fix(overlay): deduplicate secondary subtitle rendering

- Parse selected secondary tracks through the subtitle deduplication pipeline
- Fall back to live mpv text when source resolution fails

src/main/runtime/mpv-client-event-bindings.test.ts

@@ -191,6 +191,8 @@ test('mpv event bindings register all expected events', () => {
     onSubtitleAssChange: () => {},
     onSecondarySubtitleChange: () => {},
     onSubtitleTrackChange: () => {},
+    onSecondarySubtitleTrackChange: () => {},
+    onSecondarySubtitleDelayChange: () => {},
     onSubtitleTrackListChange: () => {},
     onSubtitleTiming: () => {},
     onMediaPathChange: () => {},
@@ -215,6 +217,8 @@ test('mpv event bindings register all expected events', () => {
     'subtitle-ass-change',
     'secondary-subtitle-change',
     'subtitle-track-change',
+    'secondary-subtitle-track-change',
+    'secondary-subtitle-delay-change',
     'subtitle-track-list-change',
     'subtitle-timing',
     'media-path-change',

src/main/runtime/mpv-client-event-bindings.ts

@@ -4,6 +4,8 @@ type MpvBindingEventName =
   | 'subtitle-ass-change'
   | 'secondary-subtitle-change'
   | 'subtitle-track-change'
+  | 'secondary-subtitle-track-change'
+  | 'secondary-subtitle-delay-change'
   | 'subtitle-track-list-change'
   | 'subtitle-timing'
   | 'media-path-change'
@@ -90,6 +92,8 @@ export function createBindMpvClientEventHandlers(deps: {
   onSubtitleAssChange: (payload: { text: string }) => void;
   onSecondarySubtitleChange: (payload: { text: string }) => void;
   onSubtitleTrackChange: (payload: { sid: number | null }) => void;
+  onSecondarySubtitleTrackChange: (payload: { sid: number | null }) => void;
+  onSecondarySubtitleDelayChange: (payload: { delay: number }) => void;
   onSubtitleTrackListChange: (payload: { trackList: unknown[] | null }) => void;
   onSubtitleTiming: (payload: { text: string; start: number; end: number }) => void;
   onMediaPathChange: (payload: { path: string | null }) => void;
@@ -107,6 +111,8 @@ export function createBindMpvClientEventHandlers(deps: {
     mpvClient.on('subtitle-ass-change', deps.onSubtitleAssChange);
     mpvClient.on('secondary-subtitle-change', deps.onSecondarySubtitleChange);
     mpvClient.on('subtitle-track-change', deps.onSubtitleTrackChange);
+    mpvClient.on('secondary-subtitle-track-change', deps.onSecondarySubtitleTrackChange);
+    mpvClient.on('secondary-subtitle-delay-change', deps.onSecondarySubtitleDelayChange);
     mpvClient.on('subtitle-track-list-change', deps.onSubtitleTrackListChange);
     mpvClient.on('subtitle-timing', deps.onSubtitleTiming);
     mpvClient.on('media-path-change', deps.onMediaPathChange);

src/main/runtime/mpv-main-event-bindings.test.ts

@@ -37,6 +37,9 @@ test('main mpv event binder wires callbacks through to runtime deps', () => {
     broadcastSubtitleAss: (text) => calls.push(`broadcast-ass:${text}`),
     broadcastSecondarySubtitle: (text) => calls.push(`broadcast-secondary:${text}`),
     onSubtitleTrackChange: () => calls.push('subtitle-track-change'),
+    onSecondarySubtitleTrackChange: () => calls.push('secondary-subtitle-track-change'),
+    onSecondarySubtitleDelayChange: (delay) =>
+      calls.push(`secondary-subtitle-delay-change:${delay}`),
     onSubtitleTrackListChange: () => calls.push('subtitle-track-list-change'),
 
