feat(lyrics): prefetch on play + smooth measured-position autoscroll

Warm the lrclib cache the moment a track starts (Android PlayerController
onMediaItemTransition; web onNpSongChange) so opening the Lyrics tab is
instant. Extract a DOM-free fetch helper on web for reuse.

Rewrite Android lyrics autoscroll to center on the active line's measured
position in a LazyColumn instead of a fixed line-height estimate — removes
stutter and the downward drift on wrapped lines; active line sits at a
~40%-from-top reading anchor.

CHANGELOG: fold into 1.0; drop the now-fixed lyrics known-issue note.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -64,10 +64,12 @@ Versioning: two-part `MAJOR.MINOR` — each release bumps the minor (features an
 - **Android** — Offline playback: tap ▶ on a fully-downloaded group to play locally; downloaded songs are also served from disk automatically when opened from any album or playlist view
 - **Android** — Downloaded badge: albums and playlists in the Library tabs show a ↓-in-circle badge on the cover when fully downloaded
 - **Android** — Duplicate-safe storage: a song shared by an album and a playlist is stored once (keyed by song ID) and reference-counted on delete — removing one group keeps the file if another still needs it
+- **Web + Android** — Lyrics prefetch: lyrics are fetched the moment a track starts playing (not only when the Lyrics tab is opened), so opening Lyrics is instant. The lrclib cache is warmed on every song change
 
 ### Fixed
 - **Android** — Download crash on Android 15 (targetSdk 35): `FOREGROUND_SERVICE_TYPE_DATA_SYNC` is banned for apps targeting API 35; WorkManager's expedited path used it and the resulting exception killed the app ~1.5 s after tapping download. Fixed by dropping `setExpedited()` and all foreground-service use from `DownloadWorker` — downloads run as silent background work with progress shown in the Offline tab
 - **Android** — Offline detail screen flashed open then immediately returned: its initial empty state (before Room emitted) was mistaken for "deleted". Now shows a brief spinner until loaded, and only leaves when the last song is actually removed
+- **Android** — Lyrics autoscroll: rewrote centering to use the active line's *measured* position in a `LazyColumn` instead of a fixed line-height estimate. Eliminates the stutter and the downward drift on wrapped (multi-line) lyrics — the active line now tracks smoothly and sits at a natural ~40 %-from-top reading anchor
 
 ### Internal
 - **Android** — Offline system: `Room` DB (`novasonic_offline.db`) with `downloaded_tracks`, `download_groups`, `group_track_cross_refs`; `OfflineRepository` holds hot `StateFlow` maps for O(1) local-path lookups; `DownloadWorker` streams audio + cover art via `HttpURLConnection` to `context.filesDir/offline/{audio|covers}/`. `MusicRepository.streamUrl()` checks local files first, so playback uses the `file://` URI on every entry point

android/app/src/main/kotlin/com/novasonic/app/player/PlayerController.kt

@@ -8,6 +8,7 @@ import androidx.media3.common.Timeline
 import androidx.media3.session.MediaController
 import androidx.media3.session.SessionToken
 import com.novasonic.app.NovaSonicApp
+import com.novasonic.app.api.LrcLibClient
 import com.novasonic.app.api.model.Song
 import com.novasonic.app.data.MusicRepository
 import kotlinx.coroutines.*
@@ -279,6 +280,20 @@ class PlayerController(private val context: Context, private val repo: MusicRepo
         )
     }
 
+    /** Warm the LrcLib cache as soon as a track starts, so opening Lyrics feels instant. */
+    private fun prefetchLyrics(song: Song) {
+        scope.launch {
+            runCatching {
+                LrcLibClient.getLyrics(
+                    artist       = song.artist ?: "",
+                    title        = song.title,
+                    album        = song.album,
+                    durationSecs = song.duration,
+                )
+            }
+        }
+    }
+
     private inner class PlayerListener : Player.Listener {
         override fun onIsPlayingChanged(isPlaying: Boolean) {
             if (isPlaying) startTicker() else stopTicker()
@@ -298,6 +313,7 @@ class PlayerController(private val context: Context, private val repo: MusicRepo
             val dur  = ctrl.duration.coerceAtLeast(0)
             scheduleScrobble(song, dur)
             scope.launch { runCatching { repo.saveLastSong(song) } }
+            prefetchLyrics(song)
         }
 
         override fun onShuffleModeEnabledChanged(enabled: Boolean) = syncState()

