fix(subtitles): suppress karaoke highlight sweeps from reconstructed lyrics

src/core/services/subtitle-cue-parser.test.ts

@@ -1530,3 +1530,43 @@ test('parseSubtitleCues recovers clear word gaps in a short romaji line', () =>
 
   assert.equal(parseSubtitleCues(content, 'test.ass')[0]?.text, 'boku wo yobu');
 });
+
+test('parseSubtitleCues suppresses a karaoke highlight sweep without publishing it', () => {
+  // Main lyric: per-glyph fragments alive together for the whole line.
+  const lineFragments = [
+    ['to', 972],
+    ['so', 1051],
+    ['u', 1113],
+    ['o', 1166],
+    ['mo', 1204],
+  ] as const;
+  // Highlight sweep: one syllable at a time over the same lyric, each event ending
+  // exactly as the next begins, so no two syllables are ever on screen together.
+  const sweepFragments = [
+    ['to', 972, '0:00:01.00', '0:00:01.40'],
+    ['so', 1051, '0:00:01.40', '0:00:01.80'],
+    ['u', 1113, '0:00:01.80', '0:00:02.20'],
+    ['o', 1166, '0:00:02.20', '0:00:02.60'],
+    ['mo', 1204, '0:00:02.60', '0:00:03.00'],
+  ] as const;
+  const content = [
+    '[Events]',
+    'Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text',
+    ...[0, 1].flatMap((layer) =>
+      lineFragments.map(
+        ([fragment, x], index) =>
+          `Dialogue: ${layer},0:00:01.00,0:00:04.00,ED Romaji,,0,0,0,fx,{\\pos(${x},60)\\t(${index * 2},${index * 2 + 100},\\fscx120)}${fragment}`,
+      ),
+    ),
+    ...[40, 41].flatMap((layer) =>
+      sweepFragments.map(
+        ([fragment, x, start, end]) =>
+          `Dialogue: ${layer},${start},${end},ED Romaji2,,0,0,0,fx,{\\an5\\pos(${x},60)\\t(150,290,\\1a&HFF&)}${fragment}`,
+      ),
+    ),
+  ].join('\n');
+
+  const cues = parseSubtitleCues(content, 'test.ass');
+  assert.equal(cues.length, 1);
+  assert.equal(cues[0]?.text.replace(/\s+/gu, ''), 'tosouomo');
+});

src/core/services/subtitle-cue-parser.ts

@@ -762,6 +762,37 @@ function clusterAssFragmentEvents(
   return clusters;
 }
 
+/**
+ * A karaoke highlight sweep repaints one syllable at a time over an already-visible
+ * lyric line: each event ends as the next begins, so the cluster's concatenated text is
+ * never on screen as a whole. Publishing it would emit rolling partial copies of the
+ * lyric ("to sou omo" beside "akenakute ii to sou omotteta"). Layer copies share one
+ * placement and timing, so the test is whether any two distinct placements coexist.
+ */
+function isProgressiveHighlightSweep(events: readonly AnnotatedSubtitleCue[]): boolean {
+  const intervals: { startTime: number; endTime: number }[] = [];
+  const seen = new Set<string>();
+  for (const event of events) {
+    const anchors = [...fragmentPlacementAnchors(event)].sort().join('|');
+    const key = `${compactCueMatchText(event)}\0${anchors}\0${event.startTime}\0${event.endTime}`;
+    if (seen.has(key)) continue;
+    seen.add(key);
+    intervals.push({ startTime: event.startTime, endTime: event.endTime });
+  }
+  if (intervals.length < 2) {
+    return false;
+  }
+  intervals.sort((a, b) => a.startTime - b.startTime || a.endTime - b.endTime);
+  let latestEnd = intervals[0]!.endTime;
+  for (let index = 1; index < intervals.length; index += 1) {
+    if (intervals[index]!.startTime < latestEnd - 0.001) {
+      return false;
+    }
+    latestEnd = Math.max(latestEnd, intervals[index]!.endTime);
+  }
+  return true;
+}
+
 function decodeSingleAssFragment(cue: AnnotatedSubtitleCue): string | null {
   const visibleLines = decodeSubtitleCueText(cue.rawText)
     .split('\n')
@@ -903,6 +934,12 @@ function recoverFragmentOnlyAssLines(dialogue: AnnotatedSubtitleCue[]): Annotate
       if (!line) {
         continue;
       }
+      // A sweep only re-highlights the lyric it decorates: hide its events without
+      // publishing the reconstruction.
+      if (isProgressiveHighlightSweep(cluster.events)) {
+        cluster.events.forEach((event) => suppressed.add(event));
+        continue;
+      }
       recovered.push(line);
       cluster.events.forEach((event) => suppressed.add(event));
       // Decoration is timed to the line it overlays, so it disappears with the line's
@@ -916,7 +953,7 @@ function recoverFragmentOnlyAssLines(dialogue: AnnotatedSubtitleCue[]): Annotate
       }
     }
   }
-  if (recovered.length === 0) {
+  if (recovered.length === 0 && suppressed.size === 0) {
     return dialogue;
   }
   return [...dialogue.filter((cue) => !suppressed.has(cue)), ...recovered].sort(