     updateCurrentMediaPath: (path) => calls.push(`media-path:${path}`),
@@ -73,6 +76,8 @@ test('main mpv event binder wires callbacks through to runtime deps', () => {
   handlers.get('connection-change')?.({ connected: true });
   handlers.get('subtitle-change')?.({ text: 'line' });
   handlers.get('subtitle-track-change')?.({ sid: 3 });
+  handlers.get('secondary-subtitle-track-change')?.({ sid: 4 });
+  handlers.get('secondary-subtitle-delay-change')?.({ delay: 0.5 });
   handlers.get('subtitle-track-list-change')?.({ trackList: [] });
   handlers.get('media-path-change')?.({ path: '/tmp/video.mkv' });
   handlers.get('media-path-change')?.({ path: '' });
@@ -86,6 +91,8 @@ test('main mpv event binder wires callbacks through to runtime deps', () => {
   assert.equal(calls.includes('broadcast-sub:line'), true);
   assert.ok(calls.includes('subtitle-change:line'));
   assert.ok(calls.includes('subtitle-track-change'));
+  assert.ok(calls.includes('secondary-subtitle-track-change'));
+  assert.ok(calls.includes('secondary-subtitle-delay-change:0.5'));
   assert.ok(calls.includes('subtitle-track-list-change'));
   assert.ok(calls.includes('media-title:Episode 1'));
   assert.ok(calls.includes('media-path:/tmp/video.mkv'));

src/main/runtime/mpv-main-event-bindings.ts

@@ -54,6 +54,8 @@ export function createBindMpvMainEventHandlersHandler(deps: {
   broadcastSubtitleAss: (text: string) => void;
   broadcastSecondarySubtitle: (text: string) => void;
   onSubtitleTrackChange?: (sid: number | null) => void;
+  onSecondarySubtitleTrackChange?: (sid: number | null) => void;
+  onSecondarySubtitleDelayChange?: (delay: number) => void;
   onSubtitleTrackListChange?: (trackList: unknown[] | null) => void;
 
   updateCurrentMediaPath: (path: string) => void;
@@ -189,6 +191,8 @@ export function createBindMpvMainEventHandlersHandler(deps: {
       onSubtitleAssChange: handleMpvSubtitleAssChange,
       onSecondarySubtitleChange: handleMpvSecondarySubtitleChange,
       onSubtitleTrackChange: ({ sid }) => deps.onSubtitleTrackChange?.(sid),
+      onSecondarySubtitleTrackChange: ({ sid }) => deps.onSecondarySubtitleTrackChange?.(sid),
+      onSecondarySubtitleDelayChange: ({ delay }) => deps.onSecondarySubtitleDelayChange?.(delay),
       onSubtitleTrackListChange: ({ trackList }) => deps.onSubtitleTrackListChange?.(trackList),
       onSubtitleTiming: handleMpvSubtitleTiming,
       onMediaPathChange: handleMpvMediaPathChange,

src/main/runtime/mpv-main-event-main-deps.test.ts

@@ -47,6 +47,9 @@ test('mpv main event main deps map app state updates and delegate callbacks', as
     logSubtitleTimingError: (message) => calls.push(`subtitle-error:${message}`),
     broadcastToOverlayWindows: (channel, payload) =>
       calls.push(`broadcast:${channel}:${String(payload)}`),
+    onSecondarySubtitleChange: (text) => calls.push(`secondary:${text}`),
+    onSecondarySubtitleTrackChange: (sid) => calls.push(`secondary-track:${String(sid)}`),
+    onSecondarySubtitleDelayChange: (delay) => calls.push(`secondary-delay:${delay}`),
     onSubtitleChange: (text) => calls.push(`subtitle-change:${text}`),
     ensureImmersionTrackerInitialized: () => calls.push('ensure-immersion'),
     updateCurrentMediaPath: (path) => calls.push(`path:${path}`),
@@ -86,6 +89,8 @@ test('mpv main event main deps map app state updates and delegate callbacks', as
   deps.setCurrentSubAssText('ass');
   deps.broadcastSubtitleAss('ass');
   deps.broadcastSecondarySubtitle('sec');
+  deps.onSecondarySubtitleTrackChange?.(4);
+  deps.onSecondarySubtitleDelayChange?.(0.5);
   deps.updateCurrentMediaPath('/tmp/video');
   deps.restoreMpvSubVisibility();
   deps.resetSubtitleSidebarEmbeddedLayout();
@@ -116,6 +121,10 @@ test('mpv main event main deps map app state updates and delegate callbacks', as
   assert.ok(calls.includes('sync-overlay-mpv-sub'));
   assert.ok(calls.includes('anilist-post-watch'));
   assert.ok(calls.includes('timing:y:secondary'));
+  assert.ok(calls.includes('secondary:sec'));
+  assert.ok(calls.includes('secondary-track:4'));
+  assert.ok(calls.includes('secondary-delay:0.5'));
+  assert.ok(!calls.includes('broadcast:secondary-subtitle:set:sec'));
   assert.ok(calls.includes('ensure-immersion'));
   assert.ok(calls.includes('sync-immersion'));
   assert.ok(calls.includes('autoplay:/tmp/video'));

