dark glass gets a surface ladder, and the 200-750ms stalls are dead - two sessions of uncommitted work landing together

SURFACES: panes add black, surfaces INSIDE panes add white. Every pane stands
on 0.7075 black over the acrylic (.content 0.35 over .app 0.55), so a
translucent black card nets 0.81 - darker than its own pane and inside the
~0.85 band already rejected once on the artist page. Solid mode's ladder is
measured UPWARD from #121212 (+6/+18/+24), so glass elevates with WHITE: new
--glass-surface/-hover/-press/-hairline/-well/-ring under :root.glass
(0.03/0.08/0.11/0.10/0.18), which is independently the vocabulary
.btn-ghost:hover and .ax-ghost:hover already shipped. The four solid tokens are
NEVER redefined - they are consumed inside color-mix() and gradient stops where
an alpha multiplies through and kills the call site (0.74 x 0.03 = an invisible
chip; the album row's text-overflow fade works by BEING opaque), and they paint
modal chrome that has to stay solid. 129 additive :global(:root.glass)
declarations across 18 files, all 129 verified inside a glass block by walking
the diff - solid mode byte-identical, confirmed structurally, by computed style
with the class off, and visually.

Three of them are not colour swaps: the album row's .rowbtns shim was OCCLUDING
the last ~19px of the listeners bar and nothing translucent can occlude, so
.tcount masks out just before the buttons instead; Playlist's banner scrim and
the discography's pinned ambience both landed on opaque --bg-base and now
dissolve like the album banner; and the 0.9 black hero tint on
Home/CatalogSearch/Discography paints straight onto the pane when there is no
artwork, so :first-child (the {#if} renders no element) flips it to white.
Menus, dialogs, toasts and the import surfaces stay opaque deliberately, and
the Settings copy for the mode now says so instead of claiming cards do.
Measured with a known ground substituted behind the transparent window, since
CDP never sees the acrylic: library toolbar chroma +2 as shipped, +16 with a
white fill AND its backdrop-filter dropped, +30 for the bare pane beside it.

RESIZE: Windows drops the legacy accent-policy acrylic for the whole of a live
resize and the window stays transparent through it, so the desktop punches
through sharp - measured by toggling window_glass alone, desktop icon labels
readable through the titlebar. The blur cannot be kept (documented since Win10
2004, and the modern DWM backdrop was already rejected for the
overlay-promotion blink), so armGlassResizeGuard grounds the canvas to
--bg-sidebar for the length of the gesture.

STALLS: "choppy but the FPS says 144" was 200-750ms MAIN-THREAD TASKS, not
frame pacing - idle-scheduled work fires at scroll pauses by definition. Twin
bake moved to display resolution and off the ready task (was a same-task 4K
WebGL readback plus software blur, 100-170ms per rotation AND per artist
entry); gl.finish() -> gl.flush() in buildMaster, which had been draining the
whole 4K EASU/RCAS/grade chain on the main thread; copyCanvas realloc guarded;
one app-wide scroll clock in scrollbar.js (markAppScroll/appScrollIdleFor) so
all three periscopes stop cloning mid-gesture on ANY pane rather than only
their own; bounded reclone with the querySelectorAll census deleted, the gate
widened 250->1000ms, and the build/attach split across an idle callback and a
rAF behind a generation guard; the hero rotation tick gated on scroll recency
and its payload cache sized to the working set (2->4 entries, 24->32MiB,
32->64MiB); sessionTick no longer force-rewrites every 12s while merely PAUSED.
Scroll storm went from 109-616ms long tasks to zero. Hover recalc and GC were
measured innocent - do not re-chase them.

98/98 tests throughout.

src/lib/heroGpu.js

@@ -351,8 +351,12 @@ export function createHeroGpuRenderer(canvas, bloomCanvas, previousCanvas, previ
 
     function copyCanvas(from, to) {
         if (!from.width || !from.height) return;
-        to.width = from.width;
-        to.height = from.height;
+        // Guarded like copyBloom below: assigning width/height reallocates
+        // the backing store even when the value is unchanged — two ~10-20MB
+        // reallocs per rotation for nothing. The full-surface drawImage on
+        // an alpha:false context overwrites every pixel, so reuse is clean.
+        if (to.width !== from.width) to.width = from.width;
+        if (to.height !== from.height) to.height = from.height;
         const context = to.getContext("2d", { alpha: false });
         context.imageSmoothingEnabled = true;
         context.imageSmoothingQuality = "high";
@@ -451,7 +455,19 @@ export function createHeroGpuRenderer(canvas, bloomCanvas, previousCanvas, previ
             });
             gl.bindTexture(gl.TEXTURE_2D, finished.tx);
             gl.generateMipmap(gl.TEXTURE_2D);
-            gl.finish();
+            // flush, NEVER finish — the same reasoning resize() records
+            // below: gl.finish() blocks the main thread until the GPU has
+            // drained, and it stalls hardest exactly when the GPU is busy.
+            // Here that meant the whole 4K EASU/RCAS/grade chain drained
+            // inside the rotation task — a main-thread block in the measured
+            // 100ms+ class on a 4-core iGPU, every ~20s hero rotation, worst
+            // in glass mode when the compositor shares the GPU. GL command
+            // ordering makes the temporaries' deletion in `finally` safe
+            // without draining (deletion of referenced resources is
+            // deferred), and nothing reads pixels back before the later
+            // drawImage copies impose their own sync. renderMs now reports
+            // enqueue time, not drain time — fine for the debug attribute.
+            gl.flush();
             disposeTarget(finished, true);
             return { tx: finished.tx, width: HERO_MASTER_WIDTH, height: HERO_MASTER_HEIGHT };
         } catch (error) {