fix: address coderabbit review findings

docs/architecture/2026-03-15-renderer-performance-design.md

@@ -9,7 +9,7 @@ Minimize the time between a subtitle line appearing and annotations being displa
 
 ## Current Pipeline (Warm State)
 
-```
+```text
 MPV subtitle change (0ms)
   -> IPC to main (5ms)
   -> Cache check (2ms)
@@ -25,7 +25,7 @@ MPV subtitle change (0ms)
 
 ## Target Pipeline
 
-```
+```text
 MPV subtitle change (0ms)
   -> IPC to main (5ms)
   -> Cache check (2ms)
@@ -142,7 +142,7 @@ Collapse the 4 sequential annotation passes (`applyKnownWordMarking` -> `applyFr
 
 ### Current Flow (4 passes, 4 array copies)
 
-```
+```text
 tokens (already have frequencyRank values from parser-level applyFrequencyRanks)
   -> applyKnownWordMarking()   // .map() -> new array
   -> applyFrequencyMarking()   // .map() -> new array (POS-based filtering only)

docs/superpowers/plans/2026-03-15-renderer-performance.md

@@ -17,6 +17,7 @@
 ## File Structure
 
 ### New Files
+
 | File | Responsibility |
 |------|---------------|
 | `src/core/services/subtitle-cue-parser.ts` | Parse SRT/VTT/ASS files into `SubtitleCue[]` (timing + text) |
@@ -25,6 +26,7 @@
 | `src/core/services/subtitle-prefetch.test.ts` | Tests for prefetch service |
 
 ### Modified Files
+
 | File | Change |
 |------|--------|
 | `src/core/services/subtitle-processing-controller.ts` | Add `preCacheTokenization` + `isCacheFull` to public interface |
@@ -34,6 +36,7 @@
 | `src/renderer/subtitle-render.ts` | `cloneNode` template + `replaceChildren()` |
 | `src/main.ts` | Wire up prefetch service |
 
+
 ---
 
 ## Chunk 1: Batched Annotation Pass + DOM Template Pooling
@@ -1554,17 +1557,32 @@ subtitle processing, and restarts on cache invalidation."
 
 ### Task 10: Full test suite and type check
 
-- [ ] **Step 1: Run full test suite**
+- [ ] **Step 1: Run handoff type check**
 
-Run: `bun run test`
+Run: `bun run typecheck`
-Expected: 500+ pass, 1 pre-existing fail in immersion-tracker-service.
-
-- [ ] **Step 2: Run TypeScript type check**
-
-Run: `bun run tsc`
 Expected: No type errors.
 
-- [ ] **Step 3: Review all commits on the branch**
+- [ ] **Step 2: Run fast test lane**
+
+Run: `bun run test:fast`
+Expected: Fast unit/integration tests pass.
+
+- [ ] **Step 3: Run environment-sensitive test lane**
+
+Run: `bun run test:env`
+Expected: Environment/runtime-sensitive tests pass.
+
+- [ ] **Step 4: Run production build**
+
+Run: `bun run build`
+Expected: Production build succeeds.
+
+- [ ] **Step 5: Run dist smoke test**
+
+Run: `bun run test:smoke:dist`
+Expected: Dist smoke tests pass.
+
+- [ ] **Step 6: Review all commits on the branch**
 
 Run: `git log --oneline main..HEAD`
 Expected: ~8 focused commits, one per logical change.

src/core/services/mpv-properties.ts

@@ -63,6 +63,8 @@ const MPV_SUBTITLE_PROPERTY_OBSERVATIONS: string[] = [
   'media-title',
   'secondary-sub-visibility',
   'sub-visibility',
+  'sid',
+  'track-list',
 ];
 
 const MPV_INITIAL_PROPERTY_REQUESTS: Array<MpvProtocolCommand> = [

src/core/services/mpv-protocol.test.ts

@@ -60,6 +60,8 @@ function createDeps(overrides: Partial<MpvProtocolHandleMessageDeps> = {}): {
       emitSubtitleAssChange: (payload) => state.events.push(payload),
       emitSubtitleTiming: (payload) => state.events.push(payload),
       emitSecondarySubtitleChange: (payload) => state.events.push(payload),
+      emitSubtitleTrackChange: (payload) => state.events.push(payload),
+      emitSubtitleTrackListChange: (payload) => state.events.push(payload),
       getCurrentSubText: () => state.subText,
       setCurrentSubText: (text) => {
         state.subText = text;
@@ -120,6 +122,24 @@ test('dispatchMpvProtocolMessage emits subtitle text on property change', async
   assert.deepEqual(state.events, [{ text: '字幕', isOverlayVisible: false }]);
 });
 
