GUI goldens settle on the window's idle, not two captures that agree

test/gui_golden.py hashed a scene once two captures 400 ms apart agreed;
on a loaded machine a slow frame agreed with itself before the scene's
change landed and every later hash moved up a scene. A test window now
writes a status file into its capture directory each loop step (gui.zig
writeTestStatus: step, frames captured, idle — nothing owes a frame,
nothing moves, nothing waits on the core's clock), and the harness waits
for an idle step two past the one current when it asks: a 9P write has
been served by then, so the frame is the scene's. Two things kept a
window from ever going idle, both fixed:

- Look-hover re-armed forever: resting on a word with nothing to open, the
  hover wait ran its two frames, found nothing, cancelled, and the next
  present's pointer refresh started it again: a frame every other frame
  for as long as the pointer rested there, live too (CPU). The place last
  refused is remembered (Pardes.look_hover_refused) until the pointer
  moves off it.
- The virtual clock moved only on a wait that timed out, so a steady
  trickle of events held it still; it now moves to the core's wake too
  when an event cut the wait short and nothing owes a frame.

Captures go to a directory of their own, outside the session's (a file
written there every frame is a change in the directory its panes look
at). All 17 goldens match the file as recorded at oooopqyv; eight runs,
five alongside a ReleaseFast build and three with all 16 cores saturated
(load 13-19), were byte-identical, 14 s a run unloaded (was ~30).

Tests: the resting pointer lets the core go idle.

1 files changed, 34 insertions, 1 deletions

diff --git a/src/gui/gui.zig b/src/gui/gui.zig
index 6bde8b60..8d9c29ab 100644
--- a/src/gui/gui.zig
+++ b/src/gui/gui.zig
@@ -4273,7 +4273,13 @@ fn waitInput(ctx: ?*anyopaque, timeout_ms: u32) void {
             if (core.continuous()) {
                 const display = virtual.* + std.time.ns_per_s / s.test_hz;
                 virtual.* = if (core.nextWake()) |due| @max(virtual.*, @min(due, display)) else display;
-            } else if (!got and timeout_ms != 0) virtual.* = @max(virtual.*, core.nextWake() orelse virtual.*);
+            } else if (timeout_ms != 0 and (!got or !core.needs_frame)) {
+                // A wait that ran out, or one a stray event cut short while
+                // nothing owes a frame: time moves to the core's wake all
+                // the same, so a steady trickle of events (a pty's, the
+                // test feed's) cannot hold a virtual clock still forever.
+                virtual.* = @max(virtual.*, core.nextWake() orelse virtual.*);
+            }
         }
         if (got) {
             // A run of queued motions is one move to its last position (the
@@ -4348,6 +4354,7 @@ fn pollFrame(ctx: ?*anyopaque) void {
     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);
+    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
     // early is on time: the wait that paces it is in whole milliseconds).
@@ -8235,6 +8242,32 @@ fn captureFrame(g: *Gui, gpa: std.mem.Allocator, cmd: *c.SDL_GPUCommandBuffer, t
     g.captured +%= 1;
 }
 
+/// A test window's state for a harness (test/gui_golden.py), in the
+/// capture directory's `status`: "<loop step> <frames captured> <idle>".
+/// Idle is 1 once the frame last captured is the one that stays: nothing
+/// owes a frame, nothing moves, nothing waits on a clock. A harness that
+/// sees a step past the one it acted on, idle, has its frame, however slow
+/// the machine is.
+var test_status_step: u64 = 0;
+fn writeTestStatus(g: *const Gui, core: *const pardes.Pardes, presented: bool) void {
+    test_status_step +%= 1;
+    if (g.capture_dir.len == 0) return;
+    const idle = presented and !core.needs_frame and !core.continuous() and core.nextWake() == null and
+        !g.glide.moving() and g.scroll_lag == 0 and g.scroll_delta == 0 and g.scroll_notch == 0;
+    var line_buf: [64]u8 = undefined;
+    // What keeps it busy, after: owes a frame, moves, waits, not shown.
+    const line = std.fmt.bufPrint(&line_buf, "{d} {d} {d} {d}{d}{d}{d}\n", .{ test_status_step, g.captured, @intFromBool(idle), @intFromBool(core.needs_frame), @intFromBool(core.continuous()), @intFromBool(core.nextWake() != null), @intFromBool(!presented) }) catch return;
+    var tmp_buf: [4096]u8 = undefined;
+    var final_buf: [4096]u8 = undefined;
+    const tmp_path = std.fmt.bufPrintSentinel(&tmp_buf, "{s}/status.tmp", .{g.capture_dir}, 0) catch return;
+    const final_path = std.fmt.bufPrintSentinel(&final_buf, "{s}/status", .{g.capture_dir}, 0) catch return;
+    const fd = libc.open(tmp_path, .{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true }, @as(libc.mode_t, 0o644));
+    if (fd < 0) return;
+    _ = libc.write(fd, line.ptr, line.len);
+    _ = libc.close(fd);
+    _ = libc.rename(tmp_path, final_path);
+}
+
 fn writeCapturePpm(g: *Gui, gpa: std.mem.Allocator, pixels: []const u8, width: u32, height: u32, name: []const u8) !void {
     // A latency trace times frames, and an 8 MB file write per frame would
     // stall the loop it is timing.