android/app/src/main/kotlin/com/novasonic/app/ui/screen/NowPlayingScreen.kt

@@ -8,6 +8,7 @@ import androidx.compose.foundation.gestures.detectDragGestures
 import androidx.compose.foundation.gestures.detectHorizontalDragGestures
 import androidx.compose.foundation.gestures.detectTapGestures
 import androidx.compose.foundation.gestures.detectVerticalDragGestures
+import androidx.compose.foundation.gestures.animateScrollBy
 import androidx.compose.foundation.layout.*
 import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.itemsIndexed
@@ -399,31 +400,35 @@ fun NowPlayingScreen(
             // ── LYRICS TAB ─────────────────────────────────────────────────
             if (tab == NpTab.LYRICS && song != null) {
                 val lyricsState by lyricsVm.state.collectAsStateWithLifecycle()
-                val lyricsScrollState = rememberScrollState()
+                val lyricsListState = rememberLazyListState()
                 val density = LocalDensity.current
-
-                // Pre-compute pixel constants for absolute scroll target calculation
-                val lineHeightPx = remember(density) {
-                    with(density) { 28.sp.toPx() + 12.dp.toPx() } // lineHeight + 2×vertical padding
-                }
-                val lyricsPaddingPx = remember(density) { with(density) { 200.dp.toPx() } }
                 var lyricsViewH by remember { mutableIntStateOf(0) }
+                // Anchor the active line at 40% from the top — slightly above middle reads more naturally.
+                val anchorFrac = 0.40f
 
                 // 200ms lookahead — same as web app for better sync feel
                 val activeIdx = if (lyricsState.lines.isNotEmpty())
                     lyricsState.lines.indexOfLast { it.timeMs <= state.positionMs + 200L }.coerceAtLeast(0)
                 else -1
 
-                // Center the active line when it changes
-                LaunchedEffect(activeIdx) {
+                // Center the active line using its *measured* position — no cumulative drift, so
+                // wrapped (multi-line) lyrics stay centered. Re-runs once the viewport is measured.
+                LaunchedEffect(activeIdx, lyricsViewH) {
                     if (activeIdx < 0 || lyricsState.lines.isEmpty() || lyricsViewH == 0) return@LaunchedEffect
-                    val target = (lyricsPaddingPx + activeIdx * lineHeightPx + lineHeightPx / 2f
-                        - lyricsViewH * 0.5f).coerceAtLeast(0f).toInt()
-                    // Spring handles rapid line changes gracefully — starts from current scroll velocity
-                    lyricsScrollState.animateScrollTo(
-                        target,
-                        spring(dampingRatio = Spring.DampingRatioNoBouncy, stiffness = Spring.StiffnessLow),
+                    val info = lyricsListState.layoutInfo
+                    val anchor = info.viewportStartOffset +
+                        ((info.viewportEndOffset - info.viewportStartOffset) * anchorFrac).toInt()
+                    val item = info.visibleItemsInfo.firstOrNull { it.index == activeIdx }
+                    if (item != null) {
+                        val delta = (item.offset + item.size / 2 - anchor).toFloat()
+                        lyricsListState.animateScrollBy(
+                            delta,
+                            animationSpec = tween(durationMillis = 600, easing = FastOutSlowInEasing),
                         )
+                    } else {
+                        // Big jump (instrumental gap → chorus): approximate-anchor now, refine next tick
+                        lyricsListState.animateScrollToItem(activeIdx, -anchor)
+                    }
                 }
 
                 when {
@@ -446,17 +451,20 @@ fun NowPlayingScreen(
                             color = Color.White.copy(0.4f), textAlign = TextAlign.Center)
                     }
 
-                    lyricsState.lines.isNotEmpty() -> Column(
+                    lyricsState.lines.isNotEmpty() -> LazyColumn(
+                        state = lyricsListState,
                         modifier = Modifier
                             .weight(1f)
                             .fillMaxWidth()
-                            .onSizeChanged { lyricsViewH = it.height }
-                            .verticalScroll(lyricsScrollState),
+                            .onSizeChanged { lyricsViewH = it.height },
+                        // Asymmetric padding so the first/last lines can still reach the anchor point.
+                        contentPadding = if (lyricsViewH > 0) PaddingValues(
+                            top    = with(density) { (lyricsViewH * anchorFrac).toDp() },
+                            bottom = with(density) { (lyricsViewH * (1f - anchorFrac)).toDp() },
+                        ) else PaddingValues(vertical = 220.dp),
                         horizontalAlignment = Alignment.CenterHorizontally,
                     ) {
-                        Spacer(Modifier.height(200.dp))
-                        lyricsState.lines.forEachIndexed { idx, line ->
-                            key(idx) {
+                        itemsIndexed(lyricsState.lines, key = { idx, _ -> idx }) { idx, line ->
                             LyricsLineItem(
                                 text     = line.text,
                                 isActive = idx == activeIdx,
@@ -465,8 +473,6 @@ fun NowPlayingScreen(
                             )
                         }
                     }
-                        Spacer(Modifier.height(200.dp))
-                    }
 
