perf(fs-player): cut CPU under WebKitGTK software compositing

WebKitGTK on Linux runs with WEBKIT_DISABLE_COMPOSITING_MODE=1, so every
GPU-only effect is software-rasterised per frame. The fullscreen player
piled up large blur shadows, per-line scale transforms, oversized
radial-gradients with color-mix() and a 60 fps rAF spring on top — CPU
saturated and FPS dropped to ~10.

Linux-only via existing html.no-compositing class:
- Apple-style lyrics: drop transform: scale and 48 px / 32 px text-shadow
- Rail-style lyrics: drop 28 px text-shadow + scaleX
- Track title: 40 px glow → 6 px drop-shadow
- Album art wrap + play button: drop accent-coloured outer glows
- Mesh blobs: flatten 130 % radial-gradient + color-mix to solid #06060e
  and remove the 400 ms background transition
- Portrait wrap: drop translateZ(0) GPU-layer hint
- FS art: instant cover swap (no opacity crossfade)
- SpringScroller: replace rAF spring with native scrollTo({behavior:'smooth'})
- FsSeekbar: cache last applied values, skip identical DOM writes

Dynamic accent on title / seekbar / play button is preserved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/FullscreenPlayer.tsx

@@ -434,20 +434,40 @@ const FsSeekbar = memo(function FsSeekbar({ duration }: { duration: number }) {
   const inputRef    = useRef<HTMLInputElement>(null);
 
   useEffect(() => {
-    const s = usePlayerStore.getState();
-    const pct = s.progress * 100;
-    if (timeRef.current)   timeRef.current.textContent  = formatTime(s.currentTime);
-    if (playedRef.current) playedRef.current.style.width = `${pct}%`;
-    if (bufRef.current)    bufRef.current.style.width    = `${Math.max(pct, s.buffered * 100)}%`;
-    if (inputRef.current)  inputRef.current.value        = String(s.progress);
+    // Cache the last applied values so we can skip DOM writes when nothing
+    // changed. The store subscription fires on every state change (queue,
+    // volume, currentTrack…), not just progress, so without this guard we
+    // were writing identical width/textContent strings dozens of times per
+    // second — each one triggers a style invalidation in WebKitGTK.
+    let lastTime  = -1;
+    let lastPct   = -1;
+    let lastBufW  = -1;
+    let lastProg  = -1;
 
-    return usePlayerStore.subscribe(state => {
-      const p = state.progress * 100;
-      if (timeRef.current)   timeRef.current.textContent  = formatTime(state.currentTime);
-      if (playedRef.current) playedRef.current.style.width = `${p}%`;
-      if (bufRef.current)    bufRef.current.style.width    = `${Math.max(p, state.buffered * 100)}%`;
-      if (inputRef.current)  inputRef.current.value        = String(state.progress);
-    });
+    const apply = (s: ReturnType<typeof usePlayerStore.getState>) => {
+      const pct  = s.progress * 100;
+      const bufW = Math.max(pct, s.buffered * 100);
+      const sec  = s.currentTime | 0;
+      if (sec !== lastTime) {
+        lastTime = sec;
+        if (timeRef.current) timeRef.current.textContent = formatTime(s.currentTime);
+      }
+      if (pct !== lastPct) {
+        lastPct = pct;
+        if (playedRef.current) playedRef.current.style.width = `${pct}%`;
+      }
+      if (bufW !== lastBufW) {
+        lastBufW = bufW;
+        if (bufRef.current) bufRef.current.style.width = `${bufW}%`;
+      }
+      if (s.progress !== lastProg) {
+        lastProg = s.progress;
+        if (inputRef.current) inputRef.current.value = String(s.progress);
+      }
+    };
+
+    apply(usePlayerStore.getState());
+    return usePlayerStore.subscribe(apply);
   }, []);
 
   const handleSeek = useCallback(

src/styles/components.css

@@ -3856,6 +3856,22 @@
 html.no-compositing .fsr-lyrics-overlay { -webkit-mask-image: none; mask-image: none; }
 html.no-compositing .fsr-lyrics-rail    { will-change: auto; }
 
+/* Apple-style FS lyrics — strip GPU-only effects.
+   Without compositing, transform: scale() on every line forces a per-line
+   software repaint on each active-line change, and the 48 px / 32 px blur
+   text-shadows repaint on every word tick. Replace with opacity + colour,
+   which are cheap text-only repaints. */
+html.no-compositing .fsa-lyric-line {
+  transform: none;
+  transition: color 380ms cubic-bezier(0.25, 0.46, 0.45, 0.94),
+              opacity 380ms cubic-bezier(0.25, 0.46, 0.45, 0.94);
+}
+html.no-compositing .fsa-lyric-line.fsal-past   { transform: none; }
+html.no-compositing .fsa-lyric-line.fsal-active { transform: none; text-shadow: none; }
+html.no-compositing .fsa-lyric-line.fsal-active .fsa-lyric-word.active {
+  text-shadow: none;
+}
+
 /* ── no-compositing overrides ─────────────────────────────────────────────────
    Applied only when WEBKIT_DISABLE_COMPOSITING_MODE=1 is set (Arch Linux etc).
    Replaces GPU-only effects (backdrop-filter, CSS filter, mask-image) with
@@ -3880,10 +3896,19 @@ html.no-compositing .fs-portrait {
 }
 
