Shader files compile again on save; an attached GUI runs the session's post chain

shader_build.zig takes the Shadertoy compile out of the GUI's Post: the
process that holds the core compiles each chain file behind the prefix on a
thread, watches its directory (file_watch's new shader slots), and on a
change reads and hashes each file, compiling only one whose bytes moved. A
failed compile keeps the last good SPIR-V and is said once; the same bytes
are never compiled or reported twice. A file is read with std, not fs.zig's
readFile, whose turn hand-off a worker thread does not hold.

A detached session compiles for its frontends and sends them the chain
(wire post: each pass's scene and level or the file's SPIR-V, and
ShaderAnimation) on attach and on every change, so an attached GUI runs the
same passes, levels and animation as a local one while still reading no disk
and running no program. The attached GUI describes its frames to the chain
from the session's chrome and redraws an animating chain on its own.

10 files changed, 626 insertions, 226 deletions

diff --git a/src/config.zig b/src/config.zig
index 1b432415..86f3e7eb 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -1017,7 +1017,7 @@ pub const Runtime = struct {
         .{ .word = "Bloom", .action = .{ .scene = .bloom }, .availability = .scene_shaders, .doc = "The brightest ink glows a little: a level, 0 (off) through 3." },
         .{ .word = "Vignette", .action = .{ .scene = .vignette }, .availability = .scene_shaders, .doc = "The window's corners fall into shade: a level, 0 (off) through 3." },
         .{ .word = "Grain", .action = .{ .scene = .grain }, .availability = .scene_shaders, .doc = "The page takes a fine, still grain, like paper: a level, 0 (off) through 3." },
-        .{ .word = "Shader", .action = .shader, .availability = .scene_shaders, .doc = "A Shadertoy file into the post chain, or out of it; off empties the chain." },
+        .{ .word = "Shader", .action = .shader, .availability = .scene_shaders, .doc = "A Shadertoy file into the post chain, or out of it, compiled again when saved; off empties the chain." },
         .{ .word = "ShaderAnimation", .action = .shader_animation, .availability = .scene_shaders, .doc = "When the post chain animates on its own: off, on (while focused) or always." },
         .{ .word = "Placement", .action = .{ .choice = .placement }, .doc = "Where a new pane goes: acme, as makenewwindow does, or pardes, the older rules." },
         .{ .word = "BootShell", .action = .{ .choice = .boot_shell }, .doc = "A document dropped where a lone untouched shell stands: keep the shell, or replace it." },
diff --git a/src/detached/client.zig b/src/detached/client.zig
index 8c9a2560..69ab2fdd 100644
--- a/src/detached/client.zig
+++ b/src/detached/client.zig
@@ -109,6 +109,10 @@ pub const Client = struct {
     regions: std.ArrayListUnmanaged(pardes.Region) = .empty,
     /// The palette its chrome is drawn in, as its last frame gave it.
     chrome: pardes.Chrome = .{},
+    /// The post chain its frames are drawn through, as the session last
+    /// sent it; its paths and SPIR-V owned here. `post_key` moves with it.
+    post: wire.Post = .{},
+    post_key: u32 = 0,
     in: std.ArrayListUnmanaged(u8) = .empty,
     out: std.ArrayListUnmanaged(u8) = .empty,
     /// Bytes of `in` belonging to the message `next` returned last. Compacted at
@@ -187,6 +191,7 @@ pub const Client = struct {
         }
         for (&c.tag_layers) |*layer| c.gpa.free(layer.cells);
         c.regions.deinit(c.gpa);
+        c.freePost();
         c.in.deinit(c.gpa);
         c.out.deinit(c.gpa);
     }
@@ -356,10 +361,30 @@ pub const Client = struct {
             // open a URI on this display or read this display's selection back
             // over the socket. `Ungreeted` is the same refusal a frame gets.
             .set_clipboard, .read_clipboard, .open_link => if (!c.attached()) return error.Ungreeted,
+            // Kept, like the chrome: the frames that follow are drawn
+            // through it.
+            .post => |chain| {
+                if (!c.attached()) return error.Ungreeted;
+                c.freePost();
+                c.post = chain;
+                for (c.post.passes[0..c.post.len]) |*pass| {
+                    pass.path = c.gpa.dupe(u8, pass.path) catch return error.Closed;
+                    pass.spirv = c.gpa.dupe(u8, pass.spirv) catch return error.Closed;
+                }
+                c.post_key +%= 1;
+            },
         }
         return msg;
     }
 
+    fn freePost(c: *Client) void {
+        for (c.post.passes[0..c.post.len]) |pass| {
+            c.gpa.free(pass.path);
+            c.gpa.free(pass.spirv);
+        }
+        c.post = .{};
+    }
+
     // ---- internals --------------------------------------------------------
 
     fn reshape(c: *Client, cols: u16, rows: u16) (Error || wire.Error)!void {
@@ -1131,7 +1156,7 @@ test "detached session: the seam's own routing rules, per surviving effect" {
 test "detached session: a frontend is never asked to fork, write, or watch" {
     // THE INVARIANT OF THE WHOLE DETACHED DESIGN, pinned as a property of the
     // protocol rather than of one code path: a frontend is input and screen, so
-    // the set of messages that can reach it is exactly the eight below. If a
+    // the set of messages that can reach it is exactly the nine below. If a
     // machine-local effect is ever put back on the wire, a frontend becomes the
     // process that owns a pane's shell again — and a pane whose shell belongs to
     // a frontend dies when that frontend leaves, which is the bug this replaced.
@@ -1149,6 +1174,10 @@ test "detached session: a frontend is never asked to fork, write, or watch" {
         // The three that need THIS human's display and cannot be done by a
         // daemon nobody is looking at.
         "set_clipboard", "read_clipboard", "open_link",
+        // Display state, like a frame's chrome: the passes this display
+        // draws its frames through. A file's SPIR-V arrives compiled, so the
+        // frontend still reads no disk and runs no program.
+        "post",
     };
 
     // One: the union a frontend decodes into has no other variant. Named
@@ -1183,7 +1212,7 @@ test "detached session: a frontend is never asked to fork, write, or watch" {
             if (@intFromEnum(want) == tag) break true;
         } else false;
         if (!named) {
-            std.debug.print("tag 0x{x:0>2} is decodable by a frontend and is not one of the seven\n", .{tag});
+            std.debug.print("tag 0x{x:0>2} is decodable by a frontend and is not one of the nine\n", .{tag});
             return error.MachineLocalEffectOnTheWire;
         }
     }
diff --git a/src/detached/server.zig b/src/detached/server.zig
index 13cf4488..99a2afc2 100644
--- a/src/detached/server.zig
+++ b/src/detached/server.zig
@@ -11,6 +11,7 @@ const host_io = @import("../host_io.zig");
 const selection_pipe = @import("../selection_pipe.zig");
 
 const file_watch = @import("../file_watch.zig");
+const shader_build = @import("../shader_build.zig");
 
 const ninep_io = @import("../9p_io.zig");
 