+test('dispatchMpvProtocolMessage emits subtitle track changes', async () => {
+  const { deps, state } = createDeps({
+    emitSubtitleTrackChange: (payload) => state.events.push(payload),
+    emitSubtitleTrackListChange: (payload) => state.events.push(payload),
+  });
+
+  await dispatchMpvProtocolMessage(
+    { event: 'property-change', name: 'sid', data: '3' },
+    deps,
+  );
+  await dispatchMpvProtocolMessage(
+    { event: 'property-change', name: 'track-list', data: [{ type: 'sub', id: 3 }] },
+    deps,
+  );
+
+  assert.deepEqual(state.events, [{ sid: 3 }, { trackList: [{ type: 'sub', id: 3 }] }]);
+});
+
 test('dispatchMpvProtocolMessage enforces sub-visibility hidden when overlay suppression is enabled', async () => {
   const { deps, state } = createDeps({
     isVisibleOverlayVisible: () => true,

src/core/services/mpv-protocol.ts

@@ -52,6 +52,8 @@ export interface MpvProtocolHandleMessageDeps {
   emitSubtitleAssChange: (payload: { text: string }) => void;
   emitSubtitleTiming: (payload: { text: string; start: number; end: number }) => void;
   emitSecondarySubtitleChange: (payload: { text: string }) => void;
+  emitSubtitleTrackChange: (payload: { sid: number | null }) => void;
+  emitSubtitleTrackListChange: (payload: { trackList: unknown[] | null }) => void;
   getCurrentSubText: () => string;
   setCurrentSubText: (text: string) => void;
   setCurrentSubStart: (value: number) => void;
@@ -160,6 +162,18 @@ export async function dispatchMpvProtocolMessage(
       const nextSubText = (msg.data as string) || '';
       deps.setCurrentSecondarySubText(nextSubText);
       deps.emitSecondarySubtitleChange({ text: nextSubText });
+    } else if (msg.name === 'sid') {
+      const sid =
+        typeof msg.data === 'number'
+          ? msg.data
+          : typeof msg.data === 'string'
+            ? Number(msg.data)
+            : null;
+      deps.emitSubtitleTrackChange({ sid: sid !== null && Number.isFinite(sid) ? sid : null });
+    } else if (msg.name === 'track-list') {
+      deps.emitSubtitleTrackListChange({
+        trackList: Array.isArray(msg.data) ? (msg.data as unknown[]) : null,
+      });
     } else if (msg.name === 'aid') {
       deps.setCurrentAudioTrackId(typeof msg.data === 'number' ? (msg.data as number) : null);
       deps.syncCurrentAudioStreamIndex();

src/core/services/mpv.ts

@@ -118,6 +118,8 @@ export interface MpvIpcClientEventMap {
   'duration-change': { duration: number };
   'pause-change': { paused: boolean };
   'secondary-subtitle-change': { text: string };
+  'subtitle-track-change': { sid: number | null };
+  'subtitle-track-list-change': { trackList: unknown[] | null };
   'media-path-change': { path: string };
   'media-title-change': { title: string | null };
   'subtitle-metrics-change': { patch: Partial<MpvSubtitleRenderMetrics> };
@@ -325,6 +327,12 @@ export class MpvIpcClient implements MpvClient {
       emitSecondarySubtitleChange: (payload) => {
         this.emit('secondary-subtitle-change', payload);
       },
+      emitSubtitleTrackChange: (payload) => {
+        this.emit('subtitle-track-change', payload);
+      },
+      emitSubtitleTrackListChange: (payload) => {
+        this.emit('subtitle-track-list-change', payload);
+      },
       getCurrentSubText: () => this.currentSubText,
       setCurrentSubText: (text: string) => {
         this.currentSubText = text;