 /* Portrait wrap: replace mask-image fade with a pseudo-element overlay gradient.
-   pointer-events: none on wrap means the overlay never blocks clicks. */
+   pointer-events: none on wrap means the overlay never blocks clicks.
+   Also drop translateZ(0) — useless GPU layer hint without a compositor. */
 html.no-compositing .fs-portrait-wrap {
   -webkit-mask-image: none;
   mask-image: none;
+  transform: none;
+}
+
+/* FS art crossfade: opacity transition compositing is GPU-cheap, software-expensive.
+   Snap instantly between covers; visually a hard cut, but the previous layer is
+   replaced before the user can really notice. */
+html.no-compositing .fs-art {
+  transition: none;
 }
 html.no-compositing .fs-portrait-wrap::before {
   content: '';
@@ -3901,12 +3926,16 @@ html.no-compositing .fs-portrait-wrap::before {
 
 /* Lyrics fade overlays: gradient-only, no backdrop-filter — fine in no-compositing */
 
-/* Mesh blobs: stop animations — in software mode each frame composites surfaces
-   larger than the screen (blob-a is 130 × 130 % of the viewport); on high-DPI
-   displays this saturates the CPU and drops the player to ~10 FPS.
-   Static gradients are preserved; only the slow pan animation is removed. */
+/* Mesh blobs: stop animations + flatten to a solid dark fill.
+   Even static, the 130 × 130 % radial-gradient with color-mix() is recomputed
+   on every paint, and the `transition: background 400ms` on accent change
+   re-rasterises the giant surface for nearly half a second per track switch.
+   Atmosphere is sacrificed; the dynamic accent is preserved on title /
+   seekbar / controls via their own var(--dynamic-fs-accent) bindings. */
 html.no-compositing .fs-mesh-blob {
   animation: none;
+  background: #06060e;
+  transition: none;
 }
 
 /* Seekbar played bar: remove box-shadow — its width changes on every playback
@@ -3915,6 +3944,45 @@ html.no-compositing .fs-seekbar-played {
   box-shadow: none;
 }
 
+/* Track title: drop the 40 px text-shadow glow — paints a giant blur on a
+   clamp(28–68 px) font and re-rasterises whenever anything around it
+   redraws (cluster updates, seekbar tick, dynamic accent change). */
+html.no-compositing .fs-track-title {
+  text-shadow: 0 2px 6px rgba(0, 0, 0, 0.65);
+  transition: color 200ms ease-in-out;
+}
+
+/* Album art wrap: replace the accent-coloured outer glow with a flat
+   shadow. The 28 px blur was repainting every accent change. */
+html.no-compositing .fs-art-wrap {
+  box-shadow: 0 6px 18px rgba(0, 0, 0, 0.7), 0 0 0 1px rgba(255, 255, 255, 0.07);
+  transition: none;
+}
+
+/* Play button: drop accent glow + hover filter. Keep flat dark shadow only. */
+html.no-compositing .fs-btn-play {
+  box-shadow: 0 4px 14px rgba(0, 0, 0, 0.55);
+}
+html.no-compositing .fs-btn-play:hover {
+  filter: none;
+  box-shadow: 0 6px 18px rgba(0, 0, 0, 0.65);
+}
+
+/* Rail-style FS lyrics: same problems as Apple style — 28 px text-shadow
+   blur on every active line + every active word, plus per-line transform
+   scaleX. Strip them; rail still scrolls fine via the parent translateY. */
+html.no-compositing .fsr-lyric-line {
+  transition: color 280ms ease;
+  transform: none;
+}
+html.no-compositing .fsr-lyric-line.fsrl-active {
+  text-shadow: none;
+  transform: none;
+}
+html.no-compositing .fsr-lyric-line.fsrl-active .fsr-lyric-word.active {
+  text-shadow: none;
+}
+
 /* Chat */
 .chat-popup {
   position: absolute;

src/utils/springScroll.ts

@@ -40,6 +40,15 @@ export class SpringScroller {
 
   scrollTo(targetY: number) {
     this.target = Math.max(0, targetY);
+    // Software-rendered Linux: replace our 60 fps rAF spring with the
+    // browser's native smooth scroll. The browser handles easing internally
+    // (one animation, not 60 JS callbacks per second) and we still get a
+    // smooth visual instead of a hard snap.
+    if (document.documentElement.classList.contains('no-compositing')) {
+      this.stop();
+      this.container.scrollTo({ top: this.target, behavior: 'smooth' });
+      return;
+    }
     if (this.rafId === null) this.tick();
   }