 = @max(0, @max(sl.c0, sl.c1));
        var row: u16 = 0;
        while (row < r.h) : (row += 1) {
            if (@as(i32, row) < r0 or @as(i32, row) > r1) continue;
            var col: i32 = c0;
            while (col <= c1 and col < @as(i32, r.w - GUTTER)) : (col += 1) {
                const cc: u16 = @intCast(col);
                if (text.readCell(cc, row)) |cell| {
                    var nc = cell;
                    nc.default = false; // else vaxis ignores the style on cleared cells
                    nc.style.bg = .{ .rgb = SEL_BG[b] };
                    nc.style.fg = .{ .rgb = sel_fg };
                    text.writeCell(cc, row, nc);
                }
            }
        }
    }
    // modal line selection (helix `x`): highlight the selected body rows across
    // the text width. Only in normal mode; r0/r1 are absolute body rows.
    if (t.mode == .normal and t.msel.active) {
        const lo = @min(t.msel.r0, t.msel.r1) - off + @as(i32, BOX_H);
        const hi = @max(t.msel.r0, t.msel.r1) - off + @as(i32, BOX_H);
        var row: u16 = 0;
        while (row < r.h) : (row += 1) {
            if (@as(i32, row) < lo or @as(i32, row) > hi) continue;
            var col: u16 = 0;
            while (col < r.w - GUTTER) : (col += 1) {
                if (text.readCell(col, row)) |cell| {
                    var nc = cell;
                    nc.default = false;
                    nc.style.bg = .{ .rgb = msel_bg };
                    nc.style.fg = .{ .rgb = msel_fg };
                    text.writeCell(col, row, nc);
                }
            }
        }
    }
    if (active) {
        if (t.mode != .tty) {
            // text mode (normal/insert): cursor at the (scroll-adjusted) edit
                       // position. until the user pins it (click/type) it tracks the shell cursor.
            const crow = if (t.cur_pinned) t.cur_row else @as(i32, @intCast(t.term.screens.active.cursor.y)) + off;
            const ccol = if (t.cur_pinned) t.cur_col else @as(i32, @intCast(t.term.screens.active.cursor.x));
            const coff = if (t.cur_pinned) t.cur_off else 0;
            const prow = crow - off + @as(i32, BOX_H);
            // insert mode uses the edit-run coordinate -> visual mapping; normal
                       // mode keeps cur_col already-visual; file panes add the line-number prefix.
            const cx = if (t.file != null)
                @as(i32, PREFIX_W) + ccol + @as(i32, coff)
            else if (t.mode == .insert)
                renderedCol(t, crow, ccol, coff)
            else
                ccol;
            if (prow >= 0 and cx >= 0 and prow < @as(i32, r.h) and cx < @as(i32, r.w - GUTTER))
                text.showCursor(@intCast(cx), @intCast(prow));
        } else {
            text.showCursor(t.term.screens.active.cursor.x, t.term.screens.active.cursor.y + BOX_H);
        }
    }

    // gutter: box (the drag/rearrange handle) on top, scrollbar below
    const gut = root.child(.{ .x_off = r.x, .y_off = r.y, .width = GUTTER, .height = r.h });
    const box_bg = if (active) red else red_dim;
    var bx: u16 = 0;
    while (bx < GUTTER) : (bx += 1) {
        var by: u16 = 0;
        while (by < BOX_H) : (by += 1)
            gut.writeCell(bx, by, .{ .char = .{ .grapheme = " " }, .style = .{ .bg = .{ .rgb = box_bg } } });
    }
    const track_h: usize = if (r.h > BOX_H) r.h - BOX_H else 0;
    if (track_h > 0) {
        // scrollbar thumb: from file lines for a file pane, else the emulator's.
        var sb_total: usize = undefined;
        var sb_off: usize = undefined;
        var sb_len: usize = undefined;
        if (t.file) |f| {
            sb_total = fileLineCount(f.content);
            sb_off = f.scroll;
            sb_len = t.rows;
        } else {
            const sb = t.term.screens.active.pages.scrollbar();
            sb_total = sb.total;
            sb_off = sb.offset;
            sb_len = sb.len;
        }
        const total: usize = if (sb_total == 0) 1 else sb_total;
        const pos = (track_h * sb_off) / total;
        const len = @max(1, (track_h * sb_len) / total);
        var gy: usize = 0;
        while (gy < track_h) : (gy += 1) {
            const thumb = gy >= pos and gy < pos + len;
            const st_bg = if (thumb) gray.mid else gray.bar;
            var cx: u16 = 0;
            while (cx < GUTTER) : (cx += 1)
                gut.writeCell(cx, @intCast(BOX_H + gy), .{ .char = .{ .grapheme = " " }, .style = .{ .bg = .{ .rgb = st_bg } } });
        }
    }
}

// ---- modal editing helpers (wired onto Term by the key handler) ----

const PaneLines = struct {
    lines: []const []const u8,
    row0: i32, // absolute row of lines[0]; cursor lines-row = cur_row - row0
    body: ?[]u8, // terminal only: bodyText temp; arena-owned, freed by caller
};

// The lines the cursor moves over. File: all content lines (row0 = 0). Terminal:
// the viewport rows (row0 = scroll offset). `alloc` is per-frame scratch.
fn paneCursorLines(alloc: std.mem.Allocator, t: *Term) !PaneLines {
    if (t.file) |f| {
        var ls: std.ArrayList([]const u8) = .empty;
        var it = std.mem.splitScalar(u8, f.content, '\n');
        while (it.next()) |ln| try ls.append(alloc, ln);
        return .{ .lines = try ls.toOwnedSlice(alloc), .row0 = 0, .body = null };
    }
    const body = try bodyText(alloc, t);
    var ls: std.ArrayList([]const u8) = .empty;
    var it = std.mem.splitAny(u8, body, "\n");
    while (it.next()) |ln| try ls.append(alloc, ln);
    return .{ .lines = try ls.toOwnedSlice(alloc), .row0 = paneScroll(t), .body = body };
}

fn freePaneLines(alloc: std.mem.Allocator, pl: PaneLines) void {
    alloc.free(pl.lines);
    if (pl.body) |b| alloc.free(b);
}

fn toModalCursor(t: *Term, pl: PaneLines) modal.Cursor {
    const r: i32 = t.cur_row - pl.row0;
    return .{ .row = @intCast(@max(0, r)), .col = @intCast(@max(0, t.cur_col)) };
}

// write a modal Cursor back to the Term (absolute row, pin, clear run offset)
fn fromModalCursor(t: *Term, pl: PaneLines, c: modal.Cursor) void {
    t.cur_row = @as(i32, @intCast(c.row)) + pl.row0;
    t.cur_col = @intCast(c.col);
    t.cur_off = 0;
    t.cur_pinned = true;
}

// apply a motion result: move cursor, collapse any line selection, keep it visible
fn applyMove(t: *Term, pl: PaneLines, c: modal.Cursor) void {
    fromModalCursor(t, pl, c);
    t.msel.active = false;
    t.pending = 0;
    ensureCursorVisible(t);
}

fn ensureCursorVisible(t: *Term) void {
    const off = paneScroll(t);
    const last = off + @as(i32, t.rows) - 1;
    if (t.cur_row < off) paneScrollBy(t, t.cur_row - off)
    else if (t.cur_row > last) paneScrollBy(t, t.cur_row - last);
}

fn alignViewportTop(t: *Term) void {
    paneScrollBy(t, t.cur_row - paneScroll(t));
}
fn alignViewportBottom(t: *Term) void {
    paneScrollBy(t, t.cur_row - (paneScroll(t) + @as(i32, t.rows) - 1));
}
fn alignViewportCenter(t: *Term) void {
    paneScrollBy(t, t.cur_row - (paneScroll(t) + @as(i32, t.rows / 2)));
}

// pin the cursor to the shell's current cursor (terminal) or the scroll top
// (file), the first time the user interacts in normal/insert mode.
fn pinCursor(t: *Term) void {
    if (t.cur_pinned) return;
    const off: i32 = paneScroll(t);
    if (t.file != null) {
        t.cur_row = off;
        t.cur_col = 0;
    } else {
        t.cur_row = @as(i32, @intCast(t.term.screens.active.cursor.y)) + off;
        t.cur_col = @intCast(t.term.screens.active.cursor.x);
    }
    t.cur_off = 0;
    t.cur_pinned = true;
}

// the last navigable row: file -> last content line; terminal -> viewport bottom.
fn lastNavRow(t: *Term) i32 {
    if (t.file) |f| return @intCast(modal.lineCount(f.content) -| 1);
    return paneScroll(t) + @as(i32, t.rows) - 1;
}

const InsertAt = enum { at, append, line_start, line_end, open_below, open_above };