@@ -63,6 +64,8 @@ const Client = struct {
     mirror: std.ArrayListUnmanaged(pardes.Cell) = .empty,
     need_full: bool = true,
     accepted_ms: i64 = 0,
+    /// The post chain last sent (Session.sendPost); null: none yet.
+    post_sent: ?u64 = null,
 };
 
 const Pty = struct {
@@ -173,6 +176,8 @@ pub const Session = struct {
     /// the next poll returns at once, to draw it.
     ninep_wake: std.atomic.Value(bool) = .init(false),
     watches: file_watch.Table = @splat(null),
+    /// The post chain's files, compiled here for the GUIs attached.
+    shaders: shader_build = .{},
     check_files: bool = false,
     in_loop: bool = false,
     greet_deadline_ms: u32 = greet_deadline_default_ms,
@@ -254,6 +259,8 @@ pub const Session = struct {
         for (0..pardes.MAX_PANES) |pane|
             file_watch.watchPane(s.inotify_fd, &s.watches, @intCast(pane), null, 0, .{ .text = 0 });
         _ = file_watch.applyThemeEffect(s.core, s.gpa, s.inotify_fd, &s.watches, 0, false, false);
+        // The new core's chain generations are its own.
+        s.shaders.generation = null;
         s.check_files = false;
         if (s.ninep) |listener| listener.reset(replacement);
         s.ninep_pending = false;
@@ -267,6 +274,8 @@ pub const Session = struct {
     }
 
     pub fn deinit(s: *Session) void {
+        // First: its compile thread wakes the mailbox, closed below.
+        s.shaders.deinit(s.gpa, s.inotify_fd, &s.watches);
         if (s.mailbox.wake[0] >= 0) pardes.lsp.setStatusSink(null, null);
         if (s.mailbox.wake[0] >= 0) host_io.setExitWake(null, null);
         s.cancelWorkers();
@@ -766,6 +775,7 @@ pub const Session = struct {
     fn reloadWatched(s: *Session) void {
         if (!s.check_files) return;
         s.check_files = false;
+        s.shaders.recheck();
         for (0..reload_retries) |_| {
             if (!file_watch.reloadChanged(s.core, s.io, s.gpa, &s.watches)) return;
         }
@@ -823,6 +833,8 @@ pub const Session = struct {
         const completed = s.ninep_wake.swap(false, .acq_rel) or drained;
         for (&s.clients) |*c| if (c.fd >= 0) s.flush(c);
         const regridded = s.reconcile();
+        // After the greeting reconcile sends: a chain before it is refused.
+        const compiled = s.sendPost();
         // Nobody attached, nobody to dismiss a message: the loop expires
         // them itself, between the editor's steps (expireUnattended).
         s.core.unattended = s.primary() == null;
@@ -891,7 +903,7 @@ pub const Session = struct {
         }
         var timeout: c_int = if (timeout_ms == 0) -1 else @intCast(@min(timeout_ms, std.math.maxInt(c_int)));
         if (s.nextWake(now)) |due| timeout = if (timeout < 0) due else @min(timeout, due);
-        if (completed or s.check_files or regridded or s.ninep_pending) timeout = 0;
+        if (completed or compiled or s.check_files or regridded or s.ninep_pending) timeout = 0;
         pardes.turn.rest();
         const ready = libc.poll(&fds, @intCast(n), timeout);
         pardes.turn.wake();
@@ -1102,6 +1114,28 @@ pub const Session = struct {
         return regridded;
     }
 
+    /// Compiles the post chain's files (a finished compile wakes the loop
+    /// through the mailbox) and sends each attached frontend the chain it
+    /// has not seen: on attach, and when it or a file's SPIR-V changes. True
+    /// when a compile was taken in, so the loop draws what it said.
+    /// ponytail: a terminal frontend is sent it too and drops it; a hello
+    /// saying which frontend it is would spare those bytes.
+    fn sendPost(s: *Session) bool {
+        const files = for (s.core.settings.post.list()) |entry| {
+            if (entry.scene == null) break true;
+        } else false;
+        const compiled = s.shaders.sync(s.gpa, s.io, s.core, if (files) s.inotify() else s.inotify_fd, &s.watches, .{ .ctx = &s.mailbox, .call = wakeForExit });
+        const key = @as(u64, s.shaders.revision) << 8 | @intFromEnum(s.core.settings.shader_animation);
+        var chain: wire.Post = .{ .animation = s.core.settings.shader_animation, .len = s.core.settings.post.len };
+        _ = s.shaders.view(&s.core.settings.post, &chain.passes);
+        for (&s.clients) |*c| {
+            if (!c.attached or c.greet or c.post_sent == key) continue;
+            c.post_sent = key;
+            s.send(c, .{ .post = chain });
+        }
+        return compiled;
+    }
+
     fn send(s: *Session, c: *Client, msg: wire.ServerMsg) void {
         const want = wire.serverBound(msg);
         s.scratch.ensureTotalCapacity(s.gpa, want) catch return s.close(c, .oom);
diff --git a/src/detached/wire.zig b/src/detached/wire.zig
index 6903a1e8..ee8c72fb 100644
--- a/src/detached/wire.zig
+++ b/src/detached/wire.zig
@@ -3,6 +3,9 @@
 const std = @import("std");
 const pardes = @import("../pardes.zig");
 const limits = @import("../memory.zig").limits;
+const shader_build = @import("../shader_build.zig");
+const Scene = pardes.config.Runtime.Scene;
+const ShaderAnimation = pardes.config.Runtime.ShaderAnimation;
 
 /// 7: tag layers gained the `.notice` kind, so a frame can carry a tag value
 /// an older frontend would reject mid-session. The greeting is where that has
@@ -73,6 +76,9 @@ pub const ServerTag = enum(u8) {
     set_clipboard = 0x10,
     read_clipboard = 0x11,
     open_link = 0x12,
+    /// The post chain this frontend draws its frames through: sent on
+    /// attach and whenever it or a file's SPIR-V changes.
+    post = 0x13,
 };
 
 /// Why the server hung up on a connect. Sent as a `refuse` and followed by a
@@ -353,6 +359,22 @@ pub const ServerMsg = union(enum) {
     set_clipboard: []const u8,
     read_clipboard,
     open_link: []const u8,
+    post: Post,
+};
+
+/// The session's post chain, as a GUI attached to it draws it: its passes in
+/// order, each a bundled scene at its level or a Shadertoy file's SPIR-V
+/// (compiled by the session, shader_build.zig: a frontend reads no disk and
+/// runs no program), and when it animates on its own. Slices borrow the
+/// payload.
+pub const Post = struct {
+    animation: ShaderAnimation = .on,
+    passes: [shader_build.max]shader_build.Pass = @splat(.{}),
+    len: u8 = 0,
+
+    pub fn list(p: *const Post) []const shader_build.Pass {
+        return p.passes[0..p.len];
+    }
 };
 
 // ---------------------------------------------------------------------------
@@ -1149,6 +1171,18 @@ pub fn encodeServer(out: []u8, msg: ServerMsg) Error![]const u8 {
         .set_clipboard => |t| try w.putSlice32(t),
         .read_clipboard => {},
         .open_link => |u| try w.putSlice16(u),
+        .post => |p| {
+            try w.putByte(@intFromEnum(p.animation));
+            try w.putByte(p.len);
+            for (p.list()) |pass| {
+                // 0 a file, else a bundled scene and one.
+                try w.putByte(if (pass.scene) |scene| @as(u8, @intFromEnum(scene)) + 1 else 0);
+                try w.putByte(pass.level);
+                try w.putU32(pass.revision);
+                try w.putSlice16(pass.path);
+                try w.putSlice32(pass.spirv);
+            }
+        },
     }
     try finishMessage(&w, at);
     return w.written();
@@ -1170,6 +1204,11 @@ pub fn serverBound(msg: ServerMsg) usize {
         .frame => |f| frameBound(f.cols, f.rows),
         .set_clipboard => |t| t.len,
         .open_link => |u| u.len,
+        .post => |p| blk: {
+            var n: usize = 0;
+            for (p.list()) |pass| n += 12 + pass.path.len + pass.spirv.len;
+            break :blk n;
+        },
     };
 }
 
