fix(overlay): poll cheap playback state in the open subtitle sidebar (#290)

changes/sidebar-snapshot-poll.md

@@ -0,0 +1,4 @@
+type: fixed
+area: overlay
+
+- The open subtitle sidebar no longer re-fetches the whole subtitle list and re-queries mpv about 12 times a second. It polls only the current time and line, and reloads the list when the subtitle track, file, or parsed cues change (plus a slow ~2 s fallback), cutting idle sidebar IPC and renderer work during playback.

docs/architecture/domains.md

@@ -63,7 +63,11 @@ the plugin artifact, persistent config, or session help.
 
 The subtitle sidebar consumes parsed cues through `SubtitleSidebarSnapshot`. Its `sourceKey`
 identifies the media and subtitle source so renderer selections are invalidated on source changes,
-including changes whose cue text and timings are identical. Native selection and clean clipboard
+including changes whose cue text and timings are identical. While open, the sidebar polls the
+cheap `SubtitleSidebarPlaybackState` (cached time, current line, `sourceKey`, `cuesRevision`; no
+mpv round-trips) every 80 ms for active-cue tracking. It fetches the full snapshot, which resolves
+the source over mpv IPC and carries every cue plus config, only on open, config reload, a
+`sourceKey`/`cuesRevision` change, and a ~2 s fallback. Native selection and clean clipboard
 serialization live in `src/renderer/modals/subtitle-sidebar-selection.ts`. Electron lets standard
 Copy input reach the renderer, where sidebar selection takes priority over the live-subtitle binding.
 The preload bridge writes selections through Electron's clipboard API so copying does not depend

e2e/playback.e2e.test.ts

@@ -156,6 +156,34 @@ test(
   },
 );
 
+test(
+  'open subtitle sidebar lists the parsed cues and follows playback',
+  { timeout: TEST_TIMEOUT_MS },
+  async () => {
+    const { session, overlay } = await useSession();
+    await showCue(FIXTURE_CUES[0]);
+    await session.runAppCommand('--toggle-subtitle-sidebar');
+    await overlay.waitFor(
+      `document.querySelectorAll('#subtitleSidebarList .subtitle-sidebar-item').length === ${FIXTURE_CUES.length}`,
+      { description: 'sidebar to list every fixture cue' },
+    );
+
+    for (const [index, cue] of FIXTURE_CUES.entries()) {
+      await session.mpv.command('seek', cue.at, 'absolute+exact');
+      await overlay.waitFor(
+        `document.querySelector('#subtitleSidebarList .subtitle-sidebar-item.active')?.dataset.index === '${index}'`,
+        { description: `sidebar to highlight cue ${index}` },
+      );
+    }
+
+    await session.runAppCommand('--toggle-subtitle-sidebar');
+    await overlay.waitFor(
+      `document.getElementById('subtitleSidebarModal').classList.contains('hidden')`,
+      { description: 'sidebar to close' },
+    );
+  },
+);
+
 // Runs last: it leaves the lookup popup open over the overlay.
 test(
   'holding Shift over a word opens the Yomitan popup with its dictionary entry',

src/core/services/ipc.test.ts

@@ -86,7 +86,9 @@ function createControllerConfigFixture() {
 function createSubtitleSidebarSnapshotFixture(): SubtitleSidebarSnapshot {
   return {
     sourceKey: 'test-subtitles',
+    cuesRevision: 1,
     cues: [],
+    currentTimeSec: null,
     currentSubtitle: { text: '', startTime: null, endTime: null },
     config: {
       enabled: false,