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

@@ -186,6 +186,21 @@ test('parseAssCues handles hour timestamps', () => {
   assert.equal(cues[0]!.endTime, 5405.0);
 });
 
+test('parseAssCues respects dynamic field ordering from the Format row', () => {
+  const content = [
+    '[Events]',
+    'Format: Layer, Style, Start, End, Name, MarginL, MarginR, MarginV, Effect, Text',
+    'Dialogue: 0,Default,0:00:01.00,0:00:04.00,,0,0,0,,順番が違う',
+  ].join('\n');
+
+  const cues = parseAssCues(content);
+
+  assert.equal(cues.length, 1);
+  assert.equal(cues[0]!.startTime, 1.0);
+  assert.equal(cues[0]!.endTime, 4.0);
+  assert.equal(cues[0]!.text, '順番が違う');
+});
+
 test('parseSubtitleCues auto-detects SRT format', () => {
   const content = [
     '1',
@@ -244,3 +259,16 @@ test('parseSubtitleCues returns cues sorted by start time', () => {
   assert.equal(cues[0]!.text, '一番目');
   assert.equal(cues[1]!.text, '二番目');
 });
+
+test('parseSubtitleCues detects subtitle formats from remote URLs', () => {
+  const assContent = [
+    '[Events]',
+    'Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text',
+    'Dialogue: 0,0:00:01.00,0:00:02.00,Default,,0,0,0,,URLテスト',
+  ].join('\n');
+
+  const cues = parseSubtitleCues(assContent, 'https://host/subs.ass?lang=ja#track');
+
+  assert.equal(cues.length, 1);
+  assert.equal(cues[0]!.text, 'URLテスト');
+});

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

@@ -56,6 +56,8 @@ export function parseSrtCues(content: string): SubtitleCue[] {
 const ASS_OVERRIDE_TAG_PATTERN = /\{[^}]*\}/g;
 
 const ASS_TIMING_PATTERN = /^(\d+):(\d{2}):(\d{2})\.(\d{1,2})$/;
+const ASS_FORMAT_PREFIX = 'Format:';
+const ASS_DIALOGUE_PREFIX = 'Dialogue:';
 
 function parseAssTimestamp(raw: string): number | null {
   const match = ASS_TIMING_PATTERN.exec(raw.trim());
@@ -73,12 +75,20 @@ export function parseAssCues(content: string): SubtitleCue[] {
   const cues: SubtitleCue[] = [];
   const lines = content.split(/\r?\n/);
   let inEventsSection = false;
+  let startFieldIndex = -1;
+  let endFieldIndex = -1;
+  let textFieldIndex = -1;
 
   for (const line of lines) {
     const trimmed = line.trim();
 
     if (trimmed.startsWith('[') && trimmed.endsWith(']')) {
       inEventsSection = trimmed.toLowerCase() === '[events]';
+      if (!inEventsSection) {
+        startFieldIndex = -1;
+        endFieldIndex = -1;
+        textFieldIndex = -1;
+      }
       continue;
     }
 
@@ -86,34 +96,45 @@ export function parseAssCues(content: string): SubtitleCue[] {
       continue;
     }
 
-    if (!trimmed.startsWith('Dialogue:')) {
+    if (trimmed.startsWith(ASS_FORMAT_PREFIX)) {
+      const formatFields = trimmed
+        .slice(ASS_FORMAT_PREFIX.length)
+        .split(',')
+        .map((field) => field.trim().toLowerCase());
+      startFieldIndex = formatFields.indexOf('start');
+      endFieldIndex = formatFields.indexOf('end');
+      textFieldIndex = formatFields.indexOf('text');
       continue;
     }
 