@@ -1232,6 +1271,22 @@ pub fn decodeServer(tag: u8, payload: []const u8) Error!ServerMsg {
         .set_clipboard => .{ .set_clipboard = try r.getSlice32() },
         .read_clipboard => .read_clipboard,
         .open_link => .{ .open_link = try r.getSlice16() },
+        .post => blk: {
+            var p: Post = .{ .animation = try r.getTag(ShaderAnimation) };
+            p.len = try r.getByte();
+            if (p.len > shader_build.max) return error.Overlong;
+            for (p.passes[0..p.len]) |*pass| {
+                const kind = try r.getByte();
+                pass.* = .{
+                    .scene = if (kind == 0) null else std.enums.fromInt(Scene, kind - 1) orelse return error.BadValue,
+                    .level = try r.getByte(),
+                    .revision = try r.getU32(),
+                    .path = try r.getSlice16(),
+                    .spirv = try r.getSlice32(),
+                };
+            }
+            break :blk .{ .post = p };
+        },
     };
     try r.end();
     return msg;
@@ -1349,6 +1404,18 @@ test "detached wire: every server message round-trips" {
     try testing.expectEqualStrings("yank", (try roundServer(&buf, .{ .set_clipboard = "yank" })).set_clipboard);
     try testing.expectEqual(ServerMsg.read_clipboard, try roundServer(&buf, .read_clipboard));
     try testing.expectEqualStrings("https://x", (try roundServer(&buf, .{ .open_link = "https://x" })).open_link);
+    var chain: Post = .{ .animation = .always, .len = 2 };
+    chain.passes[0] = .{ .scene = .bloom, .level = 3 };
+    chain.passes[1] = .{ .path = "~/wave.glsl", .spirv = "\x03\x02\x23\x07spirv", .revision = 7 };
+    const back = (try roundServer(&buf, .{ .post = chain })).post;
+    try testing.expectEqual(ShaderAnimation.always, back.animation);
+    try testing.expectEqual(@as(u8, 2), back.len);
+    try testing.expectEqual(Scene.bloom, back.passes[0].scene.?);
+    try testing.expectEqual(@as(u8, 3), back.passes[0].level);
+    try testing.expect(back.passes[1].scene == null);
+    try testing.expectEqualStrings("~/wave.glsl", back.passes[1].path);
+    try testing.expectEqualStrings("\x03\x02\x23\x07spirv", back.passes[1].spirv);
+    try testing.expectEqual(@as(u32, 7), back.passes[1].revision);
 }
 
 test "detached wire: only the display's own effects are on the wire" {
@@ -1357,9 +1424,9 @@ test "detached wire: only the display's own effects are on the wire" {
     // is the direction that matters: it would hand a frontend work that must
     // outlive it. The count fails if one of the three is dropped, which would
     // silently leave a clipboard or a link unanswered on every frontend.
-    try testing.expectEqual(@as(usize, 8), std.enums.values(ServerTag).len);
+    try testing.expectEqual(@as(usize, 9), std.enums.values(ServerTag).len);
     for (std.enums.values(ServerTag)) |t| switch (t) {
-        .welcome, .refuse, .frame, .quit, .detach, .set_clipboard, .read_clipboard, .open_link => {},
+        .welcome, .refuse, .frame, .quit, .detach, .set_clipboard, .read_clipboard, .open_link, .post => {},
     };
 }
 
diff --git a/src/file_watch.zig b/src/file_watch.zig
index d028488e..8bd9de8b 100644
--- a/src/file_watch.zig
+++ b/src/file_watch.zig
@@ -70,7 +70,11 @@ pub const Watch = struct {
 /// Sharing the table also shares directory descriptors correctly when a user
 /// happens to open that .zon as an ordinary document.
 pub const theme_slot = pardes.MAX_PANES;
-pub const Table = [pardes.MAX_PANES + 1]?Watch;
+/// After it, one slot a post chain file (shader_build.zig): marked only so
+/// a save there wakes the host, which has the files read again; no
+/// generation of theirs is kept here.
+pub const shader_slot = theme_slot + 1;
+pub const Table = [shader_slot + pardes.config.Runtime.Post.max]?Watch;
 
 /// Whether this OS has a watcher at all. Off it every entry point below
 /// returns the inert answer: `watchPath` installs nothing, so `watches` stays
@@ -423,6 +427,13 @@ pub fn watchPane(
     watchPath(fd, watches, id, path, serial, generation);
 }
 
+/// Mark the post chain files' directories, one slot each, dropping the
+/// marks of files no longer in it.
+pub fn watchShaders(fd: c_int, watches: *Table, paths: []const []const u8) void {
+    for (0..pardes.config.Runtime.Post.max) |i|
+        watchPath(fd, watches, shader_slot + i, if (i < paths.len) paths[i] else null, 0, .{ .text = 0 });
+}
+
 /// Reconcile one marked pane. A true result requests an immediate second pass:
 /// the PDF pathname changed between the stat before MuPDF reopened it and the
 /// stat after, so committing either identity would lose a generation.
diff --git a/src/gui/Post.zig b/src/gui/Post.zig
index 61ebc2cd..06310b7f 100644
--- a/src/gui/Post.zig
+++ b/src/gui/Post.zig
@@ -1,10 +1,10 @@
 //! The post chain (docs/render-pipeline.md §6): Shadertoy passes over the
 //! finished frame, each reading the one before it (the frame first) and the
 //! last one writing the window. A pass is the bundled Crt (shaders/post/,
-//! compiled with the build) or a user's file (Shader <path>),
-//! compiled when it joins the chain by the build's own glslc invocation on a
-//! thread of its own, never on the frame path. A file whose compile fails
-//! keeps its last good pipeline, and glslc missing is said once.
+//! compiled with the build) or a user's file (Shader <path>), whose SPIR-V
+//! comes from shader_build.zig: compiled by the process that holds the core,
+//! this one or the detached session this GUI is attached to. A file whose
+//! compile fails keeps its last good pipeline.
 //!
 //! The uniforms are ghostty's (Uniforms), so a shader written for ghostty runs
 //! here. Their clock is the shell's and moves only when the chain draws:
@@ -14,7 +14,6 @@ const std = @import("std");
 const libc = std.c;
 const ghostty_vt = @import("ghostty-vt");
 const pardes = @import("../pardes.zig");
-const filesystem = @import("../fs.zig");
 const gui = @import("gui.zig");
 const c = gui.c;
 
@@ -25,6 +24,7 @@ const Pipeline = c.SDL_GPUGraphicsPipeline;
 pub const max = Chain.max;
 
 const prefix = @embedFile("post-prefix.glsl");
+const Built = @import("../shader_build.zig").Pass;
 const vert_spv = @embedFile("post.vert.spv");
 const bundled_spv = std.EnumArray(Scene, []const u8).init(.{
     .crt = @embedFile("post-crt.frag.spv"),
@@ -92,28 +92,15 @@ pub const Pass = struct {
     /// A user's file, owned; empty for a bundled pass.
     path: []u8 = &.{},
     pipeline: ?*Pipeline = null,
-};
-
-/// One compile of a chain's new files, on its own thread.
-const Job = struct {
-    const Result = union(enum) { none, spirv: []u8, failed: []u8, missing };
-    io: std.Io,
-    gpa: std.mem.Allocator,
-    paths: [max][]u8 = @splat(&.{}),
-    results: [max]Result = @splat(.none),
-    len: usize = 0,
-    done: std.atomic.Value(bool) = .init(false),
-    thread: std.Thread = undefined,
+    /// The SPIR-V revision `pipeline` was made from (shader_build.Pass).
+    revision: u32 = 0,
 };
 
 passes: [max]Pass = @splat(.{}),
 len: usize = 0,
-/// The chain generation `passes` follow; null before the first.
-generation: ?u32 = null,
+/// The chain key `passes` follow (sync); null before the first.
+key: ?u32 = null,
 bundled: std.EnumArray(Scene, ?*Pipeline) = .initFill(null),
-job: ?*Job = null,
-/// Said once: without glslc, files never compile.
-missing_said: bool = false,
 ping: [2]?*c.SDL_GPUTexture = .{ null, null },
 ping_w: u32 = 0,
 ping_h: u32 = 0,
@@ -136,20 +123,17 @@ bloom_up: ?*Pipeline = null,
 started_ns: ?u64 = null,
 last_ns: u64 = 0,
 
