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, 4 insertions, 29 deletions

diff --git a/desktop/modules/sound/Osd.qml b/desktop/modules/sound/Osd.qml
index f60f5ce..e66d559 100644
--- a/desktop/modules/sound/Osd.qml
+++ b/desktop/modules/sound/Osd.qml
@@ -186,9 +186,10 @@ Scope {
                     color: Qt.alpha(Theme.base, 0.82)
                 }
 
-                // Frost on the scrim, faked: a faint top-down sheen and fixed
-                // grain. The art is opaque, so the compositor's frosting
-                // cannot show here without costing the scrim its legibility.
+                // Frost on the scrim, faked: a faint top-down sheen. The art is
+                // opaque, so the compositor's frosting cannot show here
+                // without costing the scrim its legibility. No grain: see
+                // createImageData in AGENTS.md.
                 Rectangle {
                     anchors.fill: parent
                     radius: bgMask.radius
@@ -197,32 +198,6 @@ Scope {
                         GradientStop { position: 0.6; color: "transparent" }
                     }
                 }
-
-                // ponytail: grain painted once with Canvas; a ShaderEffect
-                // would animate or scale better but needs a compiled .qsb.
-                Canvas {
-                    anchors.fill: parent
-                    layer.enabled: true
-                    layer.effect: MultiEffect {
-                        maskEnabled: true
-                        maskSource: bgMask
-                        maskThresholdMin: 0.5
-                        maskSpreadAtMin: 1.0
-                    }
-                    onPaint: {
-                        if (width <= 0 || height <= 0) return;
-                        const ctx = getContext("2d");
-                        const img = ctx.createImageData(width, height);
-                        for (let i = 0; i < img.data.length; i += 4) {
-                            const v = Math.random() * 255;
-                            img.data[i] = v;
-                            img.data[i + 1] = v;
-                            img.data[i + 2] = v;
-                            img.data[i + 3] = 14;
-                        }
-                        ctx.putImageData(img, 0, 0);
-                    }
-                }
             }
 
             HoverHandler {