src/core/services/ipc.ts

@@ -12,6 +12,7 @@ import type {
   RuntimeOptionId,
   RuntimeOptionValue,
   SubtitleMiningContext,
+  SubtitleSidebarPlaybackState,
   SubtitleSidebarSnapshot,
   SubtitlePosition,
   SubsyncManualRunRequest,
@@ -79,6 +80,7 @@ export interface IpcServiceDeps {
   getCurrentSubtitleRaw: () => string;
   getCurrentSubtitleAss: () => string;
   getSubtitleSidebarSnapshot?: () => Promise<SubtitleSidebarSnapshot>;
+  getSubtitleSidebarPlaybackState?: () => SubtitleSidebarPlaybackState;
   getSubtitleSidebarOpen?: () => boolean;
   getPlaybackPaused: () => boolean | null | Promise<boolean | null>;
   getSubtitlePosition: () => unknown;
@@ -369,6 +371,7 @@ export interface IpcDepsRuntimeOptions {
   getCurrentSubtitleRaw: () => string;
   getCurrentSubtitleAss: () => string;
   getSubtitleSidebarSnapshot?: () => Promise<SubtitleSidebarSnapshot>;
+  getSubtitleSidebarPlaybackState?: () => SubtitleSidebarPlaybackState;
   getSubtitleSidebarOpen?: () => boolean;
   getPlaybackPaused: () => boolean | null | Promise<boolean | null>;
   getSubtitlePosition: () => unknown;
@@ -455,6 +458,7 @@ export function createIpcDepsRuntime(options: IpcDepsRuntimeOptions): IpcService
     getCurrentSubtitleRaw: options.getCurrentSubtitleRaw,
     getCurrentSubtitleAss: options.getCurrentSubtitleAss,
     getSubtitleSidebarSnapshot: options.getSubtitleSidebarSnapshot,
+    getSubtitleSidebarPlaybackState: options.getSubtitleSidebarPlaybackState,
     getSubtitleSidebarOpen: options.getSubtitleSidebarOpen ?? (() => false),
     getPlaybackPaused: options.getPlaybackPaused,
     getSubtitlePosition: options.getSubtitlePosition,
@@ -742,6 +746,13 @@ export function registerIpcHandlers(deps: IpcServiceDeps, ipc: IpcMainRegistrar
     return await deps.getSubtitleSidebarSnapshot();
   });
 
+  ipc.handle(IPC_CHANNELS.request.getSubtitleSidebarPlaybackState, () => {
+    if (!deps.getSubtitleSidebarPlaybackState) {
+      throw new Error('Subtitle sidebar playback state is unavailable.');
+    }
+    return deps.getSubtitleSidebarPlaybackState();
+  });
+
   ipc.handle(IPC_CHANNELS.request.getSubtitleSidebarOpen, () => {
     return deps.getSubtitleSidebarOpen?.() ?? false;
   });

src/main.ts

@@ -92,8 +92,10 @@ import type {
   SessionActionDispatchRequest,
   SecondarySubMode,
   SubtitleData,
+  SubtitleCue,
   SubtitleMiningContext,
   SubtitlePosition,
+  SubtitleSidebarPlaybackState,
   WindowGeometry,
 } from './types';
 import { OPEN_ANKI_CARD_ACTION_ID } from './types';
@@ -2047,6 +2049,33 @@ const linuxOverlayModeRuntime = createLinuxOverlayModeRuntime({
   logDebug: (message) => logger.debug(message),
 });
 let subtitleSidebarRequestedOpen = false;
+let subtitleSidebarCuesRef: SubtitleCue[] | null = null;
+let subtitleSidebarCuesRevision = 0;
+
+/**
+ * Cheap sidebar state polled ~12x/s while the sidebar is open. Reads only cached state, no
+ * mpv round-trips. `cuesRevision` bumps whenever a new parsed cue array is swapped in, so a
+ * re-parse of the same source still tells the renderer to fetch the full snapshot.
+ */
+function getSubtitleSidebarPlaybackState(): SubtitleSidebarPlaybackState {
+  if (appState.activeParsedSubtitleCues !== subtitleSidebarCuesRef) {
+    subtitleSidebarCuesRef = appState.activeParsedSubtitleCues;
+    subtitleSidebarCuesRevision += 1;
+  }
+  return {
+    sourceKey: JSON.stringify([
+      appState.activeParsedSubtitleMediaPath,
+      appState.activeParsedSubtitleSource,
+    ]),
+    cuesRevision: subtitleSidebarCuesRevision,
+    currentTimeSec: appState.mpvClient?.currentTimePos ?? null,
+    currentSubtitle: {
+      text: appState.currentSubText,
+      startTime: appState.mpvClient?.currentSubStart ?? null,
+      endTime: appState.mpvClient?.currentSubEnd ?? null,
+    },
+  };
+}
 const SEEK_THRESHOLD_SECONDS = 3;
 const EXPLICIT_SEEK_INTENT_TTL_MS = 2000;
 let explicitSeekIntentExpiresAtMs = 0;
@@ -5850,26 +5879,19 @@ const { registerIpcRuntimeHandlers } = composeIpcRuntimeHandlers({
       getCurrentSubtitleRaw: () => appState.currentSubText,
       getCurrentSubtitleAss: () => appState.currentSubAssText,
       getSubtitleSidebarOpen: () => subtitleSidebarRequestedOpen,
+      getSubtitleSidebarPlaybackState: () => getSubtitleSidebarPlaybackState(),
+      // Full snapshot: resolves the active source over mpv IPC and re-parses it when it changed.
+      // The renderer fetches it on open, on source/cue changes, and on a slow fallback cadence.
       getSubtitleSidebarSnapshot: async () => {
-        const currentSubtitle = {
-          text: appState.currentSubText,
-          startTime: appState.mpvClient?.currentSubStart ?? null,
-          endTime: appState.mpvClient?.currentSubEnd ?? null,
-        };
-        const currentTimeSec = appState.mpvClient?.currentTimePos ?? null;
         const config = configService.getConfig().subtitleSidebar;
+        const cachedSnapshot = () => ({
+          ...getSubtitleSidebarPlaybackState(),
+          cues: appState.activeParsedSubtitleCues,
+          config,
+        });
         const client = appState.mpvClient;
         if (!client?.connected) {
-          return {
-            sourceKey: JSON.stringify([
-              appState.activeParsedSubtitleMediaPath,
-              appState.activeParsedSubtitleSource,
-            ]),
-            cues: appState.activeParsedSubtitleCues,
-            currentTimeSec,
-            currentSubtitle,
-            config,
-          };
+          return cachedSnapshot();
         }
 