-/// Brings the passes to the chain the core holds, and takes in a finished
-/// compile. Called each loop step, off the frame path.
-pub fn sync(post: *Post, gpa: std.mem.Allocator, io: std.Io, device: *c.SDL_GPUDevice, format: c.SDL_GPUTextureFormat, core: *pardes.Pardes) void {
-    if (post.job) |job| {
-        if (!job.done.load(.acquire)) return;
-        job.thread.join();
-        post.job = null;
-        post.collect(gpa, device, format, core, job);
-    }
-    const chain = &core.settings.post;
-    if (post.generation == chain.generation) return;
-    post.generation = chain.generation;
+/// Brings the passes to the chain a shell was given (shader_build.zig: a
+/// local core's, or an attached session's over the wire), whose `key` moves
+/// whenever the chain or a file's SPIR-V does. A file whose SPIR-V has not
+/// moved keeps its pipeline; one the GPU refuses keeps its last, and is said
+/// to `core`, or logged where there is none. Each loop step, off the frame
+/// path.
+pub fn sync(post: *Post, gpa: std.mem.Allocator, device: *c.SDL_GPUDevice, format: c.SDL_GPUTextureFormat, chain: []const Built, key: u32, core: ?*pardes.Pardes) void {
+    if (post.key == key) return;
+    post.key = key;
     var next: [max]Pass = @splat(.{});
-    for (chain.list(), 0..) |entry, i| {
+    for (chain, 0..) |entry, i| {
         if (entry.scene) |scene| {
             if (post.bundled.get(scene) == null) post.bundled.set(scene, makePipeline(device, format, bundled_spv.get(scene), if (scene == .bloom) 2 else 1, 4) catch null);
             if (scene == .bloom and post.bloom_down == null) {
@@ -161,11 +145,22 @@ pub fn sync(post: *Post, gpa: std.mem.Allocator, io: std.Io, device: *c.SDL_GPUD
         }
         // A file already in the chain keeps its pipeline and its path.
         for (post.passes[0..post.len]) |*pass| {
-            if (pass.scene != null or !std.mem.eql(u8, pass.path, entry.path.get())) continue;
+            if (pass.scene != null or !std.mem.eql(u8, pass.path, entry.path)) continue;
             next[i] = pass.*;
             pass.* = .{};
             break;
-        } else next[i] = .{ .path = gpa.dupe(u8, entry.path.get()) catch &.{} };
+        } else next[i] = .{ .path = gpa.dupe(u8, entry.path) catch &.{} };
+        const pass = &next[i];
+        if (pass.revision == entry.revision or entry.spirv.len == 0) continue;
+        pass.revision = entry.revision;
+        const made = makePipeline(device, format, entry.spirv, 1, 4) catch {
+            var buf: [512]u8 = undefined;
+            const text = std.fmt.bufPrint(&buf, "Shader {s}: the GPU refused it", .{entry.path}) catch "Shader: the GPU refused it";
+            if (core) |p| p.setMessage(p.active, text) else std.log.err("{s}", .{text});
+            continue;
+        };
+        if (pass.pipeline) |old| c.SDL_ReleaseGPUGraphicsPipeline(device, old);
+        pass.pipeline = made;
     }
     for (post.passes[0..post.len]) |pass| {
         if (pass.scene == null) if (pass.pipeline) |pipeline| c.SDL_ReleaseGPUGraphicsPipeline(device, pipeline);
@@ -179,127 +174,6 @@ pub fn sync(post: *Post, gpa: std.mem.Allocator, io: std.Io, device: *c.SDL_GPUD
         post.last_ns = 0;
         post.uniforms.frame = 0;
     }
-    // The new files compile together, on a thread of their own.
-    const job = gpa.create(Job) catch return;
-    job.* = .{ .io = io, .gpa = gpa };
-    for (post.passes[0..post.len]) |pass| {
-        if (pass.scene != null or pass.pipeline != null or pass.path.len == 0) continue;
-        job.paths[job.len] = gpa.dupe(u8, pass.path) catch continue;
-        job.len += 1;
-    }
-    if (job.len == 0) return gpa.destroy(job);
-    job.thread = std.Thread.spawn(.{}, compile, .{job}) catch {
-        for (job.paths[0..job.len]) |path| gpa.free(path);
-        return gpa.destroy(job);
-    };
-    post.job = job;
-}
-
-fn collect(post: *Post, gpa: std.mem.Allocator, device: *c.SDL_GPUDevice, format: c.SDL_GPUTextureFormat, core: *pardes.Pardes, job: *Job) void {
-    defer gpa.destroy(job);
-    for (job.paths[0..job.len], job.results[0..job.len]) |path, result| {
-        defer gpa.free(path);
-        const pass = for (post.passes[0..post.len]) |*pass| {
-            if (pass.scene == null and std.mem.eql(u8, pass.path, path)) break pass;
-        } else null;
-        var buf: [512]u8 = undefined;
-        switch (result) {
-            .none => {},
-            .missing => if (!post.missing_said) {
-                post.missing_said = true;
-                core.setMessage(core.active, "Shader: glslc is not on PATH, so Shadertoy files do not run (Crt does)");
-            },
-            .failed => |text| {
-                defer gpa.free(text);
-                // glslc's first line says where: in the file's own lines.
-                const first = std.mem.sliceTo(text, '\n');
-                core.setMessage(core.active, std.fmt.bufPrint(&buf, "Shader {s}: {s}", .{ path, first[0..@min(first.len, 400)] }) catch "Shader: compile failed");
-            },
-            .spirv => |spirv| {
-                defer gpa.free(spirv);
-                const target = pass orelse continue;
-                const made = makePipeline(device, format, spirv, 1, 4) catch {
-                    core.setMessage(core.active, std.fmt.bufPrint(&buf, "Shader {s}: the GPU refused it", .{path}) catch "Shader: the GPU refused it");
-                    continue;
-                };
-                if (target.pipeline) |old| c.SDL_ReleaseGPUGraphicsPipeline(device, old);
-                target.pipeline = made;
-            },
-        }
-    }
-}
-
-/// The compile thread: each file behind the prefix, through glslc with the
-/// build's own flags (build.zig compileGlsl), source on stdin, SPIR-V on
-/// stdout.
-fn compile(job: *Job) void {
-    defer job.done.store(true, .release);
-    defer {
-        var event = std.mem.zeroes(c.SDL_Event);
-        event.type = c.SDL_EVENT_USER;
-        _ = c.SDL_PushEvent(&event);
-    }
-    for (job.paths[0..job.len], 0..) |path, i| job.results[i] = compileOne(job.gpa, job.io, path);
-}
-
-fn compileOne(gpa: std.mem.Allocator, io: std.Io, path: []const u8) Job.Result {
-    var home_buf: [4096]u8 = undefined;
-    const expanded = if (std.mem.startsWith(u8, path, "~/")) if (libc.getenv("HOME")) |home|
-        std.fmt.bufPrint(&home_buf, "{s}/{s}", .{ std.mem.span(home), path[2..] }) catch path
-    else
-        path else path;
-    const body = filesystem.readFile(gpa, expanded) catch |err|
-        return .{ .failed = std.fmt.allocPrint(gpa, "cannot read it ({t})", .{err}) catch return .none };
-    defer gpa.free(body);
-    // The prefix and the file go to glslc on its stdin: no file of ours is
-    // written anywhere. std's spawn does every allocation before fork and
-    // searches PATH on the stack after it (Io/Threaded.zig spawnPosix,
-    // posixExecv), so a child forked from this thread cannot wedge on a
-    // lock another thread held (the forkShell hazard).
-    var child = std.process.spawn(io, .{
-        .argv = &.{ "glslc", "-fshader-stage=fragment", "-o", "-", "-" },
-        .stdin = .pipe,
-        .stdout = .pipe,
-        .stderr = .pipe,
-    }) catch |err| return switch (err) {
-        error.FileNotFound => .missing,
-        else => .{ .failed = std.fmt.allocPrint(gpa, "glslc did not run ({t})", .{err}) catch return .none },
-    };
-    defer child.kill(io);
-    // glslc reads all of its input before it writes a byte, so writing it
-    // first cannot deadlock against a full output pipe.
-    {
-        var stdin = child.stdin.?;
-        child.stdin = null;
-        defer stdin.close(io);
-        var buf: [4096]u8 = undefined;
-        var writer = stdin.writer(io, &buf);
-        writer.interface.writeAll(prefix) catch {};
-        writer.interface.writeAll(body) catch {};
-        writer.interface.flush() catch {};
-    }
-    var streams: std.Io.File.MultiReader.Buffer(2) = undefined;
-    var reader: std.Io.File.MultiReader = undefined;
-    reader.init(gpa, io, streams.toStreams(), &.{ child.stdout.?, child.stderr.? });
-    defer reader.deinit();
-    while (reader.fill(64, .none)) |_| {
-        if (reader.reader(0).buffered().len > 16 << 20 or reader.reader(1).buffered().len > 64 << 10)
-            return .{ .failed = gpa.dupe(u8, "glslc said too much") catch return .none };
-    } else |err| switch (err) {
-        error.EndOfStream => {},
-        else => return .{ .failed = gpa.dupe(u8, "reading glslc failed") catch return .none },
-    }
-    const term = child.wait(io) catch return .{ .failed = gpa.dupe(u8, "glslc did not finish") catch return .none };
-    const clean = switch (term) {
-        .exited => |code| code == 0,
-        else => false,
-    };
-    if (clean) return .{ .spirv = reader.toOwnedSlice(0) catch return .none };
-    const said = reader.toOwnedSlice(1) catch return .none;
-    // glslc calls its input <stdin>; the file is what a person knows.
-    defer gpa.free(said);
-    const named = std.mem.replaceOwned(u8, gpa, said, "<stdin>", path) catch return .{ .failed = gpa.dupe(u8, "glslc failed") catch return .none };
-    return .{ .failed = named };
 }
 
