let _rehPrefetchRunning = false;
async function _rehPrefetchLines(fromIndex) {
  if (_rehPrefetchRunning) return;
  _rehPrefetchRunning = true;
  try {
    let prefetched = 0;
    for (let i = fromIndex; i < rehState.lines.length && prefetched < 2; i++) {
      if (!rehState.playing) break;
      const l = rehState.lines[i];
      if (!l || l.ignored || l.hidden || l.type === 'pagebreak') continue;
      
      const isDialog = l.type === 'dialog';
      const cast = isDialog ? (rehState.cast[l.speaker] || { voice: '' }) : null;
      if (isDialog && (!cast.voice || cast.voice === 'me')) continue;
      if (!isDialog && (!rehState.narratorVoice || !(l.text || '').trim() || rehState.skipDescriptions)) continue;
      
      if (rehState.synthCache.has(i) && !rehState.staleLines.has(i)) {
        prefetched++;
        continue;
      }
      
      const voice = isDialog ? cast.voice : rehState.narratorVoice;
      const cleanTxt = stripMarkdown(l.text);
      const instruct = isDialog ? _buildInstruct((cast.instruct || '').trim(), l.emotion, voice) : '';
      const text = isDialog ? _rehInlineTone(cleanTxt, l.emotion) : cleanTxt;
      
      const book = _lineAudioBookName();
      const cacheKey = await _lineAudioCacheKey(text, voice, instruct);
      let blob = await _lineAudioCacheGet(book, cacheKey);
      
      if (!blob) {
        try {
          blob = await fetchTtsPreviewBlob(voice, text, 'wav', instruct, _ttsBackendForVoice(voice, rehState.backend));
          _lineAudioCachePut(book, cacheKey, blob);
        } catch(e) {
          console.warn('Prefetch failed for line', i, e);
          continue; // skip counting as prefetched if it failed
        }
      }
      
      rehState.synthCache.set(i, blob);
      rehState.staleLines.delete(i);
      _markSynthDot(i, 'ok');
      prefetched++;
    }
  } finally {
    _rehPrefetchRunning = false;
  }
}