-    // Split on first 9 commas (ASS v4+ has 10 fields; last is Text which can contain commas)
+    if (!trimmed.startsWith(ASS_DIALOGUE_PREFIX)) {
-    const afterPrefix = trimmed.slice('Dialogue:'.length);
-    const fields: string[] = [];
-    let remaining = afterPrefix;
-    for (let fieldIndex = 0; fieldIndex < 9; fieldIndex += 1) {
-      const commaIndex = remaining.indexOf(',');
-      if (commaIndex < 0) {
-        break;
-      }
-      fields.push(remaining.slice(0, commaIndex));
-      remaining = remaining.slice(commaIndex + 1);
-    }
-
-    if (fields.length < 9) {
       continue;
     }
 
-    const startTime = parseAssTimestamp(fields[1]!);
+    if (startFieldIndex < 0 || endFieldIndex < 0 || textFieldIndex < 0) {
-    const endTime = parseAssTimestamp(fields[2]!);
+      continue;
+    }
+
+    const fields = trimmed.slice(ASS_DIALOGUE_PREFIX.length).split(',');
+    if (
+      startFieldIndex >= fields.length ||
+      endFieldIndex >= fields.length ||
+      textFieldIndex >= fields.length
+    ) {
+      continue;
+    }
+
+    const startTime = parseAssTimestamp(fields[startFieldIndex]!);
+    const endTime = parseAssTimestamp(fields[endFieldIndex]!);
     if (startTime === null || endTime === null) {
       continue;
     }
 
-    const rawText = remaining.replace(ASS_OVERRIDE_TAG_PATTERN, '').trim();
+    const rawText = fields
+      .slice(textFieldIndex)
+      .join(',')
+      .replace(ASS_OVERRIDE_TAG_PATTERN, '')
+      .trim();
     if (rawText) {
       cues.push({ startTime, endTime, text: rawText });
     }
@@ -122,8 +143,15 @@ export function parseAssCues(content: string): SubtitleCue[] {
   return cues;
 }
 
-function detectSubtitleFormat(filename: string): 'srt' | 'vtt' | 'ass' | 'ssa' | null {
+function detectSubtitleFormat(source: string): 'srt' | 'vtt' | 'ass' | 'ssa' | null {
-  const ext = filename.split('.').pop()?.toLowerCase() ?? '';
+  const normalizedSource = (() => {
+    try {
+      return /^[a-z]+:\/\//i.test(source) ? new URL(source).pathname : source;
+    } catch {
+      return source;
+    }
+  })().split(/[?#]/, 1)[0];
+  const ext = normalizedSource.split('.').pop()?.toLowerCase() ?? '';
   if (ext === 'srt') return 'srt';
   if (ext === 'vtt') return 'vtt';
   if (ext === 'ass' || ext === 'ssa') return 'ass';

src/core/services/subtitle-prefetch.test.ts

@@ -173,6 +173,29 @@ test('prefetch service onSeek re-prioritizes from new position', async () => {
   assert.ok(hasPostSeekCue, 'Should have cached cues after seek position');
 });
 
+test('prefetch service still warms the priority window when cache is full', async () => {
+  const cues = makeCues(20);
+  const cachedTexts: string[] = [];
+
+  const service = createSubtitlePrefetchService({
+    cues,
+    tokenizeSubtitle: async (text) => ({ text, tokens: [] }),
+    preCacheTokenization: (text) => {
+      cachedTexts.push(text);
+    },
+    isCacheFull: () => true,
+    priorityWindowSize: 3,
+  });
+
+  service.start(0);
+  for (let i = 0; i < 10; i += 1) {
+    await flushMicrotasks();
+  }
+  service.stop();
+
+  assert.deepEqual(cachedTexts.slice(0, 3), ['line-0', 'line-1', 'line-2']);
+});
+
 test('prefetch service pause/resume halts and continues tokenization', async () => {
   const cues = makeCues(20);
   let tokenizeCalls = 0;

src/core/services/subtitle-prefetch.ts

@@ -58,6 +58,7 @@ export function createSubtitlePrefetchService(
   async function tokenizeCueList(
     cuesToProcess: SubtitleCue[],
     runId: number,
+    options: { allowWhenCacheFull?: boolean } = {},
   ): Promise<void> {
     for (const cue of cuesToProcess) {
       if (stopped || runId !== currentRunId) {
@@ -73,7 +74,7 @@ export function createSubtitlePrefetchService(
         return;
       }
 