 fn makePipeline(device: *c.SDL_GPUDevice, format: c.SDL_GPUTextureFormat, frag: []const u8, samplers: u32, uniform_buffers: u32) !*Pipeline {
@@ -686,17 +560,6 @@ fn bloomStep(cmd: *c.SDL_GPUCommandBuffer, pipeline: *Pipeline, sampler: *c.SDL_
 pub fn deinit(post: *Post, gpa: std.mem.Allocator, device: *c.SDL_GPUDevice) void {
     for (post.bloom_tex) |texture| if (texture) |t| c.SDL_ReleaseGPUTexture(device, t);
     for ([_]?*Pipeline{ post.bloom_down, post.bloom_up }) |pipeline| if (pipeline) |p| c.SDL_ReleaseGPUGraphicsPipeline(device, p);
-    if (post.job) |job| {
-        job.thread.join();
-        for (job.paths[0..job.len], job.results[0..job.len]) |path, result| {
-            gpa.free(path);
-            switch (result) {
-                .spirv, .failed => |bytes| gpa.free(bytes),
-                .none, .missing => {},
-            }
-        }
-        gpa.destroy(job);
-    }
     for (post.passes[0..post.len]) |pass| {
         if (pass.scene == null) if (pass.pipeline) |pipeline| c.SDL_ReleaseGPUGraphicsPipeline(device, pipeline);
         gpa.free(pass.path);
@@ -801,36 +664,3 @@ test "a moved cursor becomes the previous one, stamped with the time of the move
     try std.testing.expectEqual([4]f32{ 1.0 / 255.0, 2.0 / 255.0, 3.0 / 255.0, 1 }, post.uniforms.selection_foreground_color);
 }
 
-test "ghostty's test shaders compile behind the prefix, and its invalid one fails with glslc's words" {
-    const gpa = std.testing.allocator;
-    const io = std.testing.io;
-    // ghostty's own test shaders, from its package in zig-pkg.
-    var pkg = std.Io.Dir.cwd().openDir(io, "zig-pkg", .{ .iterate = true }) catch return error.SkipZigTest;
-    defer pkg.close(io);
-    var it = pkg.iterate();
-    const ghostty = while (it.next(io) catch null) |entry| {
-        if (std.mem.startsWith(u8, entry.name, "ghostty-")) break entry.name;
-    } else return error.SkipZigTest;
-    var buf: [512]u8 = undefined;
-    for ([_][]const u8{ "crt", "focus", "invalid" }) |name| {
-        const path = try std.fmt.bufPrint(&buf, "zig-pkg/{s}/src/renderer/shaders/test_shadertoy_{s}.glsl", .{ ghostty, name });
-        const result = compileOne(gpa, io, path);
-        switch (result) {
-            .missing => return error.SkipZigTest, // no glslc here
-            .spirv => |spirv| {
-                defer gpa.free(spirv);
-                try std.testing.expect(!std.mem.eql(u8, name, "invalid"));
-                // SPIR-V's magic number, little-endian.
-                try std.testing.expectEqual(@as(u32, 0x07230203), std.mem.readInt(u32, spirv[0..4], .little));
-            },
-            .failed => |text| {
-                defer gpa.free(text);
-                try std.testing.expectEqualStrings("invalid", name);
-                // Named by the file, not by glslc's <stdin>.
-                try std.testing.expect(std.mem.indexOf(u8, text, path) != null);
-                try std.testing.expect(std.mem.indexOf(u8, text, "<stdin>") == null);
-            },
-            .none => return error.TestUnexpectedResult,
-        }
-    }
-}
diff --git a/src/gui/gui.zig b/src/gui/gui.zig
index 127f9b5d..2d31416b 100644
--- a/src/gui/gui.zig
+++ b/src/gui/gui.zig
@@ -11,6 +11,7 @@ const config = @import("../config.zig");
 const look = @import("../look.zig");
 const message = pardes.Messages.Message;
 const file_watch = @import("../file_watch.zig");
+const shader_build = @import("../shader_build.zig");
 const deck = @import("deck.zig");
 const Post = @import("Post.zig");
 const pet = @import("pet.zig");
@@ -2693,6 +2694,7 @@ fn localSession(
         .test_hz = if (std.c.getenv("PARDES_TEST_HZ")) |hz| @max(1, std.fmt.parseInt(u64, std.mem.span(hz), 10) catch 60) else 60,
     };
     defer shell.shutdownPtys();
+    defer shell.shaders.deinit(gpa, inotify_fd, &watches);
     // A virtual clock never blinks: goldens and feel series hold still.
     g.no_blink = shell.test_clock != null;
     const host = shell.host();
@@ -2862,6 +2864,7 @@ fn attachedLoop(gpa: std.mem.Allocator, g: *Gui, client: *detached_client.Client
                     in.post(.{ .paste = text });
                 },
                 .open_link => |url| look.openLink(url),
+                .post => dirty = true,
             }
         }
         try link;
@@ -2877,7 +2880,15 @@ fn attachedLoop(gpa: std.mem.Allocator, g: *Gui, client: *detached_client.Client
             geom = now;
             try attachedResize(client, g, now.cols, now.rows);
         }
-        if (!dirty) continue;
+        // The session's chain, its files compiled there.
+        g.post.sync(gpa, g.device, g.swapchain_format, client.post.list(), client.post_key, null);
+        if (!dirty) {
+            // As a local window does: a chain that moves on its own redraws
+            // alone, once a refresh.
+            if (g.post.animating(client.post.animation) and g.scene_tex != null and
+                c.SDL_GetTicksNS() -| g.post.last_ns + std.time.ns_per_ms >= refreshNs(g)) redrawPost(g, gpa);
+            continue;
+        }
         dirty = false;
         paintAttached(g, gpa, client);
     }
@@ -3993,6 +4004,8 @@ const Shell = struct {
     pipe_tasks: *PipeTasks,
     inotify_fd: c_int,
     watches: *file_watch.Table,
+    /// The post chain's files as SPIR-V.
+    shaders: shader_build = .{},
     threads_ok: bool = false,
     test_mode: bool = false,
     feed: StdinFeed = .{},
@@ -4209,11 +4222,19 @@ const Shell = struct {
             .files_changed => check_files = true,
             .fs_ready => {},
         };
+        if (check_files) s.shaders.recheck();
         if (check_files and file_watch.reloadChanged(s.core, s.io, s.gpa, s.watches))
             s.queue.push(.files_changed);
     }
 };
 