// enter insert mode, positioning the cursor per `where`. For file panes,
// open_below/open_above splice a newline into the content; terminals just move.
fn enterInsert(gpa: std.mem.Allocator, talloc: std.mem.Allocator, t: *Term, where: InsertAt) void {
    pinCursor(t);
    const pl = paneCursorLines(talloc, t) catch {
        t.mode = .insert;
        t.msel.active = false;
        t.pending = 0;
        return;
    };
    defer freePaneLines(talloc, pl);
    const cur = toModalCursor(t, pl);
    const llen: usize = if (cur.row < pl.lines.len) pl.lines[cur.row].len else 0;
    switch (where) {
        .at => {},
        .append => fromModalCursor(t, pl, modal.charRight(pl.lines, cur)),
        .line_start => fromModalCursor(t, pl, modal.firstNonWsOf(pl.lines, cur)),
        .line_end => {
            t.cur_col = @intCast(llen);
            t.cur_pinned = true;
        },
        .open_below => {
            if (t.file) |*f| {
                const new = modal.insertAt(gpa, f.content, .{ .row = cur.row, .col = llen }, "\n") catch return;
                gpa.free(f.content);
                f.content = new;
                t.cur_row = @as(i32, @intCast(cur.row + 1)) + pl.row0;
                t.cur_col = 0;
            } else {
                const bottom = paneScroll(t) + @as(i32, t.rows) - 1;
                t.cur_row = @min(bottom, t.cur_row + 1);
                t.cur_col = 0;
            }
            t.cur_off = 0;
            t.cur_pinned = true;
        },
        .open_above => {
            if (t.file) |*f| {
                const new = modal.insertAt(gpa, f.content, .{ .row = cur.row, .col = 0 }, "\n") catch return;
                gpa.free(f.content);
                f.content = new;
                // current line shifts down; the new empty line is at cur.row
                t.cur_col = 0;
            } else {
                const top = paneScroll(t);
                t.cur_row = @max(top, t.cur_row - 1);
                t.cur_col = 0;
            }
            t.cur_off = 0;
            t.cur_pinned = true;
        },
    }
    // run-offset for the terminal insert model: `i` inserts before the run at the
    // cursor (off 0); the others append after any run already sitting there.
    t.cur_off = if (where == .at) 0 else if (runAt(t, t.cur_row, t.cur_col)) |i| t.edits[i].len else 0;
    t.mode = .insert;
    t.msel.active = false;
    t.pending = 0;
    ensureCursorVisible(t);
}

// helix `x`: extend the line selection down by one (or start it at the cursor).
fn lineSelect(t: *Term) void {
    if (!t.msel.active) {
        t.msel = .{ .active = true, .r0 = t.cur_row, .r1 = t.cur_row };
    } else {
        const last = lastNavRow(t);
        t.msel.r1 = @min(t.msel.r1 + 1, last);
        t.cur_row = t.msel.r1;
    }
    t.cur_col = 0;
    t.cur_off = 0;
    t.cur_pinned = true;
    t.pending = 0;
}

// drop all insertion runs whose row is in [r0, r1] (terminal delete/change).
fn clearEditsInRange(t: *Term, r0: i32, r1: i32) void {
    var w: usize = 0;
    for (t.edits[0..t.nedits]) |e| {
        if (e.row >= r0 and e.row <= r1) continue;
        t.edits[w] = e;
        w += 1;
    }
    t.nedits = w;
}

// join the rendered body rows [r0, r1] (absolute) with '\n'. `alloc` is scratch;
// caller (setYank) copies it into the gpa-owned register before the frame ends.
fn yankRows(alloc: std.mem.Allocator, t: *Term, r0: i32, r1: i32) []const u8 {
    const body = bodyText(alloc, t) catch return "";
    defer alloc.free(body);
    const off = paneScroll(t);
    var out: std.ArrayList(u8) = .empty;
    var it = std.mem.splitAny(u8, body, "\n");
    var i: i32 = 0;
    while (it.next()) |ln| : (i += 1) {
        const abs = i + off;
        if (abs < r0 or abs > r1) continue;
        if (abs > r0) out.append(alloc, '\n') catch return "";
        out.appendSlice(alloc, ln) catch return "";
    }
    return out.toOwnedSlice(alloc) catch "";
}

fn normalDelete(gpa: std.mem.Allocator, talloc: std.mem.Allocator, t: *Term) void {
    if (t.msel.active) {
        const r0 = @min(t.msel.r0, t.msel.r1);
        const r1 = @max(t.msel.r0, t.msel.r1);
        if (t.file) |*f| {
            const d = modal.deleteLines(gpa, f.content, @intCast(@max(0, r0)), @intCast(@max(0, r1))) catch {
                t.msel.active = false;
                return;
            };
            gpa.free(f.content);
            f.content = d.content;
            setYank(gpa, d.deleted);
            gpa.free(d.deleted);
            const n = modal.lineCount(f.content);
            t.cur_row = @intCast(@min(@as(usize, @intCast(r0)), if (n == 0) 0 else n - 1));
            t.cur_col = 0;
        } else {
            clearEditsInRange(t, r0, r1);
            setYank(gpa, yankRows(talloc, t, r0, r1));
        }
        t.msel.active = false;
        t.cur_off = 0;
        t.cur_pinned = true;
        ensureCursorVisible(t);
    } else if (t.file) |*f| {
        const c = modal.Cursor{ .row = @intCast(@max(0, t.cur_row)), .col = @intCast(@max(0, t.cur_col)) };
        const new = modal.deleteChar(gpa, f.content, c) catch return;
        gpa.free(f.content);
        f.content = new;
    } // terminal, no selection: can't delete shell output -> no-op
}

fn normalYank(gpa: std.mem.Allocator, talloc: std.mem.Allocator, t: *Term) void {
    if (t.msel.active) {
        const r0 = @min(t.msel.r0, t.msel.r1);
        const r1 = @max(t.msel.r0, t.msel.r1);
        if (t.file) |*f| {
            var buf: std.ArrayList(u8) = .empty;
            var r: usize = @intCast(@max(0, r0));
            const hi: usize = @intCast(@max(0, r1));
            const lo: usize = @intCast(@max(0, r0));
            while (r <= hi) : (r += 1) {
                if (r > lo) buf.append(gpa, '\n') catch break;
                buf.appendSlice(gpa, modal.lineSlice(f.content, r)) catch break;
            }
            setYank(gpa, buf.items);
            buf.deinit(gpa);
        } else {
            setYank(gpa, yankRows(talloc, t, r0, r1));
        }
        t.cur_row = r0;
        t.cur_col = 0;
        t.msel.active = false;
    } else if (t.file) |f| {
        setYank(gpa, modal.lineSlice(f.content, @intCast(@max(0, t.cur_row))));
    } else {
        setYank(gpa, yankRows(talloc, t, t.cur_row, t.cur_row));
    }
}

fn normalPaste(gpa: std.mem.Allocator, t: *Term) void {
    const y = yank_reg orelse return;
    if (t.file) |*f| {
        const new = modal.pasteLineBelow(gpa, f.content, @intCast(@max(0, t.cur_row)), y) catch return;
        gpa.free(f.content);
        f.content = new;
        t.cur_row += 1;
        t.cur_col = 0;
    } else {
        // terminal: spill the yanked text into an insertion run at the cursor
        insertRunText(t, y);
    }
    t.cur_off = if (runAt(t, t.cur_row, t.cur_col)) |i| t.edits[i].len else 0;
    t.cur_pinned = true;
    ensureCursorVisible(t);
}

fn normalChange(gpa: std.mem.Allocator, talloc: std.mem.Allocator, t: *Term) void {
    if (t.msel.active) {
        const r0 = @min(t.msel.r0, t.msel.r1);
        const r1 = @max(t.msel.r0, t.msel.r1);
        if (t.file) |*f| {
            // keep one empty line at r0, drop the rest; yank the removed text
            var buf: std.ArrayList(u8) = .empty;
            var r: usize = @intCast(@max(0, r0));
            const hi: usize = @intCast(@max(0, r1));
            const lo: usize = @intCast(@max(0, r0));
            while (r <= hi) : (r += 1) {
                if (r > lo) buf.append(gpa, '\n') catch break;
                buf.appendSlice(gpa, modal.lineSlice(f.content, r)) catch break;
            }
            setYank(gpa, buf.items);
            buf.deinit(gpa);
            if (r1 > r0) {
                const d = modal.deleteLines(gpa, f.content, @intCast(@max(0, r0 + 1)), @intCast(@max(0, r1))) catch {
                    t.msel.active = false;
                    return;
                };
                gpa.free(f.content);
                f.content = d.content;
                gpa.free(d.deleted);
            }
            const cl = modal.clearLine(gpa, f.content, @intCast(@max(0, r0))) catch {
                t.msel.active = false;
                return;
            };
            gpa.free(f.content);
            f.content = cl;
            t.cur_row = r0;
            t.cur_col = 0;
        } else {
            clearEditsInRange(t, r0, r1);
            setYank(gpa, yankRows(talloc, t, r0, r1));
            t.cur_row = r0;
            t.cur_col = 0;
        }
        t.msel.active = false;
        t.cur_off = 0;
        t.cur_pinned = true;
        t.mode = .insert;
        t.pending = 0;
        ensureCursorVisible(t);
    } else if (t.file) |*f| {
        const c = modal.Cursor{ .row = @intCast(@max(0, t.cur_row)), .col = @intCast(@max(0, t.cur_col)) };
        const new = modal.deleteChar(gpa, f.content, c) catch return;
        gpa.free(f.content);
        f.content = new;
        t.mode = .insert;
        t.cur_off = 0;
        t.pending = 0;
    } else {
        // terminal, no selection: just enter insert at the cursor
        t.mode = .insert;
        t.cur_off = 0;
        t.pending = 0;
    }
}

// insert `text` into the terminal run model at the cursor (the existing char-mode
// splice). Factored out so `p`aste reuses it.
fn insertRunText(t: *Term, text: []const u8) void {
    var idx: ?usize = runAt(t, t.cur_row, t.cur_col);
    if (idx == null and t.nedits < Term.MAX_EDITS) {
        t.edits[t.nedits] = .{ .row = t.cur_row, .col = t.cur_col };
        t.nedits += 1;
        idx = t.nedits - 1;
    }
    if (idx) |ei| {
        const e = &t.edits[ei];
        for (text) |ch| {
            if (e.len >= Term.EDIT_CAP) break;
            if (t.cur_off > e.len) t.cur_off = e.len;
            var k: usize = e.len;
            while (k > t.cur_off) : (k -= 1) e.text[k] = e.text[k - 1];
            e.text[t.cur_off] = ch;
            e.len += 1;
            t.cur_off += 1;
        }
    }
}