         try {
@@ -5883,16 +5905,7 @@ const { registerIpcRuntimeHandlers } = composeIpcRuntimeHandlers({
             ]);
           const videoPath = typeof videoPathRaw === 'string' ? videoPathRaw : '';
           if (!videoPath) {
-            return {
-              sourceKey: JSON.stringify([
-                appState.activeParsedSubtitleMediaPath,
-                appState.activeParsedSubtitleSource,
-              ]),
-              cues: appState.activeParsedSubtitleCues,
-              currentTimeSec,
-              currentSubtitle,
-              config,
-            };
+            return cachedSnapshot();
           }
           if (
             shouldUseCachedYoutubeParsedCues({
@@ -5901,16 +5914,7 @@ const { registerIpcRuntimeHandlers } = composeIpcRuntimeHandlers({
               cachedCueCount: appState.activeParsedSubtitleCues.length,
             })
           ) {
-            return {
-              sourceKey: JSON.stringify([
-                appState.activeParsedSubtitleMediaPath,
-                appState.activeParsedSubtitleSource,
-              ]),
-              cues: appState.activeParsedSubtitleCues,
-              currentTimeSec,
-              currentSubtitle,
-              config,
-            };
+            return cachedSnapshot();
           }
 
           const resolvedSource = await resolveActiveSubtitleSidebarSourceHandler({
@@ -5921,30 +5925,12 @@ const { registerIpcRuntimeHandlers } = composeIpcRuntimeHandlers({
             videoPath,
           });
           if (!resolvedSource) {
-            return {
-              sourceKey: JSON.stringify([
-                appState.activeParsedSubtitleMediaPath,
-                appState.activeParsedSubtitleSource,
-              ]),
-              cues: appState.activeParsedSubtitleCues,
-              currentTimeSec,
-              currentSubtitle,
-              config,
-            };
+            return cachedSnapshot();
           }
 
           try {
             if (appState.activeParsedSubtitleSource === resolvedSource.sourceKey) {
-              return {
-                sourceKey: JSON.stringify([
-                  appState.activeParsedSubtitleMediaPath,
-                  appState.activeParsedSubtitleSource,
-                ]),
-                cues: appState.activeParsedSubtitleCues,
-                currentTimeSec,
-                currentSubtitle,
-                config,
-              };
+              return cachedSnapshot();
             }
 
             const content = await loadSubtitleSourceText(resolvedSource.path);
@@ -5952,30 +5938,12 @@ const { registerIpcRuntimeHandlers } = composeIpcRuntimeHandlers({
             appState.activeParsedSubtitleCues = cues;
             appState.activeParsedSubtitleSource = resolvedSource.sourceKey;
             appState.activeParsedSubtitleMediaPath = videoPath || null;
-            return {
-              sourceKey: JSON.stringify([
-                appState.activeParsedSubtitleMediaPath,
-                appState.activeParsedSubtitleSource,
-              ]),
-              cues,
-              currentTimeSec,
-              currentSubtitle,
-              config,
-            };
+            return cachedSnapshot();
           } finally {
             await resolvedSource.cleanup?.();
           }
         } catch {
-          return {
-            sourceKey: JSON.stringify([
-              appState.activeParsedSubtitleMediaPath,
-              appState.activeParsedSubtitleSource,
-            ]),
-            cues: appState.activeParsedSubtitleCues,
-            currentTimeSec,
-            currentSubtitle,
-            config,
-          };
+          return cachedSnapshot();
         }
       },
       getPlaybackPaused: () =>