src/main/runtime/mpv-main-event-main-deps.ts

@@ -53,11 +53,14 @@ export function createBuildBindMpvMainEventHandlersMainDepsHandler(deps: {
   recordAnilistMediaDuration?: (durationSec: number) => void;
   logSubtitleTimingError: (message: string, error: unknown) => void;
   broadcastToOverlayWindows: (channel: string, payload: unknown) => void;
+  onSecondarySubtitleChange?: (text: string) => void;
   getImmediateSubtitlePayload?: (text: string) => SubtitleData | null;
   emitImmediateSubtitle?: (payload: SubtitleData) => void;
   onSubtitleChange: (text: string) => void;
   logSubtitleProcessingDebug?: (message: string) => void;
   onSubtitleTrackChange?: (sid: number | null) => void;
+  onSecondarySubtitleTrackChange?: (sid: number | null) => void;
+  onSecondarySubtitleDelayChange?: (delay: number) => void;
   onSubtitleTrackListChange?: (trackList: unknown[] | null) => void;
   updateCurrentMediaPath: (path: string) => void;
   restoreMpvSubVisibility: () => void;
@@ -173,6 +176,12 @@ export function createBuildBindMpvMainEventHandlersMainDepsHandler(deps: {
       immersionLineDedupGate.reset();
       deps.onSubtitleTrackChange?.(sid);
     },
+    onSecondarySubtitleTrackChange: deps.onSecondarySubtitleTrackChange
+      ? (sid: number | null) => deps.onSecondarySubtitleTrackChange!(sid)
+      : undefined,
+    onSecondarySubtitleDelayChange: deps.onSecondarySubtitleDelayChange
+      ? (delay: number) => deps.onSecondarySubtitleDelayChange!(delay)
+      : undefined,
     onSubtitleTrackListChange: deps.onSubtitleTrackListChange
       ? (trackList: unknown[] | null) => deps.onSubtitleTrackListChange!(trackList)
       : undefined,
@@ -182,8 +191,13 @@ export function createBuildBindMpvMainEventHandlersMainDepsHandler(deps: {
     },
     broadcastSubtitleAss: (text: string) =>
       deps.broadcastToOverlayWindows('subtitle-ass:set', text),
-    broadcastSecondarySubtitle: (text: string) =>
-      deps.broadcastToOverlayWindows('secondary-subtitle:set', text),
+    broadcastSecondarySubtitle: (text: string) => {
+      if (deps.onSecondarySubtitleChange) {
+        deps.onSecondarySubtitleChange(text);
+        return;
+      }
+      deps.broadcastToOverlayWindows('secondary-subtitle:set', text);
+    },
     updateCurrentMediaPath: (path: string) => {
       immersionLineDedupGate.reset();
       deps.updateCurrentMediaPath(path);