// normal-mode (helix-style) key dispatch on a pane. The cursor is a block that
// moves over the rendered text (file content lines, or the terminal viewport);
// paneCursorLines builds that view per keystroke. Bindings (helix defaults,
// adapted to the acme text model):
//   move        h j k l  (or arrows)        word  w b e  /  W B E (long)
//   line        0 $ ^  (or home/end)         goto  g g (top)  G (last)  g e g h g l
//   page        Ctrl-d/u (half)  Ctrl-f/b (full)  PgUp/PgDn
//   viewport    z t / z z / z b  (top/center/bottom)
//   insert      i  a  I  A  o  O  (at/append/line-start/line-end/below/above)
//   select line x (extend down)   delete d   change c   yank y   paste p
//   chord       Enter -> acme look (open file/dir)   Tab -> acme execute (run word)
//   mode        Esc -> insert exits to normal; Ctrl-b -> raw tty (terminals)
//   window      Ctrl-w h/j/k/l focus neighbor pane  (Alt-n new term, Alt-c new col)
// File panes mutate their content (real edits); terminals yank rendered text and
// paste it as an insertion run (shell output can't be deleted, so d/c there only
// clear pasted runs in the selection). Select mode (`v`, char ranges) is not yet
// wired; use `x` for line-oriented selection.
// Enter raw tty mode (the Ctrl-b toggle; terminals only). tty-native: if the shell is at a
// prompt and the modal cursor sits on the current input line, move the shell's
// REAL cursor there via ghostty's promptClickMove (sends left/right arrows the
// shell understands) — so dropping into tty lands where you navigated: at the
// prompt start when the input is empty, or at/after typed text at the end. Then
// hand the keyboard to the pty. If the shell isn't at a prompt (outputting) or
// the cursor is above the prompt, just switch — the shell cursor stays put.
// Requires OSC 133 with cl=line (our bash_rc emits it).
fn enterTty(t: *Term) void {
    const screen = t.term.screens.active;
    if (t.term.cursorIsAtPrompt()) {
        const off = paneScroll(t);
        const vp_row: i32 = t.cur_row - off;
        if (vp_row >= 0) {
            if (screen.pages.pin(.{ .viewport = .{ .x = @intCast(@max(0, t.cur_col)), .y = @intCast(vp_row) } })) |click_pin| {
                const cursor_pin = screen.cursor.page_pin.*;
                var pit = cursor_pin.promptIterator(.left_up, null);
                if (pit.next()) |prompt_pin| {
                    if (!click_pin.before(prompt_pin)) {
                        const m = screen.promptClickMove(click_pin);
                        var i: usize = 0;
                        while (i < m.left) : (i += 1) termWrite(t, "\x1b[D");
                        i = 0;
                        while (i < m.right) : (i += 1) termWrite(t, "\x1b[C");
                    }
                }
            }
        }
    }
    t.mode = .tty;
    t.msel.active = false;
    t.pending = 0;
}

fn handleNormal(gpa: std.mem.Allocator, talloc: std.mem.Allocator, t: *Term, key: vaxis.Key) void {
    pinCursor(t);
    const pl = paneCursorLines(talloc, t) catch return;
    defer freePaneLines(talloc, pl);
    const cur = toModalCursor(t, pl);
    const lines = pl.lines;
    const half: usize = @max(1, t.rows / 2);

    // prefix continuations
    if (t.pending == 'g') {
        t.pending = 0;
        if (key.matches('g', .{})) return applyMove(t, pl, modal.gotoFirst());
        if (key.matches('e', .{})) return applyMove(t, pl, modal.gotoLast(lines));
        if (key.matches('h', .{})) return applyMove(t, pl, modal.lineStart(cur));
        if (key.matches('l', .{})) return applyMove(t, pl, modal.lineEnd(lines, cur));
        return;
    }
    if (t.pending == 'z') {
        t.pending = 0;
        if (key.matches('t', .{})) return alignViewportTop(t);
        if (key.matches('z', .{})) return alignViewportCenter(t);
        if (key.matches('b', .{})) return alignViewportBottom(t);
        return;
    }
    if (key.matches('g', .{})) {
        t.pending = 'g';
        return;
    }
    if (key.matches('z', .{})) {
        t.pending = 'z';
        return;
    }

    // movement
    if (key.matches('h', .{}) or key.matches(vaxis.Key.left, .{})) return applyMove(t, pl, modal.charLeft(cur));
    if (key.matches('l', .{}) or key.matches(vaxis.Key.right, .{})) return applyMove(t, pl, modal.charRight(lines, cur));
    if (key.matches('j', .{}) or key.matches(vaxis.Key.down, .{})) return applyMove(t, pl, modal.lineDown(lines, cur));
    if (key.matches('k', .{}) or key.matches(vaxis.Key.up, .{})) return applyMove(t, pl, modal.lineUp(lines, cur));
    if (key.matches('w', .{})) return applyMove(t, pl, modal.nextWordStart(lines, cur, false));
    if (key.matches('b', .{})) return applyMove(t, pl, modal.prevWordStart(lines, cur, false));
    if (key.matches('e', .{})) return applyMove(t, pl, modal.nextWordEnd(lines, cur, false));
    if (key.matches('W', .{})) return applyMove(t, pl, modal.nextWordStart(lines, cur, true));
    if (key.matches('B', .{})) return applyMove(t, pl, modal.prevWordStart(lines, cur, true));
    if (key.matches('E', .{})) return applyMove(t, pl, modal.nextWordEnd(lines, cur, true));
    if (key.matches('0', .{}) or key.matches(vaxis.Key.home, .{})) return applyMove(t, pl, modal.lineStart(cur));
    if (key.matches('$', .{}) or key.matches(vaxis.Key.end, .{})) return applyMove(t, pl, modal.lineEnd(lines, cur));
    if (key.matches('^', .{})) return applyMove(t, pl, modal.firstNonWsOf(lines, cur));
    if (key.matches('G', .{})) return applyMove(t, pl, modal.gotoLast(lines));
    if (key.matches('d', .{ .ctrl = true }) or key.matches(vaxis.Key.page_down, .{}))
        return applyMove(t, pl, modal.clampToChar(lines, modal.pageDown(cur, half)));
    if (key.matches('u', .{ .ctrl = true }) or key.matches(vaxis.Key.page_up, .{}))
        return applyMove(t, pl, modal.clampToChar(lines, modal.pageUp(cur, half)));
    if (key.matches('f', .{ .ctrl = true }))
        return applyMove(t, pl, modal.clampToChar(lines, modal.pageDown(cur, @as(usize, t.rows))));
    if (key.matches('b', .{ .ctrl = true }))
        return applyMove(t, pl, modal.clampToChar(lines, modal.pageUp(cur, @as(usize, t.rows))));

    // insert entry
    if (key.matches('i', .{})) return enterInsert(gpa, talloc, t, .at);
    if (key.matches('a', .{})) return enterInsert(gpa, talloc, t, .append);
    if (key.matches('I', .{})) return enterInsert(gpa, talloc, t, .line_start);
    if (key.matches('A', .{})) return enterInsert(gpa, talloc, t, .line_end);
    if (key.matches('o', .{})) return enterInsert(gpa, talloc, t, .open_below);
    if (key.matches('O', .{})) return enterInsert(gpa, talloc, t, .open_above);

    // line selection + edit (helix `x` then d/c/y; p pastes)
    if (key.matches('x', .{})) return lineSelect(t);
    if (key.matches('d', .{})) return normalDelete(gpa, talloc, t);
    if (key.matches('c', .{})) return normalChange(gpa, talloc, t);
    if (key.matches('y', .{})) return normalYank(gpa, talloc, t);
    if (key.matches('p', .{})) return normalPaste(gpa, t);
}

// insert-mode key dispatch on a pane. File: real content editing; terminal: the
// char-mode run splice (text shifts the underlying tty row right).
fn handleInsert(gpa: std.mem.Allocator, talloc: std.mem.Allocator, t: *Term, key: vaxis.Key) void {
    if (t.file) |*f| {
        const c = modal.Cursor{ .row = @intCast(@max(0, t.cur_row)), .col = @intCast(@max(0, t.cur_col)) };
        if (key.text) |text| {
            const new = modal.insertAt(gpa, f.content, c, text) catch return;
            gpa.free(f.content);
            f.content = new;
            for (text) |ch| {
                if (ch == '\n') {
                    t.cur_row += 1;
                    t.cur_col = 0;
                } else t.cur_col += 1;
            }
            t.cur_pinned = true;
            ensureCursorVisible(t);
            return;
        }
        switch (key.codepoint) {
            vaxis.Key.enter => {
                const new = modal.insertAt(gpa, f.content, c, "\n") catch return;
                gpa.free(f.content);
                f.content = new;
                t.cur_row += 1;
                t.cur_col = 0;
                t.cur_pinned = true;
                ensureCursorVisible(t);
            },
            vaxis.Key.backspace => {
                if (t.cur_col > 0) {
                    const new = modal.deleteChar(gpa, f.content, .{ .row = c.row, .col = c.col - 1 }) catch return;
                    gpa.free(f.content);
                    f.content = new;
                    t.cur_col -= 1;
                } else if (t.cur_row > 0) {
                    const prevlen = modal.lineSlice(f.content, c.row - 1).len;
                    const new = modal.deleteChar(gpa, f.content, .{ .row = c.row - 1, .col = prevlen }) catch return;
                    gpa.free(f.content);
                    f.content = new;
                    t.cur_row -= 1;
                    t.cur_col = @intCast(prevlen);
                }
                t.cur_pinned = true;
                ensureCursorVisible(t);
            },
            vaxis.Key.left, vaxis.Key.right, vaxis.Key.up, vaxis.Key.down => {
                const pl = paneCursorLines(talloc, t) catch return;
                defer freePaneLines(talloc, pl);
                const cur0 = toModalCursor(t, pl);
                const nc = switch (key.codepoint) {
                    vaxis.Key.left => modal.charLeft(cur0),
                    vaxis.Key.right => modal.charRight(pl.lines, cur0),
                    vaxis.Key.up => modal.lineUp(pl.lines, cur0),
                    vaxis.Key.down => modal.lineDown(pl.lines, cur0),
                    else => cur0,
                };
                fromModalCursor(t, pl, nc);
                ensureCursorVisible(t);
            },
            else => {},
        }
        return;
    }