src/main/dependencies.ts

@@ -74,6 +74,7 @@ export interface MainIpcRuntimeServiceDepsParams {
   getCurrentSubtitleRaw: IpcDepsRuntimeOptions['getCurrentSubtitleRaw'];
   getCurrentSubtitleAss: IpcDepsRuntimeOptions['getCurrentSubtitleAss'];
   getSubtitleSidebarSnapshot?: IpcDepsRuntimeOptions['getSubtitleSidebarSnapshot'];
+  getSubtitleSidebarPlaybackState?: IpcDepsRuntimeOptions['getSubtitleSidebarPlaybackState'];
   getSubtitleSidebarOpen?: IpcDepsRuntimeOptions['getSubtitleSidebarOpen'];
   getPlaybackPaused: IpcDepsRuntimeOptions['getPlaybackPaused'];
   focusMainWindow?: IpcDepsRuntimeOptions['focusMainWindow'];
@@ -276,6 +277,7 @@ export function createMainIpcRuntimeServiceDeps(
     getCurrentSubtitleRaw: params.getCurrentSubtitleRaw,
     getCurrentSubtitleAss: params.getCurrentSubtitleAss,
     getSubtitleSidebarSnapshot: params.getSubtitleSidebarSnapshot,
+    getSubtitleSidebarPlaybackState: params.getSubtitleSidebarPlaybackState,
     getSubtitleSidebarOpen: params.getSubtitleSidebarOpen,
     getPlaybackPaused: params.getPlaybackPaused,
     getSubtitlePosition: params.getSubtitlePosition,

src/preload.ts

@@ -256,6 +256,8 @@ const electronAPI: ElectronAPI = {
     ipcRenderer.invoke(IPC_CHANNELS.request.getSubtitleSidebarOpen),
   getSubtitleSidebarSnapshot: () =>
     ipcRenderer.invoke(IPC_CHANNELS.request.getSubtitleSidebarSnapshot),
+  getSubtitleSidebarPlaybackState: () =>
+    ipcRenderer.invoke(IPC_CHANNELS.request.getSubtitleSidebarPlaybackState),
   copySubtitleSidebarSelection: async (text: unknown) => {
     if (typeof text !== 'string') throw new TypeError('Subtitle selection must be text.');
     clipboard.writeText(text);

src/renderer/modals/subtitle-sidebar-selection.electron-fixture.ts

@@ -15,6 +15,7 @@ export async function setup() {
   const commands: unknown[] = [];
   const snapshot: SubtitleSidebarSnapshot = {
     sourceKey: 'episode-1:track-1',
+    cuesRevision: 1,
     cues: [
       { text: '最初の台詞', startTime: 0, endTime: 1 },
       { text: '同じ台詞\n二行目', startTime: 1, endTime: 2 },
@@ -39,6 +40,7 @@ export async function setup() {
   Object.defineProperty(window, 'electronAPI', {
     value: {
       getSubtitleSidebarSnapshot: async () => snapshot,
+      getSubtitleSidebarPlaybackState: async () => snapshot,
       getSessionBindings: async () => [
         {
           sourcePath: 'keybindings[0].key',
@@ -64,6 +66,7 @@ export async function setup() {
     } satisfies Pick<
       ElectronAPI,
       | 'getSubtitleSidebarSnapshot'
+      | 'getSubtitleSidebarPlaybackState'
       | 'getSessionBindings'
       | 'getConfiguredShortcuts'
       | 'getStatsToggleKey'

src/renderer/modals/subtitle-sidebar.test.ts

@@ -1,7 +1,11 @@
 import assert from 'node:assert/strict';
 import test from 'node:test';
 