src/main/runtime/secondary-subtitle-track.test.ts

@@ -0,0 +1,250 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { parseSubtitleCues } from '../../core/services/subtitle-cue-parser';
+import {
+  createSecondarySubtitleTrackController,
+  findActiveSubtitleText,
+} from './secondary-subtitle-track';
+
+test('findActiveSubtitleText combines unique simultaneous parsed cues', () => {
+  assert.equal(
+    findActiveSubtitleText(
+      [
+        { startTime: 1, endTime: 3, text: 'Your' },
+        { startTime: 1, endTime: 3, text: 'Your' },
+        { startTime: 1, endTime: 3, text: 'mosaic' },
+      ],
+      2,
+    ),
+    'Your\nmosaic',
+  );
+});
+
+test('secondary track controller parses the selected ASS file before publishing', async () => {
+  const broadcasts: string[] = [];
+  let currentText = '';
+  const resolverInputs: Array<{ allowSelectedFallback?: boolean }> = [];
+  const ass = `[Events]
+Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
+Dialogue: 0,0:00:01.00,0:00:03.00,Sign,,0,0,0,,Your
+Dialogue: 1,0:00:01.00,0:00:03.00,Sign,,0,0,0,,Your
+Dialogue: 2,0:00:01.00,0:00:03.00,Sign,,0,0,0,,Your
+Dialogue: 3,0:00:01.00,0:00:03.00,Sign,,0,0,0,,Your
+Dialogue: 4,0:00:01.00,0:00:03.00,Sign,,0,0,0,,mosaic`;
+  const controller = createSecondarySubtitleTrackController({
+    getMpvClient: () => ({
+      connected: true,
+      requestProperty: async (name) => {
+        if (name === 'secondary-sid') return 2;
+        if (name === 'track-list') return [{ type: 'sub', id: 2 }];
+        if (name === 'path') return '/media/video.mkv';
+        if (name === 'secondary-sub-delay') return 0;
+        return null;
+      },
+    }),
+    getCurrentTimePos: () => 2,
+    resolveSubtitleSource: async (input) => {
+      resolverInputs.push(input);
+      return { path: '/subs/english.ass', sourceKey: '/subs/english.ass' };
+    },
+    loadSubtitleSourceText: async () => ass,
+    parseSubtitleCues,
+    setCurrentSecondaryText: (text) => {
+      currentText = text;
+    },
+    broadcastSecondaryText: (text) => broadcasts.push(text),
+  });
+
+  await controller.refresh();
+  controller.handleLiveText('Your\nYour\nYour\nYour\nmosaic');
+
+  assert.equal(resolverInputs[0]?.allowSelectedFallback, false);
+  assert.equal(currentText, 'Your\nmosaic');
+  assert.deepEqual(broadcasts, ['Your\nmosaic']);
+});
+
+test('secondary track controller follows parsed cue timing and subtitle delay', async () => {
+  const broadcasts: string[] = [];
+  let time = 2.25;
+  const controller = createSecondarySubtitleTrackController({
+    getMpvClient: () => ({
+      connected: true,
+      requestProperty: async (name) => {
+        if (name === 'secondary-sid') return 2;
+        if (name === 'track-list') return [{ type: 'sub', id: 2 }];
+        if (name === 'path') return '/media/video.mkv';
+        if (name === 'secondary-sub-delay') return 0.5;
+        return null;
+      },
+    }),
+    getCurrentTimePos: () => time,
+    resolveSubtitleSource: async () => ({ path: '/subs/english.srt', sourceKey: 'english' }),
+    loadSubtitleSourceText: async () => '',
+    parseSubtitleCues: () => [
+      { startTime: 1, endTime: 2, text: 'first' },
+      { startTime: 2, endTime: 3, text: 'second' },
+    ],
+    setCurrentSecondaryText: () => {},
+    broadcastSecondaryText: (text) => broadcasts.push(text),
+  });
+
+  await controller.refresh();
+  controller.handleDelayChange(0);
+  time = 3.25;
+  controller.handleTimePos(time);
+
+  assert.deepEqual(broadcasts, ['first', 'second', '']);
+});
+
+test('secondary track controller clears old parsed text immediately on a track change', async () => {
+  const broadcasts: string[] = [];
+  let currentText = '';
+  const controller = createSecondarySubtitleTrackController({
+    getMpvClient: () => ({
+      connected: true,
+      requestProperty: async (name) => {
+        if (name === 'secondary-sid') return 2;
+        if (name === 'track-list') return [{ type: 'sub', id: 2, external: true }];
+        if (name === 'path') return '/media/video.mkv';
+        if (name === 'secondary-sub-delay') return 0;
+        return null;
+      },
+    }),
+    getCurrentTimePos: () => 2,
+    resolveSubtitleSource: async () => ({ path: '/subs/old.ass', sourceKey: 'old' }),
+    loadSubtitleSourceText: async () => '',
+    parseSubtitleCues: () => [{ startTime: 1, endTime: 3, text: 'old parsed text' }],