    // terminal: char-mode run insert (unchanged from the original editor)
    if (!t.cur_pinned) pinCursor(t);
    if (key.text) |text| {
        insertRunText(t, text);
    } else switch (key.codepoint) {
        vaxis.Key.left => {
            if (t.cur_off > 0) t.cur_off -= 1 else {
                t.cur_col = @max(0, t.cur_col - 1);
                t.cur_off = if (runAt(t, t.cur_row, t.cur_col)) |i| t.edits[i].len else 0;
            }
        },
        vaxis.Key.right => {
            const rl: u16 = if (runAt(t, t.cur_row, t.cur_col)) |i| t.edits[i].len else 0;
            if (t.cur_off < rl) t.cur_off += 1 else {
                t.cur_col += 1;
                t.cur_off = 0;
            }
        },
        vaxis.Key.up => {
            const off = paneScroll(t);
            t.cur_row = @max(off, t.cur_row - 1);
            t.cur_off = 0;
        },
        vaxis.Key.down => {
            const off = paneScroll(t);
            t.cur_row = @min(off + @as(i32, t.rows) - 1, t.cur_row + 1);
            t.cur_off = 0;
        },
        else => {},
    }
}

// tty mode: forward the key to the pty as terminal input. Ctrl+letter is sent as
// the raw control byte (so the shell sees Ctrl-D EOF, Ctrl-L clear, ...).
fn forwardToPty(t: *Term, key: vaxis.Key) void {
    var cbuf: [1]u8 = undefined;
    const bytes: ?[]const u8 = blk: {
        if (key.mods.ctrl) {
            const cp = key.codepoint;
            if (cp >= 'a' and cp <= 'z') {
                cbuf[0] = @intCast(cp - 0x60);
                break :blk cbuf[0..1];
            }
            if (cp == '@') {
                cbuf[0] = 0;
                break :blk cbuf[0..1];
            }
            if (cp >= '[' and cp <= '_') {
                cbuf[0] = @intCast(cp - 0x40);
                break :blk cbuf[0..1];
            }
            if (cp >= 'A' and cp <= 'Z') {
                cbuf[0] = @intCast(cp - 0x40);
                break :blk cbuf[0..1];
            }
        }
        if (key.text) |text| break :blk text;
        break :blk switch (key.codepoint) {
            vaxis.Key.enter => "\r",
            vaxis.Key.backspace => "\x7f",
            vaxis.Key.tab => "\t",
            vaxis.Key.escape => "\x1b",
            vaxis.Key.up => "\x1b[A",
            vaxis.Key.down => "\x1b[B",
            vaxis.Key.right => "\x1b[C",
            vaxis.Key.left => "\x1b[D",
            else => null,
        };
    };
    if (bytes) |b| termWrite(t, b);
}

// move focus to the neighbor pane in `dir`, if any. Rects are the current tiling.
fn focusDir(active: *usize, terms: *const [MAX_TERMS]?*Term, rects: *const [MAX_TERMS]Rect, dir: enum { left, right, up, down }) void {
    const a = rects[active.*];
    var best: ?usize = null;
    var best_d: i32 = 0;
    for (terms, 0..) |slot, i| {
        if (slot == null or i == active.*) continue;
        const r = rects[i];
        const vov = a.y < r.y + r.h and r.y < a.y + a.h; // vertical overlap
        const hov = a.x < r.x + r.w and r.x < a.x + a.w; // horizontal overlap
        const ok = switch (dir) {
            .left => r.x + r.w <= a.x and vov,
            .right => r.x >= a.x + a.w and vov,
            .up => r.y + r.h <= a.y and hov,
            .down => r.y >= a.y + a.h and hov,
        };
        if (!ok) continue;
        const d: i32 = switch (dir) {
            .left => a.x - (r.x + r.w),
            .right => r.x - (a.x + a.w),
            .up => a.y - (r.y + r.h),
            .down => r.y - (a.y + a.h),
        };
        if (best == null or d < best_d) {
            best = i;
            best_d = d;
        }
    }
    if (best) |b| active.* = b;
}

// Perform an acme button-2/3 chord with `txt` as the text, from pane `id`:
// .right = "look" (open the file/dir under the word, peeling a :line suffix),
// .middle = "execute" (a builtin, else run the text in a shell). Shared by the
// mouse release and the normal-mode Enter (look) / Tab (execute) keys. Returns
// true if the app should quit (the last pane was closed).
fn actOnSelection(
    io: std.Io,
    gpa: std.mem.Allocator,
    loop: *Loop,
    terms: *[MAX_TERMS]?*Term,
    layout: *Layout,
    rects: *const [MAX_TERMS]Rect,
    active: *usize,
    sw: u16,
    sh: u16,
    button: Button,
    id: usize,
    txt: []const u8,
) bool {
    const t = terms[id] orelse return false;
    if (button == .right) {
        // acme "look": resolve the selected word (peeling a :line suffix) relative
        // to the window's dir. A directory opens/focuses a terminal there + ls; a
        // file opens/focuses a file pane scrolled to :line.
        const word_raw = std.mem.trim(u8, txt, " \t\r\n");
        const pl = parsePathLine(word_raw);
        const word = pl.path;
        var cwdbuf: [1024]u8 = undefined;
        var joinbuf: [2048]u8 = undefined;
        var realbuf: [4096]u8 = undefined;
        const cwd = if (t.file) |f| (std.fs.path.dirname(f.path) orelse "/") else termCwd(t, &cwdbuf);
        const joined: ?[:0]u8 = if (word.len == 0) null else if (word[0] == '/')
            (std.fmt.bufPrintZ(&joinbuf, "{s}", .{word}) catch null)
        else
            (std.fmt.bufPrintZ(&joinbuf, "{s}/{s}", .{ cwd, word }) catch null);
        if (joined) |jz| if (realpath(jz.ptr, &realbuf)) |rp| if (isDir(rp)) {
            const dir = std.mem.span(rp);
            // focus an existing terminal on this dir
            var found: ?usize = null;
            for (terms, 0..) |*slot, i| if (slot.*) |tt| {
                var cb: [1024]u8 = undefined;
                if (std.mem.eql(u8, termCwd(tt, &cb), dir)) {
                    found = i;
                    break;
                }
            };
            if (found) |fid| {
                active.* = fid;
                if (terms[fid]) |tt| termWrite(tt, "ls\r");
            } else {
                const slot: ?usize = for (terms.*, 0..) |sl, i| {
                    if (sl == null) break i;
                } else null;
                if (slot) |free| {
                    if (forkTerm(gpa, sw, sh, rp)) |nt| {
                        terms[free] = nt;
                        startReader(io, gpa, loop, nt, free) catch {};
                        const f = layoutFindTerm(layout, id).?;
                        layoutInsert(layout, f.col, f.idx + 1, free);
                        splitBelow(t, nt, rects[id].h);
                        active.* = free;
                        termWrite(nt, "ls\r");
                    } else |_| {}
                }
            }
        } else {
            const fp = std.mem.span(rp);
            // focus an existing file pane for this path
            var found: ?usize = null;
            for (terms, 0..) |*slot, i| if (slot.*) |tt| {
                if (tt.file) |ff| if (std.mem.eql(u8, ff.path, fp)) {
                    found = i;
                    break;
                };
            };
            if (found) |fid| {
                active.* = fid;
                if (pl.line > 0) if (terms[fid]) |tt| if (tt.file) |*ff|
                    if (pl.line <= fileLineCount(ff.content)) {
                        ff.scroll = pl.line - 1;
                    };
            } else {
                const slot: ?usize = for (terms.*, 0..) |sl, i| {
                    if (sl == null) break i;
                } else null;
                if (slot) |free| {
                    if (openFileView(gpa, rp, pl.line, sw, sh)) |nt| {
                        terms[free] = nt;
                        // files live in the LEFT column (acme convention). Stack at its
                        // top with a fair share of the column height, then drop a
                        // leftover unused shell.
                        const lc = 0;
                        const ln = layout.col_n[lc];
                        if (ln > 0) {
                            var vsum: f32 = 0;
                            for (0..ln) |i| if (terms[layout.col_terms[lc][i]]) |p| {
                                vsum += p.vweight;
                            };
                            nt.vweight = vsum / @as(f32, @floatFromInt(ln));
                        }
                        layoutInsert(layout, lc, 0, free);
                        active.* = free;
                        evictLonePristineTty(layout, terms, io, gpa, lc, free, active);
                    } else |_| {}
                }
            }
        };
        return false;
    }
    switch (onSelect(button, txt)) {
        .none => {},
        .send => {
            // terminal: run in itself. file pane: run in a shell sitting in the
            // file's directory (found by cwd, else a fresh tty in the right column).
            const cmd = std.mem.trim(u8, txt, " \t\r\n");
            const dst: ?*Term = if (t.file) |f|
                ttyForDir(io, gpa, loop, terms, layout, sw, sh, std.fs.path.dirname(f.path) orelse "/")
            else
                t;
            if (dst) |dt| {
                padOutputBelowEdits(dt);
                termWrite(dt, cmd);
                termWrite(dt, "\r");
            }
        },
        .newcol => {
            // new terminal in a fresh column; ls its directory.
            const slot: ?usize = for (terms.*, 0..) |sl, i| {
                if (sl == null) break i;
            } else null;
            if (slot) |free| if (layout.ncol < MAX_COLS) {
                if (forkTerm(gpa, sw, sh, null)) |nt| {
                    terms[free] = nt;
                    startReader(io, gpa, loop, nt, free) catch {};
                    layoutAppendColumn(layout, free);
                    active.* = free;
                    termWrite(nt, "ls\r");
                } else |_| {}
            };
        },
        .del => {
            // close just this window
            absorbVWeight(layout, terms, id);
            layoutRemove(layout, id);
            t.deinit(io, gpa);
            gpa.destroy(t);
            terms[id] = null;
            if (active.* == id) active.* = for (terms.*, 0..) |sl, i| {
                if (sl != null) break i;
            } else return true;
        },
        .delcol => {
            // close every terminal in this one's column
            if (layoutFindTerm(layout, id)) |f| {
                var ids: [MAX_TERMS]usize = undefined;
                const nids = layout.col_n[f.col];
                for (0..nids) |k| ids[k] = layout.col_terms[f.col][k];
                for (ids[0..nids]) |tid| {
                    if (terms[tid]) |tt| {
                        layoutRemove(layout, tid);
                        tt.deinit(io, gpa);
                        gpa.destroy(tt);
                        terms[tid] = null;
                    }
                }
                if (terms[active.*] == null) active.* = for (terms.*, 0..) |sl, i| {
                    if (sl != null) break i;
                } else return true;
            }
        },
        .tutor => {
            // spawn the Tutor as a file pane in the LEFT column (acme convention for
            // files), stacked at the top with a fair share of the column.
            const slot: ?usize = for (terms.*, 0..) |sl, i| {
                if (sl == null) break i;
            } else null;
            if (slot) |free| {
                if (openTutorView(gpa, sw, sh)) |nt| {
                    terms[free] = nt;
                    const lc = 0;
                    const ln = layout.col_n[lc];
                    if (ln > 0) {
                        var vsum: f32 = 0;
                        for (0..ln) |i| if (terms[layout.col_terms[lc][i]]) |p| {
                            vsum += p.vweight;
                        };
                        nt.vweight = vsum / @as(f32, @floatFromInt(ln));
                    }
                    layoutInsert(layout, lc, 0, free);
                    active.* = free;
                    evictLonePristineTty(layout, terms, io, gpa, lc, free, active);
                } else |_| {}
            }
        },
    }
    return false;
}