-import type { ElectronAPI, SubtitleSidebarSnapshot } from '../../types';
+import type {
+  ElectronAPI,
+  SubtitleSidebarPlaybackState,
+  SubtitleSidebarSnapshot,
+} from '../../types';
 import { createRendererState } from '../state.js';
 import {
   applySidebarCssDeclarations,
@@ -68,7 +72,12 @@ function createCueRow() {
 
 function createListStub() {
   return {
-    innerHTML: '',
+    get innerHTML() {
+      return '';
+    },
+    set innerHTML(_html: string) {
+      this.children.length = 0;
+    },
     children: [] as ReturnType<typeof createCueRow>[],
     appendChild(child: ReturnType<typeof createCueRow>) {
       child.offsetTop = this.children.length * child.clientHeight;
@@ -180,17 +189,20 @@ test.afterEach(() => {
 
 /**
  * Builds a sidebar modal over fake window/document/dom globals. The snapshot served to the
- * modal can be changed with `setSnapshot`. Globals are restored (and polling stopped) after
- * each test.
+ * modal (and the playback state derived from it) can be changed with `setSnapshot`; `poll`
+ * runs the next scheduled playback tick. Globals are restored after each test.
  */
 function createSidebarHarness(options: SidebarHarnessOptions = {}) {
   let snapshot: SubtitleSidebarSnapshot = {
     sourceKey: 'test-subtitles',
+    cuesRevision: 1,
     cues: options.cues ?? [{ startTime: 1, endTime: 2, text: 'first' }],
     currentSubtitle: options.currentSubtitle ?? { text: 'first', startTime: 1, endTime: 2 },
-    currentTimeSec: 'currentTimeSec' in options ? options.currentTimeSec : 1.1,
+    currentTimeSec: 'currentTimeSec' in options ? (options.currentTimeSec ?? null) : 1.1,
     config: { ...DEFAULT_SIDEBAR_CONFIG, ...options.config },
   };
+  const requests = { snapshot: 0, playback: 0 };
+  let nextPoll: (() => Promise<void>) | null = null;
 
   const mpvCommands: MpvCommand[] = [];
   const ignoreMouseCalls: Array<[boolean, { forward?: boolean } | undefined]> = [];
@@ -208,7 +220,15 @@ function createSidebarHarness(options: SidebarHarnessOptions = {}) {
       addEventListener: windowEvents.addEventListener,
       removeEventListener: windowEvents.removeEventListener,
       electronAPI: {
-        getSubtitleSidebarSnapshot: async () => snapshot,
+        getSubtitleSidebarSnapshot: async () => {
+          requests.snapshot += 1;
+          return snapshot;
+        },
+        getSubtitleSidebarPlaybackState: async (): Promise<SubtitleSidebarPlaybackState> => {
+          requests.playback += 1;
+          const { sourceKey, cuesRevision, currentTimeSec, currentSubtitle } = snapshot;
+          return { sourceKey, cuesRevision, currentTimeSec, currentSubtitle };
+        },
         getPlaybackPaused: async () => false,
         sendMpvCommand: (command: MpvCommand) => {
           mpvCommands.push(command);
@@ -281,11 +301,16 @@ function createSidebarHarness(options: SidebarHarnessOptions = {}) {
     onVisibilityChanged: (visible) => {
       visibilityChanges.push(visible);
     },
+    schedule: (callback) => {
+      nextPoll = callback;
+      return () => {
+        if (nextPoll === callback) nextPoll = null;
+      };
+    },
     ...options.modalOptions,
   });
 
   harnessCleanups.push(() => {
-    // Closing stops the snapshot poll so it cannot fire against the next test's globals.
     modal.closeSubtitleSidebarModal();
     modal.disposeDomEvents();
     restoreGlobals();
@@ -307,6 +332,13 @@ function createSidebarHarness(options: SidebarHarnessOptions = {}) {
     windowEvents,
     contentEvents,
     modalEvents,
+    requests,
+    /** Runs the pending playback poll tick, if one is scheduled. */
+    async poll() {
+      const tick = nextPoll;
+      nextPoll = null;
+      await tick?.();
+    },
     setSnapshot(patch: SnapshotPatch) {
       snapshot = { ...snapshot, ...patch, config: { ...snapshot.config, ...patch.config } };
     },
@@ -675,7 +707,7 @@ test('subtitle sidebar jumps to first resolved active cue, then resumes smooth a
     currentSubtitle: { text: 'line-9', startTime: 18, endTime: 19.5 },
     currentTimeSec: 18.1,
   });
-  await h.modal.refreshSubtitleSidebarSnapshot();
+  await h.poll();
 
   assert.equal(h.state.subtitleSidebarActiveCueIndex, 9);
   assert.equal(h.cueList.scrollTop, 260);
@@ -685,12 +717,94 @@ test('subtitle sidebar jumps to first resolved active cue, then resumes smooth a
     currentSubtitle: { text: 'line-10', startTime: 20, endTime: 21.5 },
     currentTimeSec: 20.1,
   });
-  await h.modal.refreshSubtitleSidebarSnapshot();
+  await h.poll();
 
   assert.equal(h.state.subtitleSidebarActiveCueIndex, 10);
   assert.deepEqual(h.cueList.scrollToCalls.at(-1), { top: 300, behavior: 'smooth' });
 });
 