+    setCurrentSecondaryText: (text) => {
+      currentText = text;
+    },
+    broadcastSecondaryText: (text) => broadcasts.push(text),
+  });
+
+  await controller.refresh();
+  controller.handleTrackChange();
+  controller.handleLiveText('new live text');
+
+  assert.equal(currentText, 'new live text');
+  assert.deepEqual(broadcasts, ['old parsed text', '', 'new live text']);
+});
+
+test('secondary track controller falls back to live mpv text without a readable source', async () => {
+  const broadcasts: string[] = [];
+  let currentText = '';
+  const controller = createSecondarySubtitleTrackController({
+    getMpvClient: () => ({
+      connected: true,
+      requestProperty: async (name) => {
+        if (name === 'secondary-sid') return 'no';
+        if (name === 'path') return '/media/video.mkv';
+        return null;
+      },
+    }),
+    getCurrentTimePos: () => 2,
+    resolveSubtitleSource: async () => null,
+    loadSubtitleSourceText: async () => '',
+    parseSubtitleCues: () => [],
+    setCurrentSecondaryText: (text) => {
+      currentText = text;
+    },
+    broadcastSecondaryText: (text) => broadcasts.push(text),
+  });
+
+  controller.handleLiveText('live fallback');
+  await controller.refresh();
+
+  assert.equal(currentText, 'live fallback');
+  assert.deepEqual(broadcasts, ['live fallback']);
+});
+
+test('secondary track controller reuses parsed cues for an unchanged embedded track', async () => {
+  let resolveCalls = 0;
+  let parseCalls = 0;
+  const controller = createSecondarySubtitleTrackController({
+    getMpvClient: () => ({
+      connected: true,
+      requestProperty: async (name) => {
+        if (name === 'secondary-sid') return 2;
+        if (name === 'track-list') {
+          return [{ type: 'sub', id: 2, external: false, 'ff-index': 3 }];
+        }
+        if (name === 'path') return '/media/video.mkv';
+        if (name === 'secondary-sub-delay') return 0;
+        return null;
+      },
+    }),
+    getCurrentTimePos: () => 2,
+    resolveSubtitleSource: async () => {
+      resolveCalls += 1;
+      return { path: `/tmp/extracted-${resolveCalls}.ass`, sourceKey: 'embedded-track-2' };
+    },
+    loadSubtitleSourceText: async () => '',
+    parseSubtitleCues: () => {
+      parseCalls += 1;
+      return [{ startTime: 1, endTime: 3, text: 'parsed' }];
+    },
+    setCurrentSecondaryText: () => {},
+    broadcastSecondaryText: () => {},
+  });
+
+  await controller.refresh();
+  await controller.refresh();
+
+  assert.equal(resolveCalls, 1);
+  assert.equal(parseCalls, 1);
+});
+
+test('secondary track controller ignores and cleans up a refresh invalidated by reset', async () => {
+  const broadcasts: string[] = [];
+  let notifyResolveStarted: (() => void) | undefined;
+  let releaseResolve: (() => void) | undefined;
+  let cleanupCalls = 0;
+  let parseCalls = 0;
+  const resolveStarted = new Promise<void>((resolve) => {
+    notifyResolveStarted = resolve;
+  });
+  const resolveGate = new Promise<void>((resolve) => {
+    releaseResolve = resolve;
+  });
+  const controller = createSecondarySubtitleTrackController({
+    getMpvClient: () => ({
+      connected: true,
+      requestProperty: async (name) => {
+        if (name === 'secondary-sid') return 2;
+        if (name === 'track-list') return [{ type: 'sub', id: 2, external: true }];
+        if (name === 'path') return '/media/video.mkv';
+        if (name === 'secondary-sub-delay') return 0;
+        return null;
+      },
+    }),
+    getCurrentTimePos: () => 2,
+    resolveSubtitleSource: async () => {
+      notifyResolveStarted?.();
+      await resolveGate;
+      return {
+        path: '/subs/secondary.ass',
+        sourceKey: 'secondary',
+        cleanup: async () => {
+          cleanupCalls += 1;
+        },
+      };
+    },
+    loadSubtitleSourceText: async () => '',
+    parseSubtitleCues: () => {
+      parseCalls += 1;
+      return [{ startTime: 1, endTime: 3, text: 'stale' }];
+    },
+    setCurrentSecondaryText: () => {},
+    broadcastSecondaryText: (text) => broadcasts.push(text),
+  });
+
+  const refresh = controller.refresh();
+  await resolveStarted;
+  controller.reset();
+  releaseResolve?.();
+  await refresh;
+
+  assert.deepEqual(broadcasts, ['']);
+  assert.equal(parseCalls, 0);
+  assert.equal(cleanupCalls, 1);
+});