// The word under the modal cursor as a pane-local selection in paneText coords
// (row 0 is the tag; file panes carry the line-number prefix). Feed it through
// expandSel + selectionText to get the acme chord text for the Enter/Tab keys.
fn cursorWordSel(t: *Term) Term.Sel {
    const vrow = (t.cur_row - paneScroll(t)) + @as(i32, BOX_H);
    const vcol = if (t.file != null) t.cur_col + @as(i32, PREFIX_W) else t.cur_col;
    return .{ .state = .done, .c0 = vcol, .c1 = vcol, .r0 = vrow, .r1 = vrow };
}

pub fn main(init: std.process.Init) !void {
    const io = init.io;
    const gpa = init.gpa;

    var screen_w: u16 = 80;
    var screen_h: u16 = 24;
    var resize_count: usize = 0;

    // per-frame scratch for plainString; reset (not freed) after each render so
    // the retained vaxis screen can keep referencing it through vx.render().
    var frame_arena: std.heap.ArenaAllocator = .init(gpa);
    defer frame_arena.deinit();
    defer freeYank(gpa);
    var ov_buf: [128]u8 = undefined; // overlay text; must outlive each vx.render()

    var tty_buf: [0x10000]u8 = undefined;
    var tty = try vaxis.Tty.init(io, &tty_buf);

    var vx = try vaxis.init(io, gpa, init.environ_map, .{});
    defer vx.deinit(gpa, tty.writer());

    try vx.setMouseMode(tty.writer(), true);
    try vx.enterAltScreen(tty.writer());
    defer vx.exitAltScreen(tty.writer()) catch {};

    // ---- terminals + layout ----
    var terms: [MAX_TERMS]?*Term = @splat(null);
    defer for (&terms) |*slot| {
        if (slot.*) |t| {
            t.deinit(io, gpa);
            gpa.destroy(t);
            slot.* = null;
        }
    };

    var layout: Layout = .{
        .ncol = 0,
        .col_weight = @splat(1),
        .col_terms = undefined,
        .col_n = @splat(0),
    };

    // drop the bash shell-integration rc so spawned shells emit OSC 133.
    writeFile(rc_path, bash_rc);

    // Fork the initial terminals BEFORE starting any io worker thread. forkpty
    // from a multithreaded process can leave the child wedged before exec; doing
    // it while still single-threaded avoids that. (initial: two columns, the
    // left one split into two windows.)
    terms[0] = try forkTerm(gpa, screen_w, screen_h, null);
    terms[1] = try forkTerm(gpa, screen_w, screen_h, null);
    terms[2] = try forkTerm(gpa, screen_w, screen_h, null);
    // a fresh shell greets you with its directory listing (the readers below pick
    // up the output once they start).
    for (terms[0..3]) |slot| if (slot) |t| termWrite(t, "ls\r");
    layout.ncol = 2;
    layout.col_n[0] = 2;
    layout.col_terms[0][0] = 0;
    layout.col_terms[0][1] = 1;
    layout.col_n[1] = 1;
    layout.col_terms[1][0] = 2;
    var active: usize = 0;

    var loop: Loop = .init(io, &tty, &vx);
    try loop.start();
    defer loop.stop();
    try loop.installResizeHandler();

    // now safe to spawn the per-terminal reader tasks (no forking here)
    for (&terms, 0..) |*slot, id| {
        if (slot.*) |t| try startReader(io, gpa, &loop, t, id);
    }

    var rects: [MAX_TERMS]Rect = undefined;
    var col_x: [MAX_COLS]u16 = undefined;
    var col_w: [MAX_COLS]u16 = undefined;
    computeGeom(&layout, &terms, screen_w, screen_h, &rects, &col_x, &col_w);

    var drag: Drag = .none;
    var hover_col: u16 = 0; // last mouse position; used to tint the resize border under it
    var hover_row: u16 = 0;
    var ctrl_w_pending: bool = false; // Ctrl-w prefix for helix-style pane focus

    while (true) {
        const event = try loop.nextEvent();
        switch (event) {
            .mouse => |mouse| {
                const mcol: u16 = if (mouse.col < 0) 0 else @min(@as(u16, @intCast(mouse.col)), screen_w -| 1);
                const mrow: u16 = if (mouse.row < 0) 0 else @min(@as(u16, @intCast(mouse.row)), screen_h -| 1);
                hover_col = mcol;
                hover_row = mrow;

                // which pane is under the cursor (if any)
                const hovered: ?usize = for (terms, 0..) |slot, i| {
                    if (slot == null) continue;
                    const r = rects[i];
                    if (mcol >= r.x and mcol < r.x + r.w and mrow >= r.y and mrow < r.y + r.h) break i;
                } else null;

                if (mouse.button == .wheel_down) {
                    if (hovered) |id| paneScrollBy(terms[id].?, 1);
                } else if (mouse.button == .wheel_up) {
                    if (hovered) |id| paneScrollBy(terms[id].?, -1);
                } else if (mouse.type == .press and mouse.button == .left) {
                    var handled = false;

                    // vertical separator (between columns)?
                    var c: usize = 0;
                    while (c + 1 < layout.ncol and !handled) : (c += 1) {
                        if (mcol == col_x[c] + col_w[c]) {
                            drag = .{ .border_v = .{ .left_col = c, .cur_x = mcol } };
                            handled = true;
                        }
                    }

                    // horizontal separator (between stacked windows)?
                    if (!handled) {
                        var cc: usize = 0;
                        outer: while (cc < layout.ncol) : (cc += 1) {
                            if (mcol < col_x[cc] or mcol >= col_x[cc] + col_w[cc]) continue;
                            var k: usize = 0;
                            while (k + 1 < layout.col_n[cc]) : (k += 1) {
                                const id = layout.col_terms[cc][k];
                                const r = rects[id];
                                if (mrow == r.y + r.h) {
                                    drag = .{ .border_h = .{ .col = cc, .top_idx = k, .cur_y = mrow } };
                                    handled = true;
                                    break :outer;
                                }
                            }
                        }
                    }

