perf(subtitle-prefetch): sweep overlap spans instead of rescanning cues

- Track active cues with a single sorted start/end sweep across boundaries
- Join simultaneous cues in cue-list order to match mpv
- Skip empty or inverted cues
- Add test for multi-cue overlap spans with unsorted cue input

src/main/runtime/subtitle-prefetch-overlaps.test.ts

@@ -22,3 +22,17 @@ test('back-to-back cues yield no overlap lines', () => {
 
   assert.deepEqual(buildOverlapPrefetchCues(cues), []);
 });
+
+test('each overlap span joins only the cues active across it, in cue-list order', () => {
+  const cues: SubtitleCue[] = [
+    { startTime: 3, endTime: 8, text: 'C' },
+    { startTime: 0, endTime: 6, text: 'A' },
+    { startTime: 2, endTime: 4, text: 'B' },
+  ];
+
+  assert.deepEqual(buildOverlapPrefetchCues(cues), [
+    { startTime: 2, endTime: 3, text: 'A\n\nB' },
+    { startTime: 3, endTime: 4, text: 'C\n\nA\n\nB' },
+    { startTime: 4, endTime: 6, text: 'C\n\nA' },
+  ]);
+});

src/main/runtime/subtitle-prefetch-overlaps.ts

@@ -13,20 +13,43 @@ export function buildOverlapPrefetchCues(cues: readonly SubtitleCue[]): Subtitle
   const boundaries = [...new Set(cues.flatMap((cue) => [cue.startTime, cue.endTime]))].sort(
     (a, b) => a - b,
   );
+  const indices = cues.map((_, index) => index);
+  const byStart = [...indices].sort((a, b) => cues[a]!.startTime - cues[b]!.startTime);
+  const byEnd = [...indices].sort((a, b) => cues[a]!.endTime - cues[b]!.endTime);
   const singleTexts = new Set(cues.map((cue) => cue.text));
   const seen = new Set<string>();
   const overlapCues: SubtitleCue[] = [];
 
+  // Sweep the boundaries once, keeping the cues active over [startTime, endTime). Every
+  // cue edge is a boundary, so a cue is active for a whole interval or not at all.
+  const active = new Set<number>();
+  let nextStart = 0;
+  let nextEnd = 0;
   for (let i = 0; i + 1 < boundaries.length; i += 1) {
     const startTime = boundaries[i]!;
     const endTime = boundaries[i + 1]!;
-    const midpoint = (startTime + endTime) / 2;
-    const active = cues.filter((cue) => cue.startTime <= midpoint && cue.endTime > midpoint);
-    if (active.length < 2) continue;
+    for (; nextEnd < byEnd.length && cues[byEnd[nextEnd]!]!.endTime <= startTime; nextEnd += 1) {
+      active.delete(byEnd[nextEnd]!);
+    }
+    for (
+      ;
+      nextStart < byStart.length && cues[byStart[nextStart]!]!.startTime <= startTime;
+      nextStart += 1
+    ) {
+      const index = byStart[nextStart]!;
+      // Empty or inverted cues are never on screen.
+      if (cues[index]!.endTime > startTime) active.add(index);
+    }
+    if (active.size < 2) continue;
 
+    // mpv joins simultaneous cues in cue-list order.
+    const liveText = [...active]
+      .sort((a, b) => a - b)
+      .map((index) => cues[index]!.text)
+      .join('\n');
     const text = resolvePrimarySubtitleText({
-      liveText: active.map((cue) => cue.text).join('\n'),
-      currentTimeSec: midpoint,
+      liveText,
+      currentTimeSec: (startTime + endTime) / 2,
       cues,
     });
     if (!text.trim() || singleTexts.has(text) || seen.has(text)) continue;