                     lyricsState.plainText.isNotBlank() -> LazyColumn(
                         modifier = Modifier.weight(1f).fillMaxWidth(),

src/player/nowplaying.js

@@ -145,7 +145,6 @@ async function _loadNpLyrics(songId) {
   }
   _lyricsSongId = songId;
   _lyricsLines = [];
-  content.innerHTML = '<div style="text-align:center;padding:60px 0;color:rgba(255,255,255,.3);font-size:14px">Lade Lyrics…</div>';
 
   const token = ++_lyricsLoadToken;
   const s      = S.queue[S.qIdx];
@@ -155,6 +154,28 @@ async function _loadNpLyrics(songId) {
   const dur    = s?.duration || 0;
   const cacheKey = `${artist}|${title}`;
 
+  // Cache hit (warmed on play, or from a previous view) — render instantly, no placeholder flash
+  if(_lyricsCache.has(cacheKey)) {
+    content.innerHTML = '';
+    _showLyricsResult(content, _lyricsCache.get(cacheKey));
+    return;
+  }
+
+  content.innerHTML = '<div style="text-align:center;padding:60px 0;color:rgba(255,255,255,.3);font-size:14px">Lade Lyrics…</div>';
+  const result = await _fetchLyricsData(artist, title, album, dur);
+  if(token !== _lyricsLoadToken) return;
+  content.innerHTML = '';
+  _showLyricsResult(content, result);
+}
+
+/**
+ * Pure lrclib fetch cascade — no DOM. Populates _lyricsCache and returns the result.
+ * Shared by the Lyrics tab and by prefetchLyrics() so opening Lyrics feels instant.
+ */
+async function _fetchLyricsData(artist, title, album, dur) {
+  const cacheKey = `${artist}|${title}`;
+  if(_lyricsCache.has(cacheKey)) return _lyricsCache.get(cacheKey);
+
   const SIG = () => AbortSignal.timeout(6000);
   const safeJson = async (promise) => { try { const r = await promise; return r.ok ? r.json() : null; } catch { return null; } };
   const bestMatch = (candidates, durSecs) => {
@@ -166,19 +187,12 @@ async function _loadNpLyrics(songId) {
     return Math.abs(pick.duration - durSecs) <= 5 ? pick : (synced[0] || candidates[0]);
   };
 
-  // Cache hit — instant
-  if(_lyricsCache.has(cacheKey)) {
-    const cached = _lyricsCache.get(cacheKey);
-    content.innerHTML = '';
-    _showLyricsResult(content, cached);
-    return;
-  }
+  let result = null;
 
   // 1a. lrclib exact match with duration
   const exactP = new URLSearchParams({ artist_name: artist, track_name: title });
   if(album) exactP.set('album_name', album);
   if(dur)   exactP.set('duration', dur);
-  let result = null;
   try {
     const r = await fetch(`https://lrclib.net/api/get?${exactP}`, { signal: SIG() });
     if(r.status === 200) {
@@ -199,8 +213,6 @@ async function _loadNpLyrics(songId) {
     } catch {}
   }
 
-  if(token !== _lyricsLoadToken) return;
-
   // 2+3. lrclib search — artist+title and title-only in parallel (2 requests, not 3)
   if(!result) {
     const artistP = new URLSearchParams({ track_name: title });
@@ -214,12 +226,15 @@ async function _loadNpLyrics(songId) {
     result = bestMatch(artistData, dur) ?? bestMatch(titleData, dur);
   }
 
-  if(token !== _lyricsLoadToken) return;
-
-  if(token !== _lyricsLoadToken) return;
   _lyricsCache.set(cacheKey, result ?? null);
-  content.innerHTML = '';
-  _showLyricsResult(content, result);
+  return result ?? null;
+}
+
+/** Warm the lyrics cache when a track starts, so opening the Lyrics tab is instant. */
+function prefetchLyrics(song) {
+  if(!song || !song.title) return;
+  const artist = (song.artist || '').split(/[/,]/)[0].trim();
+  _fetchLyricsData(artist, song.title, song.album || '', song.duration || 0).catch(() => {});
 }
 
 function _showLyricsResult(content, result) {
@@ -268,6 +283,8 @@ export function onNpSongChange(songId) {
   _lyricsLines = [];
   const content = document.getElementById('np-lyrics-content');
   if(content) content.innerHTML = '';
+  // Warm the cache the moment the song changes, so the Lyrics tab opens instantly.
+  prefetchLyrics(S.queue.find(x => x.id === songId) || S.queue[S.qIdx]);
   if(S.npLyricsMode) _loadNpLyrics(songId);
 }