                    // inside a pane?
                    if (!handled) {
                        if (hovered) |id| {
                            const r = rects[id];
                            const in_gutter = mcol < r.x + GUTTER;
                            if (in_gutter and mrow < r.y + BOX_H) {
                                drag = .{ .move = .{ .id = id, .cur_x = mcol, .cur_y = mrow } };
                            } else if (in_gutter) {
                                active = id;
                                if (terms[id]) |t| {
                                    const local: i32 = @as(i32, mrow) - @as(i32, r.y + BOX_H);
                                    paneScrollBy(t, -local); // left: scroll up to here
                                }
                            } else {
                                active = id;
                                if (terms[id]) |t| {
                                    const sc: i32 = @as(i32, mcol) - @as(i32, r.x + GUTTER);
                                    const v: i32 = @as(i32, mrow) - @as(i32, r.y);
                                    t.sel[@intFromEnum(Button.left)] = .{ .state = .dragging, .c0 = sc, .c1 = sc, .r0 = v, .r1 = v };
                                    drag = .{ .select = .{ .id = id, .button = .left } };
                                }
                            }
                        }
                    }
                } else if (mouse.type == .press and (mouse.button == .middle or mouse.button == .right)) {
                    // middle/right select text too (own state + color), text area only.
                    // Right focuses the pane (acme "look" lands you there); middle
                    // (execute) does NOT — you stay where you were editing.
                    const b: Button = if (mouse.button == .middle) .middle else .right;
                    if (hovered) |id| {
                        const r = rects[id];
                        if (mcol < r.x + GUTTER and mrow >= r.y + BOX_H) {
                            // gutter (scrollbar): right scrolls down, mirroring left's up
                            if (b == .right) active = id;
                            if (terms[id]) |t| {
                                const local: i32 = @as(i32, mrow) - @as(i32, r.y + BOX_H);
                                paneScrollBy(t, if (b == .right) local else -local);
                            }
                        } else if (mcol >= r.x + GUTTER) {
                            if (b == .right) active = id;
                            if (terms[id]) |t| {
                                const c: i32 = @as(i32, mcol) - @as(i32, r.x + GUTTER);
                                const v: i32 = @as(i32, mrow) - @as(i32, r.y);
                                t.sel[@intFromEnum(b)] = .{ .state = .dragging, .c0 = c, .c1 = c, .r0 = v, .r1 = v };
                                drag = .{ .select = .{ .id = id, .button = b } };
                            }
                        }
                    }
                } else if (mouse.type == .drag) {
                    switch (drag) {
                        .border_v => |*d| {
                            const c = d.left_col;
                            if (c + 1 < layout.ncol) {
                                const lo = col_x[c] + MINW;
                                const hi = col_x[c] +| col_w[c] +| col_w[c + 1] -| MINW;
                                d.cur_x = if (lo <= hi) std.math.clamp(mcol, lo, hi) else mcol;
                            } else d.cur_x = mcol;
                        },
                        .border_h => |*d| {
                            const cc = d.col;
                            const k = d.top_idx;
                            if (k + 1 < layout.col_n[cc]) {
                                const a = layout.col_terms[cc][k];
                                const b = layout.col_terms[cc][k + 1];
                                const lo = rects[a].y + MINH;
                                const hi = rects[a].y +| rects[a].h +| rects[b].h -| MINH;
                                d.cur_y = if (lo <= hi) std.math.clamp(mrow, lo, hi) else mrow;
                            } else d.cur_y = mrow;
                        },
                        .move => |*d| {
                            d.cur_x = mcol;
                            d.cur_y = mrow;
                        },
                        .select => |s| {
                            if (terms[s.id]) |t| {
                                const r = rects[s.id];
                                t.sel[@intFromEnum(s.button)].c1 = @as(i32, mcol) - @as(i32, r.x + GUTTER);
                                t.sel[@intFromEnum(s.button)].r1 = @as(i32, mrow) - @as(i32, r.y);
                            }
                        },
                        .none => {},
                    }
                } else if (mouse.type == .release) {
                    switch (drag) {
                        .border_v => |d| {
                            const c = d.left_col;
                            if (c + 1 < layout.ncol) {
                                const combined: f32 = @floatFromInt(col_w[c] + col_w[c + 1]);
                                var nl: f32 = @floatFromInt(d.cur_x -| col_x[c]);
                                const minw: f32 = MINW;
                                nl = std.math.clamp(nl, minw, @max(minw, combined - minw));
                                const pair = layout.col_weight[c] + layout.col_weight[c + 1];
                                layout.col_weight[c] = pair * (nl / combined);
                                layout.col_weight[c + 1] = pair - layout.col_weight[c];
                            }
                        },
                        .border_h => |d| {
                            const c = d.col;
                            const k = d.top_idx;
                            if (k + 1 < layout.col_n[c]) {
                                const a = layout.col_terms[c][k];
                                const b = layout.col_terms[c][k + 1];
                                if (terms[a]) |ta| if (terms[b]) |tb| {
                                    const combined: f32 = @floatFromInt(rects[a].h + rects[b].h);
                                    var nt: f32 = @floatFromInt(d.cur_y -| rects[a].y);
                                    const minh: f32 = MINH;
                                    nt = std.math.clamp(nt, minh, @max(minh, combined - minh));
                                    const pair = ta.vweight + tb.vweight;
                                    ta.vweight = pair * (nt / combined);
                                    tb.vweight = pair - ta.vweight;
                                };
                            }
                        },
                        .move => |d| {
                            moveTerm(&layout, &rects, &col_x, &col_w, d.id, d.cur_x, d.cur_y);
                            // a file moved into the left column evicts a lone unused shell there
                            if (terms[d.id]) |mt| if (mt.file != null) {
                                if (layoutFindTerm(&layout, d.id)) |f| if (f.col == 0)
                                    evictLonePristineTty(&layout, &terms, io, gpa, 0, d.id, &active);
                            };
                        },
                        .select => |s| {
                            if (terms[s.id]) |t| {
                                t.sel[@intFromEnum(s.button)].state = .done;
                                if (s.button == .left) {
                                    // left-click in the body enters NORMAL mode and
                                    // anchors the cursor at the selection end. The row is
                                    // stored absolute (visual + scroll off) so it rides
                                    // the scrollback. tag clicks (row < BOX_H) don't edit.
                                    const sl = t.sel[@intFromEnum(Button.left)];
                                    const body_vis = sl.r1 - @as(i32, BOX_H);
                                    if (body_vis >= 0) {
                                        const off: i32 = paneScroll(t);
                                        t.cur_row = body_vis + off;
                                        // file panes render a line-number prefix; cur_col
                                        // is content-relative, so peel PREFIX_W off the click.
                                        t.cur_col = if (t.file != null) @max(0, sl.c1 - @as(i32, PREFIX_W)) else sl.c1;
                                        t.cur_off = if (runAt(t, t.cur_row, t.cur_col)) |i| t.edits[i].len else 0;
                                        t.cur_pinned = true;
                                        t.mode = .normal;
                                        t.msel.active = false;
                                        t.pending = 0;
                                    }
                                } else {
                                    // middle/right: acme execute/look. A no-drag click
                                    // expands to the file-ish word under the cursor first.
                                    expandSel(gpa, t, &t.sel[@intFromEnum(s.button)]);
                                    if (selectionText(gpa, t, t.sel[@intFromEnum(s.button)])) |txt| {
                                        defer gpa.free(txt);
                                        if (actOnSelection(io, gpa, &loop, &terms, &layout, &rects, &active, screen_w, screen_h, s.button, s.id, txt)) break;
                                    } else |_| {}
                                }
                            }
                        },
                        .none => {},
                    }
                    drag = .none;
                }
            },
            .key_press => |key| kpress: {
                if (key.matches('c', .{ .ctrl = true })) break;

                if (terms[active]) |at| {
                    // Ctrl-w prefix: helix-style pane focus (Ctrl-w h/j/k/l).
                    if (ctrl_w_pending) {
                        ctrl_w_pending = false;
                        if (key.matches('h', .{}) or key.matches(vaxis.Key.left, .{})) focusDir(&active, &terms, &rects, .left)
                        else if (key.matches('l', .{}) or key.matches(vaxis.Key.right, .{})) focusDir(&active, &terms, &rects, .right)
                        else if (key.matches('k', .{}) or key.matches(vaxis.Key.up, .{})) focusDir(&active, &terms, &rects, .up)
                        else if (key.matches('j', .{}) or key.matches(vaxis.Key.down, .{})) focusDir(&active, &terms, &rects, .down);
                        if (terms[active]) |nt| nt.pending = 0;
                        break :kpress;
                    }
                    if (key.matches('w', .{ .ctrl = true })) {
                        ctrl_w_pending = true;
                        break :kpress;
                    }

                    // global window ops (any mode): Alt-n new terminal below,
                    // Alt-c move active terminal into a fresh column.
                    if (key.matches('n', .{ .alt = true })) {
                        // new terminal below the active one, in its column.
                        // ponytail: forks from the now-multithreaded process;
                        // execv (not execvp) keeps the child async-signal-safe.
                        const free_slot: ?usize = for (terms, 0..) |s, i| {
                            if (s == null) break i;
                        } else null;
                        if (free_slot) |slot| {
                            if (forkTerm(gpa, screen_w, screen_h, null)) |nt| {
                                terms[slot] = nt;
                                startReader(io, gpa, &loop, nt, slot) catch {};
                                const f = layoutFindTerm(&layout, active).?;
                                layoutInsert(&layout, f.col, f.idx + 1, slot);
                                active = slot;
                                termWrite(nt, "ls\r");
                            } else |_| {}
                        }
                        break :kpress;
                    }
                    if (key.matches('c', .{ .alt = true })) {
                        // move active terminal into a fresh column
                        const f = layoutFindTerm(&layout, active).?;
                        if (layout.ncol < MAX_COLS and layout.col_n[f.col] > 1) {
                            layoutRemove(&layout, active);
                            layoutAppendColumn(&layout, active);
                        }
                        break :kpress;
                    }