src/main/runtime/secondary-subtitle-track.ts

@@ -0,0 +1,233 @@
+import type { SubtitleCue } from '../../types/subtitle';
+
+type SecondarySubtitleMpvClient = {
+  connected?: boolean;
+  requestProperty: (name: string) => Promise<unknown>;
+};
+
+type ResolvedSubtitleSource = {
+  path: string;
+  sourceKey: string;
+  cleanup?: () => Promise<void>;
+};
+
+type SecondarySubtitleSourceInput = {
+  currentExternalFilenameRaw: unknown;
+  currentTrackRaw: unknown;
+  trackListRaw: unknown;
+  sidRaw: unknown;
+  videoPath: string;
+  allowSelectedFallback?: boolean;
+};
+
+const DEFAULT_REFRESH_DELAY_MS = 500;
+
+function finiteNumber(value: unknown, fallback = 0): number {
+  const number = typeof value === 'number' ? value : Number(value);
+  return Number.isFinite(number) ? number : fallback;
+}
+
+function trackId(value: unknown): number | null {
+  if (typeof value !== 'number' && typeof value !== 'string') return null;
+  const number = typeof value === 'number' ? value : Number(value.trim());
+  return Number.isInteger(number) ? number : null;
+}
+
+function buildSelectedTrackIdentity(
+  trackListRaw: unknown,
+  sidRaw: unknown,
+  videoPath: string,
+): string | null {
+  if (!Array.isArray(trackListRaw)) return null;
+  const sid = trackId(sidRaw);
+  if (sid === null) return null;
+
+  const selectedTrack = trackListRaw.find((entry: unknown) => {
+    if (!entry || typeof entry !== 'object') return false;
+    const track = entry as Record<string, unknown>;
+    return track.type === 'sub' && trackId(track.id) === sid;
+  }) as Record<string, unknown> | undefined;
+  if (!selectedTrack) return null;
+
+  return JSON.stringify([
+    videoPath,
+    sid,
+    selectedTrack.external === true,
+    selectedTrack['external-filename'] ?? null,
+    trackId(selectedTrack['ff-index']),
+  ]);
+}
+
+export function findActiveSubtitleText(cues: readonly SubtitleCue[], timeSeconds: number): string {
+  if (!Number.isFinite(timeSeconds)) return '';
+
+  const seen = new Set<string>();
+  const activeText: string[] = [];
+  for (const cue of cues) {
+    if (cue.startTime > timeSeconds || cue.endTime <= timeSeconds) continue;
+    const text = cue.text.trim();
+    if (!text || seen.has(text)) continue;
+    seen.add(text);
+    activeText.push(text);
+  }
+  return activeText.join('\n');
+}
+
+export function createSecondarySubtitleTrackController(deps: {
+  getMpvClient: () => SecondarySubtitleMpvClient | null;
+  getCurrentTimePos: () => number;
+  resolveSubtitleSource: (
+    input: SecondarySubtitleSourceInput,
+  ) => Promise<ResolvedSubtitleSource | null>;
+  loadSubtitleSourceText: (source: string) => Promise<string>;
+  parseSubtitleCues: (content: string, filename: string) => SubtitleCue[];
+  setCurrentSecondaryText: (text: string) => void;
+  broadcastSecondaryText: (text: string) => void;
+  logDebug?: (message: string) => void;
+  logWarn?: (message: string, error: unknown) => void;
+}) {
+  let parsedCues: SubtitleCue[] | null = null;
+  let parsedSourceKey: string | null = null;
+  let parsedTrackIdentity: string | null = null;
+  let secondaryDelaySeconds = 0;
+  let lastLiveText = '';
+  let lastBroadcastText: string | null = null;
+  let refreshGeneration = 0;
+  let refreshTimer: ReturnType<typeof setTimeout> | null = null;
+
+  const publish = (text: string): void => {
+    deps.setCurrentSecondaryText(text);
+    if (text === lastBroadcastText) return;
+    lastBroadcastText = text;
+    deps.broadcastSecondaryText(text);
+  };
+
+  const resolveAtTime = (timeSeconds: number): string => {
+    if (!parsedCues) return lastLiveText;
+    return findActiveSubtitleText(parsedCues, timeSeconds - secondaryDelaySeconds);
+  };
+
+  const useLiveFallback = (): void => {
+    parsedCues = null;
+    parsedSourceKey = null;
+    parsedTrackIdentity = null;
+    publish(lastLiveText);
+  };
+
+  const refresh = async (): Promise<void> => {
+    const generation = ++refreshGeneration;