+/// A post chain compile is done (shader_build.zig): the loop's SDL wait ends.
+fn wakeSdl(_: ?*anyopaque) void {
+    var event = std.mem.zeroes(c.SDL_Event);
+    event.type = c.SDL_EVENT_USER;
+    _ = c.SDL_PushEvent(&event);
+}
+
 fn wakeFs(ctx: ?*anyopaque) void {
     const q: *Queue = @ptrCast(@alignCast(ctx.?));
     q.push(.fs_ready);
@@ -4356,7 +4377,11 @@ fn pollFrame(ctx: ?*anyopaque) void {
     const geom = windowCells(g);
     if (updateCoreResize(core, geom.cols, geom.rows, g.cell_w, g.cell_h, g.tagline_width, g.tagline_height)) resetScroll(g);
     stepScroll(g, core, s.gpa);
-    g.post.sync(s.gpa, s.io, g.device, g.swapchain_format, core);
+    _ = s.shaders.sync(s.gpa, s.io, core, s.inotify_fd, s.watches, .{ .call = wakeSdl });
+    {
+        var passes: [Post.max]shader_build.Pass = undefined;
+        g.post.sync(s.gpa, g.device, g.swapchain_format, s.shaders.view(&core.settings.post, &passes), s.shaders.revision, core);
+    }
     if (s.test_mode) writeTestStatus(g, core, s.presented);
     // Between the core's frames, a chain that moves on its own redraws
     // alone, once a refresh of the display the window is on (a millisecond
@@ -5805,8 +5830,9 @@ fn renderFrame(
     const win_w: f32 = @floatFromInt(sw);
     const win_h: f32 = @floatFromInt(sh);
 
-    // The post chain runs over a local frame, from the scene texture.
-    var scene_on = core != null and g.post.ready();
+    // The post chain runs from the scene texture, over a local frame or an
+    // attached session's alike.
+    var scene_on = g.post.ready();
     if (scene_on) ensureSceneTexture(g, sw, sh) catch {
         g.scene_target_failed = true;
         scene_on = false;
@@ -6179,19 +6205,17 @@ fn renderFrame(
     c.SDL_EndGPURenderPass(rp);
 
