sound: drop the OSD's canvas grain, it broke Qt's garbage collector

The desktop shell's recurring 100% CPU spin is a Qt bug set off by the
volume OSD. Its grain Canvas called createImageData on every paint, and
Qt 6.11 subtracts that pixel buffer from the V4 unmanaged-heap counter
without having added it. The first GC after an OSD show wrapped the
size_t counter, and from then on every JS allocation ran a full GC until
a restart. The OSD shows on volume keys, track changes and resume, which
is why the spin followed all three.

Found by running each always-on unit of the desktop in its own qs with
GC stats: only the sound module wrapped, 3 s after a resume, and a
volume nudge reproduced it on demand. Qt's qml tool with a bare Canvas
confirms it is createImageData alone: 17,856 of 17,856 GCs wrapped, the
same canvas drawing with fillRect 0 of 5. With the grain removed, 8 OSD
shows under forced GCs left the counter intact.

The grain was alpha 14/255, so the OSD looks nearly the same. vram.sh's
comment blamed the fdinfo parse for the underflow; corrected, and the
lesson is in AGENTS.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/AGENTS.md b/AGENTS.md
index e089254..ca862dd 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -402,6 +402,20 @@ changing that component. The ones that generalise:
   is not a function`, which shows only in `qs log`; a Repeater bound to it
   simply builds nothing. Use `reduce((a, x) => a.concat(...), [])`. Seen
   with the assistant page's cloud Prefer rows (2026-10-23).
+- **Canvas `createImageData` breaks Qt's garbage collector for good.** Qt
+  6.11 subtracts the pixel buffer from the V4 engine's unmanaged-heap counter
+  without ever having added it, so the first GC after one is created wraps
+  the `size_t` counter below zero, and from then on every JS allocation runs
+  a full GC: a core pinned, keybinds and the drawer lagging, until a
+  restart. Reproduced in Qt's own `qml` tool with nothing but a `Canvas`
+  (17,856 of 17,856 GCs wrapped; the same canvas drawing with `fillRect`, 0
+  of 5). The volume OSD's grain did it on every show, which is why the
+  desktop shell spun after volume keys, track changes and resumes alike.
+  `QV4_GC_TIMELIMIT=0`, set earlier for a misdiagnosed "stuck incremental
+  GC", does not help. Never use `createImageData`, `getImageData` or
+  `putImageData`. Found by running each always-on unit in its own `qs` with
+  `--log-rules qt.qml.gc.allocatorStats.debug=true` and reading the
+  unmanaged counter after each GC.
 - **A `Socket` whose connect failed never retries.** Quickshell 0.3.1 keeps
   the failed `QLocalSocket`, so setting `connected = true` again (or toggling
   it, or changing `path`) is a no-op: `setConnected` only connects when no