-      if (deps.isCacheFull()) {
+      if (!options.allowWhenCacheFull && deps.isCacheFull()) {
         return;
       }
 
@@ -96,7 +97,7 @@ export function createSubtitlePrefetchService(
 
     // Phase 1: Priority window
     const priorityCues = computePriorityWindow(cues, currentTimeSeconds, windowSize);
-    await tokenizeCueList(priorityCues, runId);
+    await tokenizeCueList(priorityCues, runId, { allowWhenCacheFull: true });
 
     if (stopped || runId !== currentRunId) {
       return;

src/main.ts

@@ -421,6 +421,10 @@ import { resolveConfigDir } from './config/path-resolution';
 import { parseSubtitleCues } from './core/services/subtitle-cue-parser';
 import { createSubtitlePrefetchService } from './core/services/subtitle-prefetch';
 import type { SubtitlePrefetchService } from './core/services/subtitle-prefetch';
+import {
+  getActiveExternalSubtitleSource,
+  resolveSubtitleSourcePath,
+} from './main/runtime/subtitle-prefetch-source';
 
 if (process.platform === 'linux') {
   app.commandLine.appendSwitch('enable-features', 'GlobalShortcutsPortal');
@@ -1077,9 +1081,17 @@ const subtitleProcessingController = createSubtitleProcessingController(
 );
 
 let subtitlePrefetchService: SubtitlePrefetchService | null = null;
+let subtitlePrefetchRefreshTimer: ReturnType<typeof setTimeout> | null = null;
 let lastObservedTimePos = 0;
 const SEEK_THRESHOLD_SECONDS = 3;
 
+function clearScheduledSubtitlePrefetchRefresh(): void {
+  if (subtitlePrefetchRefreshTimer) {
+    clearTimeout(subtitlePrefetchRefreshTimer);
+    subtitlePrefetchRefreshTimer = null;
+  }
+}
+
 async function initSubtitlePrefetch(
   externalFilename: string,
   currentTimePos: number,
@@ -1111,6 +1123,37 @@ async function initSubtitlePrefetch(
   }
 }
 