     if (scene_on) {
-        if (core) |p| {
-            var boxes: [max_cursors]CursorBox = undefined;
-            const cursors = if (cursors_shown) cursorBoxes(g, surface, layout, win_h, &boxes) else 0;
-            var spare: [Post.max_spare][4]f32 = undefined;
-            g.post.describe(.{
-                .cursor = if (cursors > 0) boxes[0] else null,
-                .spare = spare[0..postSpare(surface, layout, &spare)],
-                .background = chrome.page orelse bg_default,
-                .foreground = chrome.fg orelse fg_default,
-                .selection_foreground = p.theme().sel_fg,
-                .selection_background = p.theme().sel_bg,
-            });
-        }
+        var boxes: [max_cursors]CursorBox = undefined;
+        const cursors = if (cursors_shown) cursorBoxes(g, surface, layout, win_h, &boxes) else 0;
+        var spare: [Post.max_spare][4]f32 = undefined;
+        g.post.describe(.{
+            .cursor = if (cursors > 0) boxes[0] else null,
+            .spare = spare[0..postSpare(surface, layout, &spare)],
+            .background = chrome.page orelse bg_default,
+            .foreground = chrome.fg orelse fg_default,
+            .selection_foreground = if (core) |p| p.theme().sel_fg else chrome.sel_fg,
+            .selection_background = if (core) |p| p.theme().sel_bg else chrome.sel_bg,
+        });
         const chain_zone = tracy.zone(@src(), "post chain");
         defer chain_zone.end();
         g.post.scale = display_scale;
diff --git a/src/main.zig b/src/main.zig
index 86c302fe..c23f8183 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -410,6 +410,7 @@ test {
     _ = @import("locations_cache.zig");
     _ = @import("fs.zig");
     _ = @import("detached/wire.zig");
+    _ = @import("shader_build.zig");
     _ = @import("detached/server.zig");
     _ = @import("detached/client.zig");
     _ = @import("9p_io.zig");
diff --git a/src/shader_build.zig b/src/shader_build.zig
new file mode 100644
index 00000000..8aa7eb5e
--- /dev/null
+++ b/src/shader_build.zig
@@ -0,0 +1,402 @@
+//! The post chain's Shadertoy files as SPIR-V (docs/render-pipeline.md §6),
+//! for whichever process holds the core: a local GUI makes its pipelines
+//! from them, a detached session sends them to the GUIs attached to it
+//! (wire `post`), which never read a disk or run a program themselves.
+//!
+//! Each file compiles behind the prefix by the build's own glslc invocation,
+//! on a thread of its own and never on the frame path, when it joins the
+//! chain and again whenever a save changes its bytes: the files' directories
+//! are watched (file_watch.watchShaders) and a change there has every file
+//! read and hashed, an unchanged one compiling nothing. A compile that fails
+//! keeps the last good SPIR-V and is said once, and glslc missing is said
+//! once for good.
+const std = @import("std");
+const libc = std.c;
+const pardes = @import("pardes.zig");
+const file_watch = @import("file_watch.zig");
+
+const Build = @This();
+const Chain = pardes.config.Runtime.Post;
+const Scene = pardes.config.Runtime.Scene;
+pub const max = Chain.max;
+
+const prefix = @embedFile("post-prefix.glsl");
+
+/// A pass as a shell draws it: a bundled scene at its level, or a file with
+/// its last good SPIR-V (empty until a compile succeeds), whose `revision`
+/// moves each time that SPIR-V does. Slices borrow the build's (or, in an
+/// attached GUI, the client's) storage until its next change.
+pub const Pass = struct {
+    scene: ?Scene = null,
+    level: u8 = 0,
+    path: []const u8 = &.{},
+    spirv: []const u8 = &.{},
+    revision: u32 = 0,
+};
+
+/// Who to wake when a compile is done: the host's loop.
+pub const Wake = struct {
+    ctx: ?*anyopaque = null,
+    call: ?*const fn (?*anyopaque) void = null,
+};
+
+const File = struct {
+    path: []u8 = &.{},
+    /// The hash of the source last compiled, good or not: the same bytes
+    /// saved again compile nothing and say nothing twice.
+    source: ?u64 = null,
+    spirv: []u8 = &.{},
+    revision: u32 = 0,
+    /// Wants a compile: new to the chain, or its directory changed.
+    stale: bool = true,
+};
+
+const Result = union(enum) {
+    none,
+    unchanged,
+    missing,
+    failed: struct { text: []u8, source: u64 },
+    spirv: struct { bytes: []u8, source: u64 },
+};
+
+/// One compile of the stale files, on its own thread.
+const Job = struct {
+    io: std.Io,
+    gpa: std.mem.Allocator,
+    wake: Wake,
+    paths: [max][]u8 = @splat(&.{}),
+    known: [max]?u64 = @splat(null),
+    results: [max]Result = @splat(.none),
+    len: usize = 0,
+    done: std.atomic.Value(bool) = .init(false),
+    thread: std.Thread = undefined,
+};
+
+files: [max]File = @splat(.{}),
+len: usize = 0,
+/// The chain generation `files` follow; null before the first.
+generation: ?u32 = null,
+/// Moves whenever what `view` gives does: the chain or a file's SPIR-V.
+revision: u32 = 0,
+job: ?*Job = null,
+missing_said: bool = false,
+
+/// Brings the files to the chain the core holds, watches their directories,
+/// takes in a finished compile and starts the next. Each loop step, off the
+/// frame path. True when a compile was taken in: what it said wants a frame.
+pub fn sync(b: *Build, gpa: std.mem.Allocator, io: std.Io, core: *pardes.Pardes, fd: c_int, watches: *file_watch.Table, wake: Wake) bool {
+    var collected = false;
+    if (b.job) |job| {
+        if (!job.done.load(.acquire)) return false;
+        job.thread.join();
+        b.job = null;
+        b.collect(gpa, core, job);
+        collected = true;
+    }
+    const chain = &core.settings.post;
+    if (b.generation != chain.generation) {
+        b.generation = chain.generation;
+        b.revision +%= 1;
+        var next: [max]File = @splat(.{});
+        var len: usize = 0;
+        var paths: [max][]const u8 = undefined;
+        for (chain.list()) |entry| {
+            if (entry.scene != null) continue;
+            next[len] = for (b.files[0..b.len]) |*file| {
+                if (!std.mem.eql(u8, file.path, entry.path.get())) continue;
+                const kept = file.*;
+                file.* = .{};
+                break kept;
+            } else .{ .path = gpa.dupe(u8, entry.path.get()) catch continue };
+            paths[len] = next[len].path;
+            len += 1;
+        }
+        for (b.files[0..b.len]) |file| free(gpa, file);
+        b.files = next;
+        b.len = len;
+        var bufs: [max][4096]u8 = undefined;
+        var expanded: [max][]const u8 = undefined;
+        for (paths[0..len], 0..) |path, i| expanded[i] = expand(&bufs[i], path);
+        file_watch.watchShaders(fd, watches, expanded[0..len]);
+    }
+    var stale = false;
+    for (b.files[0..b.len]) |file| stale = stale or file.stale;
+    if (!stale) return collected;
+    const job = gpa.create(Job) catch return collected;
+    job.* = .{ .io = io, .gpa = gpa, .wake = wake };
+    for (b.files[0..b.len]) |*file| {
+        if (!file.stale) continue;
+        job.paths[job.len] = gpa.dupe(u8, file.path) catch continue;
+        job.known[job.len] = file.source;
+        job.len += 1;
+        file.stale = false;
+    }
+    job.thread = std.Thread.spawn(.{}, compile, .{job}) catch {
+        for (job.paths[0..job.len]) |path| gpa.free(path);
+        gpa.destroy(job);
+        return collected;
+    };
+    b.job = job;
+    return collected;
+}
+
+/// A watched directory changed: every file is read again at the next sync.
+pub fn recheck(b: *Build) void {
+    for (b.files[0..b.len]) |*file| file.stale = true;
+}
+
+/// The chain as passes, in its order.
+pub fn view(b: *const Build, chain: *const Chain, out: *[max]Pass) []const Pass {
+    for (chain.list(), 0..) |entry, i| {
+        out[i] = if (entry.scene) |scene| .{ .scene = scene, .level = entry.level } else for (b.files[0..b.len]) |file| {
+            if (std.mem.eql(u8, file.path, entry.path.get())) break .{ .path = file.path, .spirv = file.spirv, .revision = file.revision };
+        } else .{ .path = entry.path.get() };
+    }
+    return out[0..chain.len];
+}
+
+pub fn deinit(b: *Build, gpa: std.mem.Allocator, fd: c_int, watches: *file_watch.Table) void {
+    if (b.job) |job| {
+        job.thread.join();
+        freeJob(gpa, job);
+    }
+    for (b.files[0..b.len]) |file| free(gpa, file);
+    b.len = 0;
+    file_watch.watchShaders(fd, watches, &.{});
+}
+
+fn free(gpa: std.mem.Allocator, file: File) void {
+    gpa.free(file.path);
+    gpa.free(file.spirv);
+}
+
+fn freeJob(gpa: std.mem.Allocator, job: *Job) void {
+    for (job.paths[0..job.len], job.results[0..job.len]) |path, result| {
+        gpa.free(path);
+        switch (result) {
+            .spirv => |r| gpa.free(r.bytes),
+            .failed => |r| gpa.free(r.text),
+            .none, .unchanged, .missing => {},
+        }
+    }
+    gpa.destroy(job);
+}
+
+fn collect(b: *Build, gpa: std.mem.Allocator, core: *pardes.Pardes, job: *Job) void {
+    defer gpa.destroy(job);
+    for (job.paths[0..job.len], job.results[0..job.len]) |path, result| {
+        defer gpa.free(path);
+        const file = for (b.files[0..b.len]) |*file| {
+            if (std.mem.eql(u8, file.path, path)) break file;
+        } else null;
+        var buf: [512]u8 = undefined;
+        switch (result) {
+            .none, .unchanged => {},
+            .missing => if (!b.missing_said) {
+                b.missing_said = true;
+                core.setMessage(core.active, "Shader: glslc is not on PATH, so Shadertoy files do not run (Crt does)");
+            },
+            .failed => |r| {
+                defer gpa.free(r.text);
+                if (file) |f| f.source = r.source;
+                // glslc's first line says where: in the file's own lines.
+                const first = std.mem.sliceTo(r.text, '\n');
+                core.setMessage(core.active, std.fmt.bufPrint(&buf, "Shader {s}: {s}", .{ path, first[0..@min(first.len, 400)] }) catch "Shader: compile failed");
