fix: eliminate NP overlay stale-song on autoplay and cover-load delay

Three improvements:
- openNowPlaying(): reads NSBridge.getState() (always-fresh @Volatile snapshot)
  via ID search to reconcile S.qIdx before render; fixes NP showing old song
  when deferred evaluateJavascript callbacks haven't fired yet after backgrounding
- onPositionUpdate(): periodic check (~every 2s) detects track mismatch via
  snapshot ID; catches missed/delayed onTrackChanged when NP is open
- updatePlayerUI(): pre-warms next song's 1024px cover so auto-advance NP
  swap is instant from browser cache instead of a 1-second load delay
- styles: add backface-visibility:hidden + isolation:isolate to .np-overlay
  to reduce GPU composite flicker on Android WebView on manual skip

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/nowplaying.js

@@ -2,12 +2,29 @@ import { S } from './state.js';
 import { esc } from './utils.js';
 import { coverUrl } from './api.js';
 // Circular imports — safe (only used inside function bodies)
-import { playCurrent, _syncNpDirect, dlSongLocal, npToggleStar, _syncOverlayState, isPlaying, seekToMs } from './player.js';
+import { playCurrent, _syncNpDirect, dlSongLocal, npToggleStar, _syncOverlayState, isPlaying, seekToMs, updatePlayerUI } from './player.js';
 import { navigate } from './views.js';
 import { openPlModal, showSongInfo } from './ui.js';
 
 // ─── NOW PLAYING OVERLAY ───
 export function openNowPlaying() {
+  // Reconcile S.qIdx with native state before rendering.
+  // NSBridge.getState() reads a @Volatile snapshot updated synchronously before every callJs(),
+  // so it is always fresh even when evaluateJavascript callbacks are still queued (backgrounded app).
+  if (typeof window.NSBridge !== 'undefined') {
+    try {
+      const live = JSON.parse(window.NSBridge.getState() || '{}');
+      const liveId = String(live.currentId || '');
+      if (liveId && liveId !== String(S.queue[S.qIdx]?.id || '')) {
+        const found = S.queue.findIndex(s => String(s.id) === liveId);
+        if (found >= 0) {
+          S.qIdx = found;
+          const s = S.queue[S.qIdx];
+          if (s) { updatePlayerUI(s, true); onNpSongChange(s.id); }
+        }
+      }
+    } catch {}
+  }
   S.npOpen = true;
   const overlay = document.getElementById('np-overlay');
   overlay.classList.add('open');

src/player.js

@@ -16,6 +16,7 @@ let _lastPositionMs = 0;   // ms — set by onPositionUpdate, used for prev() de
 let _lastDurationMs = 0;   // ms — set by onPositionUpdate, used for seek calculations
 let _skipping       = false;  // true briefly after a skip — suppresses the transient isPlaying:false
 let _skipTimer      = null;
+let _reconTick      = 0;   // counter for periodic track-reconciliation inside onPositionUpdate
 
 // ─── SCROBBLE ─────────────────────────────────────────────────────────────
 let _scrobbleTimer  = null;
@@ -221,6 +222,24 @@ window.NSPlayer = {
     _lastPositionMs = posMs;
     _lastDurationMs = durMs;
     if (S._scrubbing) return;
+    // Every 8 ticks (~2 s): verify JS track matches native snapshot.
+    // Catches the gap between auto-advance and onTrackChanged arriving in JS
+    // (evaluateJavascript is deferred while WebView is backgrounded/resuming).
+    if (HAS_BRIDGE && ++_reconTick >= 8) {
+      _reconTick = 0;
+      try {
+        const snap   = JSON.parse(window.NSBridge.getState() || '{}');
+        const snapId = String(snap.currentId || '');
+        if (snapId && snapId !== String(S.queue[S.qIdx]?.id || '')) {
+          const found = S.queue.findIndex(s => String(s.id) === snapId);
+          if (found >= 0) {
+            S.qIdx = found;
+            const s = S.queue[S.qIdx];
+            if (s) { updatePlayerUI(s, true); syncNowPlaying(); onNpSongChange(s.id); renderQueue(); }
+          }
+        }
+      } catch {}
+    }
     const dur = durMs / 1000;
     const cur = posMs / 1000;
     if (dur <= 0) return;
@@ -496,6 +515,12 @@ export function updatePlayerUI(s, _skipMediaSession = false) {
   // Preload the hi-res NP cover now so opening the overlay is instant
   const _npCv = coverUrl(s.coverArt, 1024);
   if (_npCv && _npCv !== _npBgLastUrl) { const _pre = new Image(); _pre.src = _npCv; }
+  // Pre-warm the next song's 1024 cover so auto-advance NP swap is instant from cache
+  const _nextForPreload = S.qIdx + 1 < S.queue.length ? S.queue[S.qIdx + 1] : null;
+  if (_nextForPreload?.coverArt) {
+    const _nextCv = coverUrl(_nextForPreload.coverArt, 1024);
+    if (_nextCv) { const _preN = new Image(); _preN.src = _nextCv; }
+  }
 }
 
 // ── Solid cover-color background ────────────────────────────────────────────