+async function refreshSubtitlePrefetchFromActiveTrack(): Promise<void> {
+  const client = appState.mpvClient;
+  if (!client?.connected) {
+    return;
+  }
+
+  try {
+    const [trackListRaw, sidRaw] = await Promise.all([
+      client.requestProperty('track-list'),
+      client.requestProperty('sid'),
+    ]);
+    const externalFilename = getActiveExternalSubtitleSource(trackListRaw, sidRaw);
+    if (!externalFilename) {
+      subtitlePrefetchService?.stop();
+      subtitlePrefetchService = null;
+      return;
+    }
+    await initSubtitlePrefetch(externalFilename, lastObservedTimePos);
+  } catch {
+    // Track list query failed; skip subtitle prefetch refresh.
+  }
+}
+
+function scheduleSubtitlePrefetchRefresh(delayMs = 0): void {
+  clearScheduledSubtitlePrefetchRefresh();
+  subtitlePrefetchRefreshTimer = setTimeout(() => {
+    subtitlePrefetchRefreshTimer = null;
+    void refreshSubtitlePrefetchFromActiveTrack();
+  }, delayMs);
+}
+
 const overlayShortcutsRuntime = createOverlayShortcutsRuntimeService(
   createBuildOverlayShortcutsRuntimeMainDepsHandler({
     getConfiguredShortcuts: () => getConfiguredShortcuts(),
@@ -2896,42 +2939,13 @@ const {
       autoPlayReadySignalMediaPath = null;
       currentMediaTokenizationGate.updateCurrentMediaPath(path);
       startupOsdSequencer.reset();
+      clearScheduledSubtitlePrefetchRefresh();
       subtitlePrefetchService?.stop();
       subtitlePrefetchService = null;
       if (path) {
         ensureImmersionTrackerStarted();
-        // Attempt to initialize subtitle prefetch for external subtitle tracks.
         // Delay slightly to allow MPV's track-list to be populated.
-        setTimeout(() => {
+        scheduleSubtitlePrefetchRefresh(500);
-          const client = appState.mpvClient;
-          if (!client?.connected) return;
-          void (async () => {
-            try {
-              const [trackListRaw, sidRaw] = await Promise.all([
-                client.requestProperty('track-list'),
-                client.requestProperty('sid'),
-              ]);
-              if (!Array.isArray(trackListRaw) || sidRaw == null) return;
-              const sid = typeof sidRaw === 'number' ? sidRaw : typeof sidRaw === 'string' ? Number(sidRaw) : null;
-              if (sid == null || !Number.isFinite(sid)) return;
-              const activeTrack = trackListRaw.find(
-                (entry: unknown) => {
-                  if (!entry || typeof entry !== 'object') return false;
-                  const t = entry as Record<string, unknown>;
-                  return t.type === 'sub' && t.id === sid && t.external === true;
-                },
-              ) as Record<string, unknown> | undefined;
-              if (!activeTrack) return;
-              const externalFilename = typeof activeTrack['external-filename'] === 'string'
-                ? (activeTrack['external-filename'] as string).trim()
-                : '';
-              if (!externalFilename) return;
-              void initSubtitlePrefetch(externalFilename, lastObservedTimePos);
-            } catch {
-              // Track list query failed — not critical, skip prefetch.
-            }
-          })();
-        }, 500);
       }
       mediaRuntime.updateCurrentMediaPath(path);
     },
@@ -2982,6 +2996,12 @@ const {
       }
       lastObservedTimePos = time;
     },
+    onSubtitleTrackChange: () => {
+      scheduleSubtitlePrefetchRefresh();
+    },
+    onSubtitleTrackListChange: () => {
+      scheduleSubtitlePrefetchRefresh();
+    },
     updateSubtitleRenderMetrics: (patch) => {
       updateMpvSubtitleRenderMetrics(patch as Partial<MpvSubtitleRenderMetrics>);
     },
@@ -3590,7 +3610,7 @@ async function loadSubtitleSourceText(source: string): Promise<string> {
     }
   }
 
-  const filePath = source.startsWith('file://') ? decodeURI(new URL(source).pathname) : source;
+  const filePath = resolveSubtitleSourcePath(source);
   return fs.promises.readFile(filePath, 'utf8');
 }
 

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

@@ -71,6 +71,8 @@ test('mpv event bindings register all expected events', () => {
     onSubtitleChange: () => {},
     onSubtitleAssChange: () => {},
     onSecondarySubtitleChange: () => {},
+    onSubtitleTrackChange: () => {},
+    onSubtitleTrackListChange: () => {},
     onSubtitleTiming: () => {},
     onMediaPathChange: () => {},
     onMediaTitleChange: () => {},
@@ -92,6 +94,8 @@ test('mpv event bindings register all expected events', () => {
     'subtitle-change',
     'subtitle-ass-change',
     'secondary-subtitle-change',
+    'subtitle-track-change',
+    'subtitle-track-list-change',
     'subtitle-timing',
     'media-path-change',
     'media-title-change',

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

@@ -3,6 +3,8 @@ type MpvBindingEventName =
   | 'subtitle-change'
   | 'subtitle-ass-change'
   | 'secondary-subtitle-change'
+  | 'subtitle-track-change'
+  | 'subtitle-track-list-change'
   | 'subtitle-timing'
   | 'media-path-change'
   | 'media-title-change'
@@ -69,6 +71,8 @@ export function createBindMpvClientEventHandlers(deps: {
   onSubtitleChange: (payload: { text: string }) => void;
   onSubtitleAssChange: (payload: { text: string }) => void;
   onSecondarySubtitleChange: (payload: { text: string }) => void;
+  onSubtitleTrackChange: (payload: { sid: number | null }) => void;
+  onSubtitleTrackListChange: (payload: { trackList: unknown[] | null }) => void;
   onSubtitleTiming: (payload: { text: string; start: number; end: number }) => void;
   onMediaPathChange: (payload: { path: string | null }) => void;
   onMediaTitleChange: (payload: { title: string | null }) => void;
@@ -83,6 +87,8 @@ export function createBindMpvClientEventHandlers(deps: {
     mpvClient.on('subtitle-change', deps.onSubtitleChange);
     mpvClient.on('subtitle-ass-change', deps.onSubtitleAssChange);
     mpvClient.on('secondary-subtitle-change', deps.onSecondarySubtitleChange);
+    mpvClient.on('subtitle-track-change', deps.onSubtitleTrackChange);
+    mpvClient.on('subtitle-track-list-change', deps.onSubtitleTrackListChange);
     mpvClient.on('subtitle-timing', deps.onSubtitleTiming);
     mpvClient.on('media-path-change', deps.onMediaPathChange);
     mpvClient.on('media-title-change', deps.onMediaTitleChange);

