perf(back-to-top): keep button mounted, swap mount/unmount for opacity

The previous `{#if showGlobalBackToTop}` block mounted/unmounted a
`position: fixed` button (with box-shadow) every time the user scrolled
past the 800px threshold. On 4K under software compositing the
unmount path (going up across the threshold) registered as a visible
hiccup — the user perceived it as a stutter halfway through scrolling
back to the top.

The button is now always mounted. Visibility is driven by a `.hidden`
class with `opacity: 0; pointer-events: none` and a 150ms opacity
transition. `tabindex` and `aria-hidden` reflect the boolean for a11y.

Also rewrites the scroll listener: previously each scroll event wrote
to a `$state<number>` and triggered a `$derived` recompute (60-120
times/second during active scrolling). Now the listener
rAF-coalesces and only assigns when the threshold actually flips.
Default scroll cost drops from "per-event reactive churn" to
"one boolean flip per threshold cross".

cc

src/routes/+page.svelte

@@ -601,30 +601,52 @@
   let isAutoUpdating = $state(false);
   let autoUpdateProgress = $state<AutoUpdateProgress>({ state: 'checking' });
 
-  // Global back-to-top button
+  // Global back-to-top button.
+  //
+  // The previous implementation registered a capture-phase scroll listener
+  // on .main-content that wrote `globalScrollTop = target.scrollTop` to a
+  // `$state` on every scroll event, and used `$derived(globalScrollTop > 800)`
+  // to gate the button. That fired Svelte reactivity 60-120 times/second
+  // during scroll, and when the derived crossed the 800px threshold the
+  // button mount/unmount work added an extra repaint that registered as
+  // a visible hiccup at 4K under software compositing — especially
+  // pronounced when scrolling up (button unmount).
+  //
+  // New impl: track only the boolean visibility state, coalesce reads via
+  // requestAnimationFrame, and only assign when the threshold flips. This
+  // takes the per-scroll-tick reactive work from "every event" down to
+  // "at most once per animation frame, only on state change".
   let mainContentEl: HTMLElement | null = $state(null);
-  let globalScrollTop = $state(0);
+  let showGlobalBackToTop = $state(false);
   let activeScrollTarget: HTMLElement | null = null;
-  const showGlobalBackToTop = $derived(globalScrollTop > 800);
+  const BACK_TO_TOP_THRESHOLD_PX = 800;
 
   $effect(() => {
     if (!mainContentEl) return;
+    let rafId = 0;
     const handler = (e: Event) => {
       const target = e.target as HTMLElement;
-      if (target !== mainContentEl) {
-        globalScrollTop = target.scrollTop;
-        activeScrollTarget = target;
-      }
+      if (target === mainContentEl) return;
+      activeScrollTarget = target;
+      if (rafId !== 0) return;
+      rafId = requestAnimationFrame(() => {
+        rafId = 0;
+        const next = target.scrollTop > BACK_TO_TOP_THRESHOLD_PX;
+        if (next !== showGlobalBackToTop) showGlobalBackToTop = next;
+      });
     };
     mainContentEl.addEventListener('scroll', handler, true);
-    return () => mainContentEl?.removeEventListener('scroll', handler, true);
+    return () => {
+      mainContentEl?.removeEventListener('scroll', handler, true);
+      if (rafId !== 0) cancelAnimationFrame(rafId);
+    };
   });
 
   // Reset scroll state on view or item change (but not on back/forward — that restores saved position)
   $effect(() => {
     void activeView;
     void currentNavItemId;
-    globalScrollTop = 0;
+    showGlobalBackToTop = false;
     // Scroll to top for forward navigation (not back/forward, which restores saved position)
     if (!isBackForward()) {
       tick().then(() => {
@@ -4927,7 +4949,7 @@
 
       // Save scroll position of the view we're leaving
       if (prevView !== navState.activeView || prevItemId !== navState.activeItemId) {
-        const scrollTopToSave = activeScrollTarget?.scrollTop ?? globalScrollTop;
+        const scrollTopToSave = activeScrollTarget?.scrollTop ?? 0;
         saveScrollPosition(prevView, scrollTopToSave, prevItemId);
       }
 
@@ -4946,7 +4968,7 @@
         tick().then(() => {
           if (activeScrollTarget) {
             activeScrollTarget.scrollTop = savedScroll;
-            globalScrollTop = savedScroll;
+            showGlobalBackToTop = savedScroll > BACK_TO_TOP_THRESHOLD_PX;
           }
         });
       }
@@ -6642,11 +6664,19 @@
       {/if}
     </main>
 
-    {#if showGlobalBackToTop}
-      <button class="back-to-top-global" onclick={globalScrollToTop} title="Back to top">
-        <ChevronUp size={20} />
-      </button>
-    {/if}
+    <!-- Always mounted to avoid the mount/unmount paint cost when crossing
+         the 800px threshold (visible hiccup on scroll-up at 4K under software
+         compositing). Visibility is toggled via class + opacity transition. -->
+    <button
+      class="back-to-top-global"
+      class:hidden={!showGlobalBackToTop}
+      onclick={globalScrollToTop}
+      title="Back to top"
+      tabindex={showGlobalBackToTop ? 0 : -1}
+      aria-hidden={!showGlobalBackToTop}
+    >
+      <ChevronUp size={20} />
+    </button>
 
     <!-- Lyrics Sidebar -->
     {#if lyricsSidebarVisible && !isQueueOpen}
@@ -7263,10 +7293,19 @@
     align-items: center;
     justify-content: center;
     box-shadow: 0 2px 8px rgba(0, 0, 0, 0.3);
-    transition: background 150ms ease, color 150ms ease;
+    /* Opacity-only transition for show/hide so the button can stay mounted
+       at all times. Mounting/unmounting a position:fixed element on every
+       scroll-threshold crossing forced an extra paint event that the user
+       perceived as a stutter, especially on scroll-up at 4K. */
+    transition: background 150ms ease, color 150ms ease, opacity 150ms ease;
     z-index: 200;
   }
 
+  .back-to-top-global.hidden {
+    opacity: 0;
+    pointer-events: none;
+  }
+
   .back-to-top-global:hover {
     background: var(--bg-tertiary);
     color: var(--text-primary);