+test('open subtitle sidebar follows playback from cheap polls without refetching cues or re-measuring', async () => {
+  let measurements = 0;
+  const h = createSidebarHarness({
+    cues: Array.from({ length: 4 }, (_, index) => ({
+      startTime: index * 2,
+      endTime: index * 2 + 1.5,
+      text: `line-${index}`,
+    })),
+    currentSubtitle: { text: 'line-0', startTime: 0, endTime: 1.5 },
+    currentTimeSec: 0.1,
+    config: { layout: 'embedded', maxWidth: 360 },
+    contentWidth: () => {
+      measurements += 1;
+      return 360;
+    },
+  });
+  await h.modal.openSubtitleSidebarModal();
+  const snapshotsAfterOpen = h.requests.snapshot;
+  const measurementsAfterOpen = measurements;
+
+  // Inside the lookahead window the upcoming cue activates before mpv reports its text.
+  h.setSnapshot({
+    currentSubtitle: { text: '', startTime: null, endTime: null },
+    currentTimeSec: 1.9,
+  });
+  await h.poll();
+  assert.equal(h.state.subtitleSidebarActiveCueIndex, 1);
+
+  h.setSnapshot({
+    currentSubtitle: { text: 'line-2', startTime: 4, endTime: 5.5 },
+    currentTimeSec: 4.1,
+  });
+  await h.poll();
+  assert.equal(h.state.subtitleSidebarActiveCueIndex, 2);
+  assert.equal(h.cueList.children[2]!.classList.contains('active'), true);
+  assert.equal(h.cueList.children[1]!.classList.contains('active'), false);
+
+  assert.equal(h.requests.playback, 2);
+  assert.equal(h.requests.snapshot, snapshotsAfterOpen);
+  assert.equal(measurements, measurementsAfterOpen);
+});
+
+for (const change of [
+  { name: 'subtitle source', patch: { sourceKey: 'other-subtitles' } },
+  { name: 'parsed cue revision', patch: { cuesRevision: 2 } },
+]) {
+  test(`open subtitle sidebar refetches and re-renders cues when the ${change.name} changes`, async () => {
+    const h = createSidebarHarness();
+    await h.modal.openSubtitleSidebarModal();
+    const snapshotsAfterOpen = h.requests.snapshot;
+
+    h.setSnapshot({
+      ...change.patch,
+      cues: [
+        { startTime: 1, endTime: 2, text: 'replacement' },
+        { startTime: 3, endTime: 4, text: 'second' },
+      ],
+    });
+    await h.poll();
+
+    assert.equal(h.requests.snapshot, snapshotsAfterOpen + 1);
+    assert.deepEqual(
+      h.cueList.children.map((row) => (row.children[1] as { textContent: string }).textContent),
+      ['replacement', 'second'],
+    );
+  });
+}
+
+test('open subtitle sidebar picks up changes main did not flag on a slow full-snapshot fallback', async () => {
+  const h = createSidebarHarness();
+  await h.modal.openSubtitleSidebarModal();
+  h.setSnapshot({ config: { enabled: false } });
+
+  for (let polls = 0; h.state.subtitleSidebarModalOpen && polls < 100; polls += 1) {
+    await h.poll();
+  }
+
+  assert.equal(h.state.subtitleSidebarModalOpen, false);
+  assert.equal(h.requests.snapshot, 2);
+  assert.ok(h.requests.playback > 1);
+});
+
 test('subtitle sidebar closes and resumes a hover pause', async () => {
   const h = createSidebarHarness({ config: { pauseVideoOnHover: true } });
   h.modal.wireDomEvents();