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

@@ -34,6 +34,8 @@ test('main mpv event binder wires callbacks through to runtime deps', () => {
     setCurrentSubAssText: (text) => calls.push(`set-ass:${text}`),
     broadcastSubtitleAss: (text) => calls.push(`broadcast-ass:${text}`),
     broadcastSecondarySubtitle: (text) => calls.push(`broadcast-secondary:${text}`),
+    onSubtitleTrackChange: () => calls.push('subtitle-track-change'),
+    onSubtitleTrackListChange: () => calls.push('subtitle-track-list-change'),
 
     updateCurrentMediaPath: (path) => calls.push(`media-path:${path}`),
     restoreMpvSubVisibility: () => calls.push('restore-mpv-sub'),
@@ -65,6 +67,8 @@ test('main mpv event binder wires callbacks through to runtime deps', () => {
   });
 
   handlers.get('subtitle-change')?.({ text: 'line' });
+  handlers.get('subtitle-track-change')?.({ sid: 3 });
+  handlers.get('subtitle-track-list-change')?.({ trackList: [] });
   handlers.get('media-path-change')?.({ path: '' });
   handlers.get('media-title-change')?.({ title: 'Episode 1' });
   handlers.get('time-pos-change')?.({ time: 2.5 });
@@ -73,6 +77,8 @@ test('main mpv event binder wires callbacks through to runtime deps', () => {
   assert.ok(calls.includes('set-sub:line'));
   assert.ok(calls.includes('broadcast-sub:line'));
   assert.ok(calls.includes('subtitle-change:line'));
+  assert.ok(calls.includes('subtitle-track-change'));
+  assert.ok(calls.includes('subtitle-track-list-change'));
   assert.ok(calls.includes('media-title:Episode 1'));
   assert.ok(calls.includes('restore-mpv-sub'));
   assert.ok(calls.includes('reset-guess-state'));

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

@@ -42,6 +42,8 @@ export function createBindMpvMainEventHandlersHandler(deps: {
   setCurrentSubAssText: (text: string) => void;
   broadcastSubtitleAss: (text: string) => void;
   broadcastSecondarySubtitle: (text: string) => void;
+  onSubtitleTrackChange?: (sid: number | null) => void;
+  onSubtitleTrackListChange?: (trackList: unknown[] | null) => void;
 
   updateCurrentMediaPath: (path: string) => void;
   restoreMpvSubVisibility: () => void;
@@ -146,6 +148,8 @@ export function createBindMpvMainEventHandlersHandler(deps: {
       onSubtitleChange: handleMpvSubtitleChange,
       onSubtitleAssChange: handleMpvSubtitleAssChange,
       onSecondarySubtitleChange: handleMpvSecondarySubtitleChange,
+      onSubtitleTrackChange: ({ sid }) => deps.onSubtitleTrackChange?.(sid),
+      onSubtitleTrackListChange: ({ trackList }) => deps.onSubtitleTrackListChange?.(trackList),
       onSubtitleTiming: handleMpvSubtitleTiming,
       onMediaPathChange: handleMpvMediaPathChange,
       onMediaTitleChange: handleMpvMediaTitleChange,