                    // Ctrl-b: hidden toggle in/out of raw tty mode (terminals only).
                    // tty forwards every key straight to the pty, so it's deliberately
                    // off the normal editing path — this toggle is the only way in/out.
                    if (key.matches('b', .{ .ctrl = true })) {
                        if (at.file == null) {
                            if (at.mode == .tty) {
                                at.mode = .normal;
                                at.pending = 0;
                            } else enterTty(at);
                        }
                        break :kpress;
                    }

                    // Normal-mode Enter/Tab mirror the acme mouse chords on the word
                    // under the cursor: Enter = right-click "look" (open the file/dir),
                    // Tab = middle-click "execute" (run it / builtin).
                    if (at.mode == .normal and (key.matches(vaxis.Key.enter, .{}) or key.matches(vaxis.Key.tab, .{}))) {
                        pinCursor(at);
                        const btn: Button = if (key.matches(vaxis.Key.tab, .{})) .middle else .right;
                        var sel = cursorWordSel(at);
                        expandSel(gpa, at, &sel);
                        if (selectionText(gpa, at, sel)) |word| {
                            defer gpa.free(word);
                            if (actOnSelection(io, gpa, &loop, &terms, &layout, &rects, &active, screen_w, screen_h, btn, active, word)) break;
                        } else |_| {}
                        break :kpress;
                    }

                    // per-mode dispatch. Esc: insert -> normal (normal Esc is a
                    // no-op, like helix). Ctrl-b toggles raw tty on terminals (above).
                    // tty forwards every key to the pty.
                    switch (at.mode) {
                        .normal => handleNormal(gpa, frame_arena.allocator(), at, key),
                        .insert => {
                            if (key.matches(vaxis.Key.escape, .{})) {
                                at.mode = .normal;
                                at.msel.active = false;
                                at.pending = 0;
                            } else handleInsert(gpa, frame_arena.allocator(), at, key);
                        },
                        .tty => forwardToPty(at, key),
                    }
                }
            },
            .pty_read => |pr| {
                defer gpa.free(pr.bytes);
                if (terms[pr.id]) |t| {
                    t.stream.nextSlice(pr.bytes);
                    // follow new output to the bottom, so a command's result shows
                    // even in a pane you didn't focus. Exception: the pane you're
                    // actively navigating in a text mode — don't yank the cursor out
                    // from under you (it still tracks the bottom on its own there).
                    if (!(pr.id == active and t.mode != .tty))
                        t.term.screens.active.scroll(.active);
                }
            },
            .pty_eof => |id| {
                if (terms[id]) |t| {
                    absorbVWeight(&layout, &terms, id);
                    layoutRemove(&layout, id);
                    t.deinit(io, gpa);
                    gpa.destroy(t);
                    terms[id] = null;
                    // refocus the first surviving terminal; quit if none remain.
                    if (active == id) active = for (terms, 0..) |s, i| {
                        if (s != null) break i;
                    } else break;
                }
            },
            .winsize => |ws| {
                resize_count += 1;
                screen_w = ws.cols;
                screen_h = ws.rows;
                try vx.resize(gpa, tty.writer(), ws);
            },
            .quit => break,
            else => {},
        }

        // recompute geometry from the (possibly mutated) layout, then push any
        // grid-size change to each emulator + pty. Weights only change on a drag
        // *release*, so mid-drag the rects are unchanged and nothing resizes.
        computeGeom(&layout, &terms, screen_w, screen_h, &rects, &col_x, &col_w);
        for (&terms, 0..) |*slot, id| {
            if (slot.*) |t| {
                const r = rects[id];
                const cols = @max(1, r.w -| GUTTER);
                const rows = @max(1, r.h -| BOX_H); // the tag steals the top row
                if (cols != t.cols or rows != t.rows) {
                    // file panes have no pty/emulator grid to reflow; just record
                    // the size so bodyText renders the right number of rows.
                    if (t.file == null) {
                        t.term.resize(gpa, cols, rows) catch {};
                        pty_set_size(t.pty, rows, cols);
                    }
                    t.cols = cols;
                    t.rows = rows;
                }
            }
        }

        // ---- render ----
        const win = vx.window();
        win.clear();

        for (&terms, 0..) |*slot, id| {
            if (slot.*) |t| try renderPane(win, frame_arena.allocator(), t, rects[id], id == active);
        }

        // No static separators: each pane's tagline (top) and scrollbar gutter
        // (left) already read as borders. The 1-cell gap between panes stays as
        // an (invisible) drag zone; the dashed preview below shows it on drag.

        // dashed preview while dragging; commit happens on release.
        switch (drag) {
            .border_v => |d| {
                var ry: u16 = 0;
                while (ry < screen_h) : (ry += 1)
                    win.writeCell(d.cur_x, ry, .{ .char = .{ .grapheme = "╎" }, .style = .{ .fg = .{ .rgb = gray.lit } } });
            },
            .border_h => |d| {
                const c = d.col;
                var rx: u16 = col_x[c];
                while (rx < col_x[c] + col_w[c]) : (rx += 1)
                    win.writeCell(rx, d.cur_y, .{ .char = .{ .grapheme = "╌" }, .style = .{ .fg = .{ .rgb = gray.lit } } });
            },
            .move => |d| {
                // insertion indicator: dashed line at cur_y across the target column
                var tc: usize = if (layout.ncol > 0) layout.ncol - 1 else 0;
                for (0..layout.ncol) |c| {
                    if (d.cur_x >= col_x[c] and d.cur_x < col_x[c] + col_w[c]) {
                        tc = c;
                        break;
                    }
                }
                var rx: u16 = col_x[tc];
                while (rx < col_x[tc] + col_w[tc]) : (rx += 1)
                    win.writeCell(rx, d.cur_y, .{ .char = .{ .grapheme = "╌" }, .style = .{ .fg = .{ .rgb = gray.mid } } });
                win.writeCell(d.cur_x, d.cur_y, .{ .char = .{ .grapheme = "▌" }, .style = .{ .fg = .{ .rgb = gray.mid } } });
            },
            else => {
                // not dragging: tint the resize border under the cursor (matching
                // the press-handler hit tests) so the invisible drag zones show.
                var c: usize = 0;
                while (c + 1 < layout.ncol) : (c += 1) {
                    if (hover_col != col_x[c] + col_w[c]) continue;
                    var ry: u16 = 0;
                    while (ry < screen_h) : (ry += 1)
                        win.writeCell(hover_col, ry, .{ .char = .{ .grapheme = " " }, .style = .{ .bg = .{ .rgb = hover_bg } } });
                }
                var cc: usize = 0;
                while (cc < layout.ncol) : (cc += 1) {
                    if (hover_col < col_x[cc] or hover_col >= col_x[cc] + col_w[cc]) continue;
                    var k: usize = 0;
                    while (k + 1 < layout.col_n[cc]) : (k += 1) {
                        const r = rects[layout.col_terms[cc][k]];
                        if (hover_row != r.y + r.h) continue;
                        var rx: u16 = col_x[cc];
                        while (rx < col_x[cc] + col_w[cc]) : (rx += 1)
                            win.writeCell(rx, hover_row, .{ .char = .{ .grapheme = " " }, .style = .{ .bg = .{ .rgb = hover_bg } } });
                    }
                }
            },
        }

        // debug overlay: a small virtual-text box drawn last, on top of everything.
        // ov_buf lives across vx.render() — vaxis keeps slices into it (retained).
        if (terms[active]) |at| {
            var nlive: usize = 0;
            for (terms) |slot| {
                if (slot != null) nlive += 1;
            }
            var sb_off: usize = undefined;
            var sb_total: usize = undefined;
            if (at.file) |f| {
                sb_off = f.scroll;
                sb_total = fileLineCount(f.content);
            } else {
                const sb = at.term.screens.active.pages.scrollbar();
                sb_off = sb.offset;
                sb_total = sb.total;
            }
            const s = std.fmt.bufPrint(
                &ov_buf,
                "pardes  resizes={d}\nterms {d}  cols {d}\nactive #{d}  {d}x{d}\nscroll {d}/{d}",
                .{ resize_count, nlive, layout.ncol, active, at.cols, at.rows, sb_off, sb_total },
            ) catch "";
            const ow: u16 = 26;
            const oh: u16 = 6;
            const ox: u16 = if (screen_w > ow) screen_w - ow else 0;
            const ov = win.child(.{ .x_off = ox, .y_off = 0, .width = ow, .height = oh, .border = .{ .where = .all } });
            ov.fill(.{ .char = .{ .grapheme = " " }, .style = .{ .bg = .{ .rgb = gray.bar } } });
            _ = ov.printSegment(.{ .text = s, .style = .{ .fg = .{ .rgb = gray.txt } } }, .{});
        }

        try vx.render(tty.writer());
        _ = frame_arena.reset(.retain_capacity);
    }
}

// On box-drag release: drop the window into the column under cur_x at the
// vertical slot picked by cur_y (acme's coldragwin, minus the live retile).
fn moveTerm(
    l: *Layout,
    rects: *const [MAX_TERMS]Rect,
    col_x: *const [MAX_COLS]u16,
    col_w: *const [MAX_COLS]u16,
    id: usize,
    cur_x: u16,
    cur_y: u16,
) void {
    const src = layoutFindTerm(l, id) orelse return;
    // target column under cur_x (last column absorbs anything past the right edge)
    var tc: usize = if (l.ncol > 0) l.ncol - 1 else 0;
    for (0..l.ncol) |c| {
        if (cur_x >= col_x[c] and cur_x < col_x[c] + col_w[c]) {
            tc = c;
            break;
        }
    }