src/renderer/modals/subtitle-sidebar.ts

@@ -19,7 +19,11 @@ import {
 
 const MANUAL_SCROLL_HOLD_MS = 1500;
 const ACTIVE_CUE_LOOKAHEAD_SEC = 0.18;
-const SNAPSHOT_POLL_INTERVAL_MS = 80;
+// While open, the sidebar polls cheap cached playback state for active-cue tracking and only
+// fetches the full snapshot (mpv source resolution + every cue + config) when the source or
+// parsed cues changed, plus this fallback in case main missed a change.
+const PLAYBACK_POLL_INTERVAL_MS = 80;
+const FULL_SNAPSHOT_FALLBACK_POLLS = 25;
 const EMBEDDED_SIDEBAR_MIN_WIDTH_PX = 240;
 const EMBEDDED_SIDEBAR_MAX_RATIO = 0.45;
 const appliedSidebarCssKeys = new WeakMap<HTMLElement, Set<string>>();
@@ -206,9 +210,18 @@ export function createSubtitleSidebarModal(
     modalStateReader: Pick<ModalStateReader, 'isAnyModalOpen'>;
     onVisibilityChanged?: (visible: boolean) => void;
     shouldRestoreOpenOnStartup?: () => Promise<boolean>;
+    /** Schedules a poll tick and returns its cancel function. Defaults to `setTimeout`. */
+    schedule?: (callback: () => Promise<void>, delayMs: number) => () => void;
   },
 ) {
-  let snapshotPollInterval: ReturnType<typeof setTimeout> | null = null;
+  const schedule =
+    options.schedule ??
+    ((callback: () => Promise<void>, delayMs: number) => {
+      const timer = setTimeout(callback, delayMs);
+      return () => clearTimeout(timer);
+    });
+  let cancelPlaybackPoll: (() => void) | null = null;
+  let pollsSinceFullSnapshot = 0;
   let lastAppliedVideoMarginRatio: number | null = null;
   let subtitleSidebarHoverRequestId = 0;
   let disposeDomEvents: (() => void) | null = null;
@@ -218,6 +231,7 @@ export function createSubtitleSidebarModal(
   let subtitleSidebarPauseHeldByYomitanPopup = false;
   let lastSubtitleSidebarLookupCueIndex = -1;
   let subtitleSourceKey: string | null = null;
+  let subtitleCuesRevision: number | null = null;
 
   function restoreEmbeddedSidebarPassthrough(): void {
     syncOverlayMouseIgnoreState(ctx);
@@ -576,6 +590,8 @@ export function createSubtitleSidebarModal(
 
   async function refreshSnapshot(): Promise<SubtitleSidebarSnapshot> {
     const snapshot = await window.electronAPI.getSubtitleSidebarSnapshot();
+    pollsSinceFullSnapshot = 0;
+    subtitleCuesRevision = snapshot.cuesRevision;
     if (snapshot.sourceKey !== subtitleSourceKey) {
       clearSubtitleSidebarSelection(ctx.dom.subtitleSidebarList);
       lastSubtitleSidebarLookupCueIndex = -1;
@@ -590,8 +606,8 @@ export function createSubtitleSidebarModal(
       ctx.state.subtitleSidebarModalOpen = false;
       ctx.dom.subtitleSidebarModal.classList.add('hidden');
       ctx.dom.subtitleSidebarModal.setAttribute('aria-hidden', 'true');
-      stopSnapshotPolling();
-      updateActiveCue(null, snapshot.currentTimeSec ?? null);
+      stopPlaybackPolling();
+      updateActiveCue(null, snapshot.currentTimeSec);
       setStatus('Subtitle sidebar disabled in config.');
       syncEmbeddedSidebarLayout();
       if (!ctx.state.isOverSubtitle && !options.modalStateReader.isAnyModalOpen()) {
@@ -609,38 +625,49 @@ export function createSubtitleSidebarModal(
         renderCueList();
       }
     }
-    updateActiveCue(snapshot.currentSubtitle, snapshot.currentTimeSec ?? null);
+    updateActiveCue(snapshot.currentSubtitle, snapshot.currentTimeSec);
     syncEmbeddedSidebarLayout();
     return snapshot;
   }
 
