Perf/UI cover cache mainstage (#468)

* Enhance CachedImage and ArtistDetail components with improved image caching and priority handling

- Refactor CachedImage to utilize a priority system for image loading based on viewport visibility, improving performance during scrolling.
- Update useCachedUrl to accept an optional getPriority function for better cache management.
- Optimize ArtistDetail and Artists components by using useMemo for cover art URLs, reducing redundant calculations and improving rendering efficiency.
- Adjust image loading logic in CachedImage to ensure smoother transitions and avoid unnecessary fetch requests.

* perf(ui): unblock IDB cover art, stabilize mainstage rails and virtual lists

Let IndexedDB reads bypass the network concurrency slot so cached thumbnails
paint without queueing behind remote fetches; debounce disk eviction during
heavy scrolling.

Fix mainstage horizontal rails: dedupe album/song ids for React keys, widen
artwork budget overscan, avoid resetting the budget on list append, and raise
Home initial artwork budgets. CachedImage treats already-decoded images as
loaded; rail cards load cover images eagerly.

Refresh dynamic color extraction and extend virtual scrolling / scroll roots on
Albums, Artists, Playlists, and related surfaces.


Remove local agent-only commit instructions from the repository tree.

* perf(virtual): viewport-based overscan for main scroll lists

Drive TanStack Virtual overscan from measured scroll height so each list
renders about one screen of extra rows above and below the viewport for
snappier scrolling on Albums, Artists (list mode), and Tracks virtual song list.

Introduce useResizeClientHeight helpers (ID + ref) for ResizeObserver-based
clientHeight tracking.

* docs(changelog): note PR #468 UI cover cache, rails, and virtual lists

Add a coarse summary under 1.46.0 Changed for cover-art pipeline,
mainstage rails, viewport-based overscan, and library/chrome polish.

src/components/AlbumRow.tsx

@@ -1,10 +1,11 @@
-import React, { useRef, useState, useEffect } from 'react';
+import React, { useRef, useState, useEffect, useMemo } from 'react';
 import { SubsonicAlbum } from '../api/subsonic';
 import AlbumCard from './AlbumCard';
 import { ChevronLeft, ChevronRight, ArrowRight } from 'lucide-react';
 import { NavLink, useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import { usePerfProbeFlags } from '../utils/perfFlags';
+import { dedupeById } from '../utils/dedupeById';
 
 interface Props {
   title: string;
@@ -50,6 +51,7 @@ export default function AlbumRow({
   const [artworkBudget, setArtworkBudget] = useState(initialArtworkBudget);
 
   const loadingRef = useRef(false);
+  const uniqueAlbums = useMemo(() => dedupeById(albums), [albums]);
 
   const recomputeArtworkBudget = () => {
     if (!windowArtworkByViewport) return;
@@ -62,19 +64,23 @@ export default function AlbumRow({
     const gap = Number.parseFloat(gridStyles.columnGap || gridStyles.gap || '16') || 16;
     const step = Math.max(1, cardW + gap);
     const visibleCount = Math.ceil((scrollLeft + clientWidth) / step);
-    const nextBudget = Math.max(initialArtworkBudget, visibleCount + 4);
+    // Extra slack so fast horizontal scroll doesn’t hit the idx≥budget cliff between frames.
+    const nextBudget = Math.max(initialArtworkBudget, visibleCount + 12);
     setArtworkBudget(prev => (nextBudget > prev ? nextBudget : prev));
   };
 
   const handleScroll = () => {
-    if (interactivityDisabled) return;
+    if (windowArtworkByViewport) recomputeArtworkBudget();
+
     if (!scrollRef.current) return;
     const { scrollLeft, scrollWidth, clientWidth } = scrollRef.current;
-    setShowLeft(scrollLeft > 0);
-    setShowRight(scrollLeft < scrollWidth - clientWidth - 5);
-    recomputeArtworkBudget();
 
-    // Auto-load trigger
+    if (!interactivityDisabled) {
+      setShowLeft(scrollLeft > 0);
+      setShowRight(scrollLeft < scrollWidth - clientWidth - 5);
+    }
+
+    // Auto-load trigger (native horizontal scroll still works when rail buttons are perf-disabled)
     if (onLoadMore && !loadingRef.current && scrollLeft > 0 && scrollLeft + clientWidth >= scrollWidth - 300) {
       triggerLoadMore();
     }
@@ -90,15 +96,13 @@ export default function AlbumRow({
   };
 
   useEffect(() => {
-    if (interactivityDisabled) return;
     handleScroll();
     const raf = window.requestAnimationFrame(() => {
-      // One post-layout pass ensures we account for final grid/card geometry.
-      recomputeArtworkBudget();
+      if (windowArtworkByViewport) recomputeArtworkBudget();
     });
     window.addEventListener('resize', handleScroll);
     const ro = new ResizeObserver(() => {
-      recomputeArtworkBudget();
+      if (windowArtworkByViewport) recomputeArtworkBudget();
     });
     if (scrollRef.current) ro.observe(scrollRef.current);
     return () => {
@@ -106,11 +110,14 @@ export default function AlbumRow({
       window.removeEventListener('resize', handleScroll);
       ro.disconnect();
     };
-  }, [albums, interactivityDisabled, windowArtworkByViewport, initialArtworkBudget]);
+  }, [uniqueAlbums, interactivityDisabled, windowArtworkByViewport, initialArtworkBudget]);
 
+  // Reset when the row’s identity changes (new data / server), not when the list grows via
+  // “load more” — reusing albums.length would shrink the budget mid-scroll and flash placeholders.
+  const rowArtworkResetKey = uniqueAlbums[0]?.id ?? '';
   useEffect(() => {
     setArtworkBudget(initialArtworkBudget);
-  }, [initialArtworkBudget, albums.length]);
+  }, [initialArtworkBudget, rowArtworkResetKey]);
 
   const scroll = (dir: 'left' | 'right') => {
     if (!scrollRef.current) return;
@@ -118,7 +125,7 @@ export default function AlbumRow({
     scrollRef.current.scrollBy({ left: dir === 'left' ? -amount : amount, behavior: 'smooth' });
   };
 
-  if (albums.length === 0) return null;
+  if (uniqueAlbums.length === 0) return null;
 
   return (
     <section className="album-row-section">
@@ -154,8 +161,8 @@ export default function AlbumRow({
       </div>
       
       <div className="album-grid-wrapper">
-        <div className="album-grid" ref={scrollRef} onScroll={interactivityDisabled ? undefined : handleScroll}>
-          {albums.map((a, idx) => (
+        <div className="album-grid" ref={scrollRef} onScroll={handleScroll}>
+          {uniqueAlbums.map((a, idx) => (
             <AlbumCard
               key={a.id}
               album={a}