+    const client = deps.getMpvClient();
+    if (!client?.connected) {
+      useLiveFallback();
+      return;
+    }
+
+    let resolvedSource: ResolvedSubtitleSource | null = null;
+    try {
+      const [secondarySid, trackList, videoPathRaw, secondaryDelayRaw] = await Promise.all([
+        client.requestProperty('secondary-sid').catch(() => null),
+        client.requestProperty('track-list').catch(() => null),
+        client.requestProperty('path').catch(() => null),
+        client.requestProperty('secondary-sub-delay').catch(() => 0),
+      ]);
+      if (generation !== refreshGeneration) return;
+
+      const videoPath = typeof videoPathRaw === 'string' ? videoPathRaw.trim() : '';
+      if (!videoPath || secondarySid === null || secondarySid === 'no') {
+        useLiveFallback();
+        return;
+      }
+
+      secondaryDelaySeconds = finiteNumber(secondaryDelayRaw);
+      const selectedTrackIdentity = buildSelectedTrackIdentity(trackList, secondarySid, videoPath);
+      if (selectedTrackIdentity && selectedTrackIdentity === parsedTrackIdentity && parsedCues) {
+        publish(resolveAtTime(deps.getCurrentTimePos()));
+        return;
+      }
+
+      resolvedSource = await deps.resolveSubtitleSource({
+        currentExternalFilenameRaw: null,
+        currentTrackRaw: null,
+        trackListRaw: trackList,
+        sidRaw: secondarySid,
+        videoPath,
+        allowSelectedFallback: false,
+      });
+      if (generation !== refreshGeneration) return;
+      if (!resolvedSource) {
+        deps.logDebug?.('[secondary-subtitle-track] selected source is not readable');
+        useLiveFallback();
+        return;
+      }
+
+      if (resolvedSource.sourceKey === parsedSourceKey && parsedCues) {
+        parsedTrackIdentity = selectedTrackIdentity;
+        publish(resolveAtTime(deps.getCurrentTimePos()));
+        return;
+      }
+
+      const content = await deps.loadSubtitleSourceText(resolvedSource.path);
+      const cues = deps.parseSubtitleCues(content, resolvedSource.path);
+      if (generation !== refreshGeneration) return;
+      if (cues.length === 0) {
+        deps.logDebug?.('[secondary-subtitle-track] selected source contained no parsed cues');
+        useLiveFallback();
+        return;
+      }
+
+      parsedCues = cues;
+      parsedSourceKey = resolvedSource.sourceKey;
+      parsedTrackIdentity = selectedTrackIdentity;
+      publish(resolveAtTime(deps.getCurrentTimePos()));
+    } catch (error) {
+      if (generation !== refreshGeneration) return;
+      deps.logWarn?.('[secondary-subtitle-track] failed to parse selected source', error);
+      useLiveFallback();
+    } finally {
+      await resolvedSource?.cleanup?.().catch(() => undefined);
+    }
+  };
+
+  const scheduleRefresh = (delayMs = DEFAULT_REFRESH_DELAY_MS): void => {
+    if (refreshTimer) clearTimeout(refreshTimer);
+    refreshTimer = setTimeout(() => {
+      refreshTimer = null;
+      void refresh();
+    }, delayMs);
+  };
+
+  const clearSelectedTrack = (): void => {
+    refreshGeneration += 1;
+    if (refreshTimer) clearTimeout(refreshTimer);
+    refreshTimer = null;
+    parsedCues = null;
+    parsedSourceKey = null;
+    parsedTrackIdentity = null;
+    secondaryDelaySeconds = 0;
+    lastLiveText = '';
+    publish('');
+  };
+
+  return {
+    refresh,
+    scheduleRefresh,
+    handleLiveText(text: string): void {
+      lastLiveText = text;
+      publish(resolveAtTime(deps.getCurrentTimePos()));
+    },
+    handleTimePos(timeSeconds: number): void {
+      if (!parsedCues) return;
+      publish(resolveAtTime(timeSeconds));
+    },
+    handleTrackChange(): void {
+      clearSelectedTrack();
+    },
+    handleDelayChange(delaySeconds: number): void {
+      secondaryDelaySeconds = finiteNumber(delaySeconds);
+      if (parsedCues) {
+        publish(resolveAtTime(deps.getCurrentTimePos()));
+      }
+    },
+    reset: clearSelectedTrack,
+  };
+}