    // insertion index in the target column, ignoring the moving window itself
    var ti: usize = 0;
    var k: usize = 0;
    while (k < l.col_n[tc]) : (k += 1) {
        const cid = l.col_terms[tc][k];
        if (cid == id) continue;
        const rr = rects[cid];
        if (cur_y < rr.y + rr.h / 2) break;
        ti += 1;
    }

    const src_solo = l.col_n[src.col] == 1;
    if (tc == src.col and src_solo) return; // dragging the only window of its column: nothing to do

    layoutRemove(l, id);
    if (src_solo and src.col < tc) tc -= 1; // removing src dropped a column to our left
    if (tc >= l.ncol) tc = l.ncol - 1;
    if (ti > l.col_n[tc]) ti = l.col_n[tc];
    layoutInsert(l, tc, ti, id);
}

// index of the run anchored exactly at (row, col), if any.
fn runAt(t: *Term, row: i32, col: i32) ?usize {
    for (t.edits[0..t.nedits], 0..) |e, i| if (e.row == row and e.col == col) return i;
    return null;
}

// rendered column of a position: original col + earlier-on-row insertions + the
// offset into the run at this col. (horizontal only; scrolling doesn't matter.)
fn renderedCol(t: *Term, row: i32, col: i32, off: u16) i32 {
    var rc: i32 = col + @as(i32, off);
    for (t.edits[0..t.nedits]) |e| if (e.row == row and e.col < col) {
        rc += @as(i32, e.len);
    };
    return rc;
}

// Splice this row's insertion runs into `line` (the raw tty text for absolute
// row `abs_row`), shifting the underlying text right rather than overwriting it.
fn spliceRow(alloc: std.mem.Allocator, out: *std.ArrayList(u8), t: *Term, abs_row: i32, line: []const u8) !void {
    var idxs: [Term.MAX_EDITS]usize = undefined;
    var n: usize = 0;
    for (t.edits[0..t.nedits], 0..) |e, i| if (e.row == abs_row and e.len > 0) {
        idxs[n] = i;
        n += 1;
    };
    // insertion-sort the runs by column (n is tiny)
    var a: usize = 1;
    while (a < n) : (a += 1) {
        const key = idxs[a];
        var b = a;
        while (b > 0 and t.edits[idxs[b - 1]].col > t.edits[key].col) : (b -= 1) idxs[b] = idxs[b - 1];
        idxs[b] = key;
    }
    var vpos: usize = 0; // original-line columns consumed so far
    for (idxs[0..n]) |i| {
        const e = &t.edits[i];
        const c: usize = @intCast(@max(0, e.col));
        if (c <= line.len) {
            if (c > vpos) try out.appendSlice(alloc, line[vpos..c]);
            vpos = c;
        } else {
            // run anchored past the text: emit the rest, then pad with spaces
            if (vpos < line.len) {
                try out.appendSlice(alloc, line[vpos..]);
                vpos = line.len;
            }
            while (vpos < c) : (vpos += 1) try out.append(alloc, ' ');
        }
        try out.appendSlice(alloc, e.text[0..e.len]);
    }
    if (vpos < line.len) try out.appendSlice(alloc, line[vpos..]);
}

// The terminal body with all insertion runs spliced in, newline-joined. This is
// the actual on-screen text: render and selection both read it, so there's no
// tty-vs-typed distinction. Caller frees.
// Per visible row, whether it's a shell prompt/input row (OSC 133). Used to hide
// prompts in text mode. Rows beyond the iterator (blank tail) stay false.
fn promptFlags(t: *Term, flags: []bool) void {
    @memset(flags, false);
    var it = t.term.screens.active.pages.rowIterator(.right_down, .{ .viewport = .{} }, null);
    var i: usize = 0;
    while (it.next()) |pin| : (i += 1) {
        if (i >= flags.len) break;
        flags[i] = pin.rowAndCell().row.semantic_prompt != .none;
    }
}

fn bodyText(alloc: std.mem.Allocator, t: *Term) ![]u8 {
    // file pane: line-numbered content from f.scroll, edits spliced like the tty.
    if (t.file) |f| {
        var out: std.ArrayList(u8) = .empty;
        errdefer out.deinit(alloc);
        var lines = std.mem.splitScalar(u8, f.content, '\n');
        var skipped: usize = 0;
        while (skipped < f.scroll) : (skipped += 1) {
            if (lines.next() == null) break;
        }
        var i: usize = 0;
        while (i < t.rows) : (i += 1) {
            if (i > 0) try out.append(alloc, '\n');
            const abs: i32 = @as(i32, @intCast(f.scroll)) + @as(i32, @intCast(i));
            if (lines.next()) |text| {
                var lbuf: [16]u8 = undefined;
                const prefix = std.fmt.bufPrint(&lbuf, "{d: >4} ", .{abs + 1}) catch "     ";
                var base: std.ArrayList(u8) = .empty;
                defer base.deinit(alloc);
                try base.appendSlice(alloc, prefix);
                try base.appendSlice(alloc, text);
                try spliceRow(alloc, &out, t, abs, base.items);
            } else try spliceRow(alloc, &out, t, abs, "");
        }
        return out.toOwnedSlice(alloc);
    }
    const off: i32 = @intCast(t.term.screens.active.pages.scrollbar().offset);
    const body = try t.term.plainString(alloc);
    defer alloc.free(body);
    // text mode (normal AND insert) hides shell prompt rows: a clean acme-style
    // page to navigate and type on. Only raw tty mode (the Ctrl-b toggle) shows
    // the real terminal with its prompts + typed input.
    var flagbuf: [512]bool = undefined;
    const nrows: usize = @min(t.rows, flagbuf.len);
    const flags = flagbuf[0..nrows];
    if (t.mode != .tty) promptFlags(t, flags) else @memset(flags, false);
    var out: std.ArrayList(u8) = .empty;
    errdefer out.deinit(alloc);
    // pad to the full grid height: plainString trims trailing blank rows, but we
    // must still be able to type (and splice) onto those blank rows.
    var it = std.mem.splitAny(u8, body, "\n");
    var i: usize = 0;
    while (i < nrows) : (i += 1) {
        if (i > 0) try out.append(alloc, '\n');
        const raw = it.next() orelse "";
        const line = if (flags[i]) "" else raw; // hidden prompt -> blank (edits still splice)
        try spliceRow(alloc, &out, t, @as(i32, @intCast(i)) + off, line);
    }
    return out.toOwnedSlice(alloc);
}

// The full rendered pane: tag at visual row 0, spliced body below. Caller frees.
fn paneText(gpa: std.mem.Allocator, t: *Term) ![]u8 {
    const tag = try tagLine(gpa, t);
    defer gpa.free(tag);
    const body = try bodyText(gpa, t);
    defer gpa.free(body);
    return std.fmt.allocPrint(gpa, "{s}\n{s}", .{ tag, body });
}

// Block-selected text, newline-joined per row; caller frees. Reads the rendered
// screen, so selecting typed text and tty text works identically.
fn selectionText(gpa: std.mem.Allocator, t: *Term, sl: Term.Sel) ![]u8 {
    const r0 = @min(sl.r0, sl.r1);
    const r1 = @max(sl.r0, sl.r1);
    const c0: usize = @intCast(@max(0, @min(sl.c0, sl.c1)));
    const c1: usize = @intCast(@max(0, @max(sl.c0, sl.c1)));
    const pane = try paneText(gpa, t);
    defer gpa.free(pane);
    var out: std.ArrayList(u8) = .empty;
    errdefer out.deinit(gpa);
    var it = std.mem.splitAny(u8, pane, "\n");
    var v: i32 = 0;
    var first = true;
    while (it.next()) |line| : (v += 1) {
        if (v < r0 or v > r1) continue;
        if (!first) try out.append(gpa, '\n');
        first = false;
        const b0 = @min(c0, line.len);
        const b1 = @min(c1 + 1, line.len);
        try out.appendSlice(gpa, line[b0..b1]);
    }
    return out.toOwnedSlice(gpa);
}

// file-ish word chars (acme isfilec): alnum + . - + / : @ _ ~
fn isFileChar(c: u8) bool {
    return std.ascii.isAlphanumeric(c) or switch (c) {
        '.', '-', '+', '/', ':', '@', '_', '~' => true,
        else => false,
    };
}

// acme-style: a no-drag middle/right click expands to the file-ish word under the
// cursor, in place. A real (dragged) selection is left untouched.
fn expandSel(gpa: std.mem.Allocator, t: *Term, sl: *Term.Sel) void {
    if (sl.c0 != sl.c1 or sl.r0 != sl.r1) return;
    const pane = paneText(gpa, t) catch return;
    defer gpa.free(pane);
    var it = std.mem.splitAny(u8, pane, "\n");
    var v: i32 = 0;
    while (it.next()) |line| : (v += 1) {
        if (v != sl.r0) continue;
        const col: usize = @intCast(@max(0, sl.c0));
        if (col > line.len) return;
        var hi = col;
        while (hi < line.len and isFileChar(line[hi])) hi += 1;
        var lo = col;
        while (lo > 0 and isFileChar(line[lo - 1])) lo -= 1;
        if (hi > lo) {
            sl.c0 = @intCast(lo);
            sl.c1 = @intCast(hi - 1);
        }
        return;
    }
}

// Decide what a selection means. Middle button: a builtin name (comptime-matched
// against `builtins`) runs that builtin; anything else is sent to the terminal.
fn onSelect(button: Button, text: []const u8) SelectAction {
    if (button != .middle) return .none;
    const cmd = std.mem.trim(u8, text, " \t\r\n");
    if (cmd.len == 0) return .none;
    inline for (builtins) |b| {
        if (std.mem.eql(u8, cmd, b.name)) return b.action;
    }
    return .send;
}