+            },
+            .spirv => |r| {
+                const f = file orelse {
+                    gpa.free(r.bytes);
+                    continue;
+                };
+                gpa.free(f.spirv);
+                f.spirv = r.bytes;
+                f.source = r.source;
+                f.revision +%= 1;
+                b.revision +%= 1;
+            },
+        }
+    }
+}
+
+/// `~/` is the home directory.
+fn expand(buf: []u8, path: []const u8) []const u8 {
+    if (!std.mem.startsWith(u8, path, "~/")) return path;
+    const home = libc.getenv("HOME") orelse return path;
+    return std.fmt.bufPrint(buf, "{s}/{s}", .{ std.mem.span(home), path[2..] }) catch path;
+}
+
+/// The compile thread: each file behind the prefix, through glslc with the
+/// build's own flags (build.zig compileGlsl), source on stdin, SPIR-V on
+/// stdout.
+fn compile(job: *Job) void {
+    for (job.paths[0..job.len], 0..) |path, i| job.results[i] = compileOne(job.gpa, job.io, path, job.known[i]);
+    job.done.store(true, .release);
+    if (job.wake.call) |call| call(job.wake.ctx);
+}
+
+fn compileOne(gpa: std.mem.Allocator, io: std.Io, path: []const u8, known: ?u64) Result {
+    var home_buf: [4096]u8 = undefined;
+    // Not fs.zig's readFile: that hands the core's turn over for the read,
+    // a turn this thread never holds.
+    const body = std.Io.Dir.cwd().readFileAlloc(io, expand(&home_buf, path), gpa, .limited(1 << 20)) catch |err|
+        return .{ .failed = .{ .text = std.fmt.allocPrint(gpa, "cannot read it ({t})", .{err}) catch return .none, .source = 0 } };
+    defer gpa.free(body);
+    const source = std.hash.Wyhash.hash(0, body);
+    if (known == source) return .unchanged;
+    // The prefix and the file go to glslc on its stdin: no file of ours is
+    // written anywhere. std's spawn does every allocation before fork and
+    // searches PATH on the stack after it (Io/Threaded.zig spawnPosix,
+    // posixExecv), so a child forked from this thread cannot wedge on a
+    // lock another thread held (the forkShell hazard).
+    var child = std.process.spawn(io, .{
+        .argv = &.{ "glslc", "-fshader-stage=fragment", "-o", "-", "-" },
+        .stdin = .pipe,
+        .stdout = .pipe,
+        .stderr = .pipe,
+    }) catch |err| return switch (err) {
+        error.FileNotFound => .missing,
+        else => .{ .failed = .{ .text = std.fmt.allocPrint(gpa, "glslc did not run ({t})", .{err}) catch return .none, .source = source } },
+    };
+    defer child.kill(io);
+    // glslc reads all of its input before it writes a byte, so writing it
+    // first cannot deadlock against a full output pipe.
+    {
+        var stdin = child.stdin.?;
+        child.stdin = null;
+        defer stdin.close(io);
+        var buf: [4096]u8 = undefined;
+        var writer = stdin.writer(io, &buf);
+        writer.interface.writeAll(prefix) catch {};
+        writer.interface.writeAll(body) catch {};
+        writer.interface.flush() catch {};
+    }
+    var streams: std.Io.File.MultiReader.Buffer(2) = undefined;
+    var reader: std.Io.File.MultiReader = undefined;
+    reader.init(gpa, io, streams.toStreams(), &.{ child.stdout.?, child.stderr.? });
+    defer reader.deinit();
+    const failed = struct {
+        fn of(a: std.mem.Allocator, text: []const u8, hash: u64) Result {
+            return .{ .failed = .{ .text = a.dupe(u8, text) catch return .none, .source = hash } };
+        }
+    }.of;
+    while (reader.fill(64, .none)) |_| {
+        if (reader.reader(0).buffered().len > 16 << 20 or reader.reader(1).buffered().len > 64 << 10)
+            return failed(gpa, "glslc said too much", source);
+    } else |err| switch (err) {
+        error.EndOfStream => {},
+        else => return failed(gpa, "reading glslc failed", source),
+    }
+    const term = child.wait(io) catch return failed(gpa, "glslc did not finish", source);
+    const clean = switch (term) {
+        .exited => |code| code == 0,
+        else => false,
+    };
+    if (clean) return .{ .spirv = .{ .bytes = reader.toOwnedSlice(0) catch return .none, .source = source } };
+    const said = reader.toOwnedSlice(1) catch return .none;
+    // glslc calls its input <stdin>; the file is what a person knows.
+    defer gpa.free(said);
+    const named = std.mem.replaceOwned(u8, gpa, said, "<stdin>", path) catch return failed(gpa, "glslc failed", source);
+    return .{ .failed = .{ .text = named, .source = source } };
+}
+
+test "ghostty's test shaders compile behind the prefix, and its invalid one fails with glslc's words" {
+    const gpa = std.testing.allocator;
+    const io = std.testing.io;
+    // ghostty's own test shaders, from its package in zig-pkg.
+    var pkg = std.Io.Dir.cwd().openDir(io, "zig-pkg", .{ .iterate = true }) catch return error.SkipZigTest;
+    defer pkg.close(io);
+    var it = pkg.iterate();
+    const ghostty = while (it.next(io) catch null) |entry| {
+        if (std.mem.startsWith(u8, entry.name, "ghostty-")) break entry.name;
+    } else return error.SkipZigTest;
+    var buf: [512]u8 = undefined;
+    for ([_][]const u8{ "crt", "focus", "invalid" }) |name| {
+        const path = try std.fmt.bufPrint(&buf, "zig-pkg/{s}/src/renderer/shaders/test_shadertoy_{s}.glsl", .{ ghostty, name });
+        switch (compileOne(gpa, io, path, null)) {
+            .missing => return error.SkipZigTest, // no glslc here
+            .spirv => |r| {
+                defer gpa.free(r.bytes);
+                try std.testing.expect(!std.mem.eql(u8, name, "invalid"));
+                // SPIR-V's magic number, little-endian.
+                try std.testing.expectEqual(@as(u32, 0x07230203), std.mem.readInt(u32, r.bytes[0..4], .little));
+                // The same bytes again compile nothing.
+                try std.testing.expectEqual(Result.unchanged, compileOne(gpa, io, path, r.source));
+            },
+            .failed => |r| {
+                defer gpa.free(r.text);
+                try std.testing.expectEqualStrings("invalid", name);
+                // Named by the file, not by glslc's <stdin>.
+                try std.testing.expect(std.mem.indexOf(u8, r.text, path) != null);
+                try std.testing.expect(std.mem.indexOf(u8, r.text, "<stdin>") == null);
+            },
+            .none, .unchanged => return error.TestUnexpectedResult,
+        }
+    }
+}
+
+test "a save that changes a file compiles it again, a failed one keeps the last good SPIR-V and is said once" {
+    const gpa = std.testing.allocator;
+    const io = std.testing.io;
+    const core = try pardes.Pardes.init(gpa, .{ .cols = 80, .rows = 24 });
+    defer core.deinit();
+    _ = try core.newShell(1, "");
+    const pane = core.panes[core.active].?;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var dir_buf: [4096]u8 = undefined;
+    const dir = dir_buf[0..try tmp.dir.realPath(io, &dir_buf)];
+    var path_buf: [4200]u8 = undefined;
+    const path = try std.fmt.bufPrint(&path_buf, "{s}/wave.glsl", .{dir});
+    const good = "void mainImage(out vec4 o, in vec2 p) { o = texture(iChannel0, p / iResolution.xy); }\n";
+    try tmp.dir.writeFile(io, .{ .sub_path = "wave.glsl", .data = good });
+    try std.testing.expect(core.settings.post.set(null, path, true));
+
+    const fd = file_watch.init(true);
+    defer if (fd >= 0) {
+        _ = libc.close(fd);
+    };
+    var watches: file_watch.Table = @splat(null);
+    var b: Build = .{};
+    defer b.deinit(gpa, fd, &watches);
+    const settle = struct {
+        fn run(build: *Build, c: *pardes.Pardes, f: c_int, w: *file_watch.Table) void {
+            _ = build.sync(std.testing.allocator, std.testing.io, c, f, w, .{});
+            while (build.job) |job| {
+                while (!job.done.load(.acquire)) std.Thread.yield() catch {};
+                _ = build.sync(std.testing.allocator, std.testing.io, c, f, w, .{});
+            }
+        }
+    }.run;
+    settle(&b, core, fd, &watches);
+    if (b.missing_said) return error.SkipZigTest; // no glslc here
+    var out: [max]Pass = undefined;
+    const first = b.view(&core.settings.post, &out)[0];
+    try std.testing.expect(first.spirv.len != 0);
+    if (file_watch.supported) try std.testing.expect(watches[file_watch.shader_slot] != null);
+
+    // Unchanged bytes: nothing moves.
+    const at = b.revision;
+    b.recheck();
+    settle(&b, core, fd, &watches);
+    try std.testing.expectEqual(at, b.revision);
+
+    // A bad save: the last good SPIR-V stays, the error is said, and the
+    // same bad bytes again say nothing more.
+    try tmp.dir.writeFile(io, .{ .sub_path = "wave.glsl", .data = "void mainImage(" });
+    pane.msg_len = 0;
+    b.recheck();
+    settle(&b, core, fd, &watches);
+    try std.testing.expectEqual(at, b.revision);
+    try std.testing.expect(std.mem.startsWith(u8, pane.msg[0..pane.msg_len], "Shader "));
+    pane.msg_len = 0;
+    b.recheck();
+    settle(&b, core, fd, &watches);
+    try std.testing.expectEqual(@as(u16, 0), pane.msg_len);
+
+    // A good save again: new SPIR-V, a new revision.
+    try tmp.dir.writeFile(io, .{ .sub_path = "wave.glsl", .data = "// saved again\n" ++ good });
+    b.recheck();
+    settle(&b, core, fd, &watches);
+    const second = b.view(&core.settings.post, &out)[0];
+    try std.testing.expect(second.revision != first.revision and b.revision != at);
+}
diff --git a/src/tty/tty.zig b/src/tty/tty.zig
index c4096faf..c5491a73 100644
--- a/src/tty/tty.zig
+++ b/src/tty/tty.zig
@@ -2157,6 +2157,8 @@ const Attach = struct {
             .set_clipboard => |text| copyToClipboard(a.vx, a.tty, a.gpa, text),
             .read_clipboard => requestClipboard(a.vx, a.tty),
             .open_link => |url| look.openLink(url),
+            // A terminal has no post chain to draw through.
+            .post => {},
         }
         return null;
     }