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

@@ -248,3 +248,31 @@ test('subtitle source resolver logs debug when no active subtitle track is selec
   assert.equal(debugs.length, 1);
   assert.match(debugs[0]!, /\[subtitle-prefetch\].*no active subtitle track/);
 });
+
+test('subtitle source resolver does not fall back to the primary selected track for secondary', async () => {
+  const resolveSource = createResolveActiveSubtitleSidebarSourceHandler({
+    getFfmpegPath: () => 'ffmpeg',
+    extractInternalSubtitleTrack: async () => {
+      throw new Error('should not extract the primary track');
+    },
+  });
+
+  const resolved = await resolveSource({
+    currentExternalFilenameRaw: null,
+    currentTrackRaw: null,
+    trackListRaw: [
+      {
+        type: 'sub',
+        id: 1,
+        selected: true,
+        external: true,
+        'external-filename': '/subs/primary.ass',
+      },
+    ],
+    sidRaw: null,
+    videoPath: '/media/video.mkv',
+    allowSelectedFallback: false,
+  });
+
+  assert.equal(resolved, null);
+});

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

@@ -41,6 +41,7 @@ function getActiveSubtitleTrack(
   currentTrackRaw: unknown,
   trackListRaw: unknown,
   sidRaw: unknown,
+  allowSelectedFallback: boolean,
 ): MpvSubtitleTrackLike | null {
   if (currentTrackRaw && typeof currentTrackRaw === 'object') {
     const track = currentTrackRaw as MpvSubtitleTrackLike;
@@ -68,6 +69,10 @@ function getActiveSubtitleTrack(
     return bySid;
   }
 
+  if (!allowSelectedFallback) {
+    return null;
+  }
+
   return (
     (trackListRaw.find((entry: unknown) => {
       if (!entry || typeof entry !== 'object') {
@@ -94,6 +99,7 @@ export function createResolveActiveSubtitleSidebarSourceHandler(deps: {
     trackListRaw: unknown;
     sidRaw: unknown;
     videoPath: string;
+    allowSelectedFallback?: boolean;
   }): Promise<ActiveSubtitleSidebarSource | null> => {
     const currentExternalFilename =
       typeof input.currentExternalFilenameRaw === 'string'
@@ -103,7 +109,12 @@ export function createResolveActiveSubtitleSidebarSourceHandler(deps: {
       return { path: currentExternalFilename, sourceKey: currentExternalFilename };
     }
 
-    const track = getActiveSubtitleTrack(input.currentTrackRaw, input.trackListRaw, input.sidRaw);
+    const track = getActiveSubtitleTrack(
+      input.currentTrackRaw,
+      input.trackListRaw,
+      input.sidRaw,
+      input.allowSelectedFallback !== false,
+    );
     if (!track) {
       deps.logDebug?.('[subtitle-prefetch] no active subtitle track selected yet');
       return null;