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

@@ -35,6 +35,8 @@ export function createBuildBindMpvMainEventHandlersMainDepsHandler(deps: {
   logSubtitleTimingError: (message: string, error: unknown) => void;
   broadcastToOverlayWindows: (channel: string, payload: unknown) => void;
   onSubtitleChange: (text: string) => void;
+  onSubtitleTrackChange?: (sid: number | null) => void;
+  onSubtitleTrackListChange?: (trackList: unknown[] | null) => void;
   updateCurrentMediaPath: (path: string) => void;
   restoreMpvSubVisibility: () => void;
   getCurrentAnilistMediaKey: () => string | null;
@@ -101,6 +103,12 @@ export function createBuildBindMpvMainEventHandlersMainDepsHandler(deps: {
     broadcastSubtitle: (payload: { text: string; tokens: null }) =>
       deps.broadcastToOverlayWindows('subtitle:set', payload),
     onSubtitleChange: (text: string) => deps.onSubtitleChange(text),
+    onSubtitleTrackChange: deps.onSubtitleTrackChange
+      ? (sid: number | null) => deps.onSubtitleTrackChange!(sid)
+      : undefined,
+    onSubtitleTrackListChange: deps.onSubtitleTrackListChange
+      ? (trackList: unknown[] | null) => deps.onSubtitleTrackListChange!(trackList)
+      : undefined,
     refreshDiscordPresence: () => deps.refreshDiscordPresence(),
     setCurrentSubAssText: (text: string) => {
       deps.appState.currentSubAssText = text;

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

@@ -0,0 +1,41 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  getActiveExternalSubtitleSource,
+  resolveSubtitleSourcePath,
+} from './subtitle-prefetch-source';
+
+test('getActiveExternalSubtitleSource returns the active external subtitle path', () => {
+  const source = getActiveExternalSubtitleSource(
+    [
+      { type: 'sub', id: 1, external: false },
+      { type: 'sub', id: 2, external: true, 'external-filename': ' https://host/subs.ass ' },
+    ],
+    '2',
+  );
+
+  assert.equal(source, 'https://host/subs.ass');
+});
+
+test('getActiveExternalSubtitleSource returns null when the selected track is not external', () => {
+  const source = getActiveExternalSubtitleSource(
+    [{ type: 'sub', id: 2, external: false, 'external-filename': '/tmp/subs.ass' }],
+    2,
+  );
+
+  assert.equal(source, null);
+});
+
+test('resolveSubtitleSourcePath converts file URLs with spaces into filesystem paths', () => {
+  const fileUrl = process.platform === 'win32'
+    ? 'file:///C:/Users/test/Sub%20Folder/subs.ass'
+    : 'file:///tmp/Sub%20Folder/subs.ass';
+
+  const resolved = resolveSubtitleSourcePath(fileUrl);
+
+  assert.ok(resolved.endsWith('/Sub Folder/subs.ass') || resolved.endsWith('\\Sub Folder\\subs.ass'));
+});
+
+test('resolveSubtitleSourcePath leaves non-file sources unchanged', () => {
+  assert.equal(resolveSubtitleSourcePath('/tmp/subs.ass'), '/tmp/subs.ass');
+});

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

@@ -0,0 +1,34 @@
+import { fileURLToPath } from 'node:url';
+
+export function getActiveExternalSubtitleSource(
+  trackListRaw: unknown,
+  sidRaw: unknown,
+): string | null {
+  if (!Array.isArray(trackListRaw) || sidRaw == null) {
+    return null;
+  }
+
+  const sid =
+    typeof sidRaw === 'number' ? sidRaw : typeof sidRaw === 'string' ? Number(sidRaw) : null;
+  if (sid == null || !Number.isFinite(sid)) {
+    return null;
+  }
+
+  const activeTrack = trackListRaw.find((entry: unknown) => {
+    if (!entry || typeof entry !== 'object') {
+      return false;
+    }
+    const track = entry as Record<string, unknown>;
+    return track.type === 'sub' && track.id === sid && track.external === true;
+  }) as Record<string, unknown> | undefined;
+
+  const externalFilename =
+    typeof activeTrack?.['external-filename'] === 'string'
+      ? activeTrack['external-filename'].trim()
+      : '';
+  return externalFilename || null;
+}
+
+export function resolveSubtitleSourcePath(source: string): string {
+  return source.startsWith('file://') ? fileURLToPath(new URL(source)) : source;
+}