-  function startSnapshotPolling(): void {
-    stopSnapshotPolling();
+  async function pollPlaybackOnce(): Promise<void> {
+    pollsSinceFullSnapshot += 1;
+    const playback = await window.electronAPI.getSubtitleSidebarPlaybackState();
+    if (
+      playback.sourceKey !== subtitleSourceKey ||
+      playback.cuesRevision !== subtitleCuesRevision ||
+      pollsSinceFullSnapshot >= FULL_SNAPSHOT_FALLBACK_POLLS
+    ) {
+      await refreshSnapshot();
+      return;
+    }
+    updateActiveCue(playback.currentSubtitle, playback.currentTimeSec);
+  }
+
+  function startPlaybackPolling(): void {
+    stopPlaybackPolling();
 
     const pollOnce = async (): Promise<void> => {
       try {
-        await refreshSnapshot();
+        await pollPlaybackOnce();
       } catch {
         // Keep polling; a transient IPC failure should not stop updates.
       }
 
       if (!ctx.state.subtitleSidebarModalOpen) {
-        snapshotPollInterval = null;
+        cancelPlaybackPoll = null;
         return;
       }
 
-      snapshotPollInterval = setTimeout(pollOnce, SNAPSHOT_POLL_INTERVAL_MS);
+      cancelPlaybackPoll = schedule(pollOnce, PLAYBACK_POLL_INTERVAL_MS);
     };
 
-    snapshotPollInterval = setTimeout(pollOnce, SNAPSHOT_POLL_INTERVAL_MS);
+    cancelPlaybackPoll = schedule(pollOnce, PLAYBACK_POLL_INTERVAL_MS);
   }
 
-  function stopSnapshotPolling(): void {
-    if (!snapshotPollInterval) {
-      return;
-    }
-    clearTimeout(snapshotPollInterval);
-    snapshotPollInterval = null;
+  function stopPlaybackPolling(): void {
+    cancelPlaybackPoll?.();
+    cancelPlaybackPoll = null;
   }
 
   async function openSubtitleSidebarModal(): Promise<void> {
@@ -664,7 +691,7 @@ export function createSubtitleSidebarModal(
     renderCueList();
     syncActiveCueClasses(-1);
     maybeAutoScrollActiveCue(-1, 'auto', true);
-    startSnapshotPolling();
+    startPlaybackPolling();
     syncEmbeddedSidebarLayout();
     restoreEmbeddedSidebarPassthrough();
     window.electronAPI.notifyOverlayModalOpened?.('subtitle-sidebar');
@@ -694,7 +721,7 @@ export function createSubtitleSidebarModal(
     ctx.state.subtitleSidebarModalOpen = false;
     ctx.dom.subtitleSidebarModal.classList.add('hidden');
     ctx.dom.subtitleSidebarModal.setAttribute('aria-hidden', 'true');
-    stopSnapshotPolling();
+    stopPlaybackPolling();
     syncEmbeddedSidebarLayout();
     if (!ctx.state.isOverSubtitle && !options.modalStateReader.isAnyModalOpen()) {
       ctx.dom.overlay.classList.remove('interactive');

src/shared/ipc/contracts.ts

@@ -68,6 +68,7 @@ export const IPC_CHANNELS = {
     getCurrentSubtitleRaw: 'get-current-subtitle-raw',
     getCurrentSubtitleAss: 'get-current-subtitle-ass',
     getSubtitleSidebarSnapshot: 'get-subtitle-sidebar-snapshot',
+    getSubtitleSidebarPlaybackState: 'get-subtitle-sidebar-playback-state',
     getSubtitleSidebarOpen: 'get-subtitle-sidebar-open',
     getPlaybackPaused: 'get-playback-paused',
     getSubtitlePosition: 'get-subtitle-position',

src/types/runtime.ts

@@ -55,6 +55,7 @@ import type {
   SubtitleData,
   SubtitleMiningContext,
   SubtitlePosition,
+  SubtitleSidebarPlaybackState,
   SubtitleSidebarSnapshot,
   SubtitleRendererStyleConfig,
   SubtitleStyleConfig,
@@ -477,6 +478,7 @@ export interface ElectronAPI {
   getCurrentSubtitleRaw: () => Promise<string>;
   getCurrentSubtitleAss: () => Promise<string>;
   getSubtitleSidebarSnapshot: () => Promise<SubtitleSidebarSnapshot>;
+  getSubtitleSidebarPlaybackState: () => Promise<SubtitleSidebarPlaybackState>;
   copySubtitleSidebarSelection: (text: string) => Promise<void>;
   getSubtitleSidebarOpen: () => Promise<boolean>;
   getPlaybackPaused: () => Promise<boolean | null>;

src/types/subtitle.ts

@@ -226,15 +226,25 @@ export interface SubtitleData {
   endTime?: number | null;
 }
 
-export interface SubtitleSidebarSnapshot {
+/**
+ * Per-tick sidebar state, served from main's cached state without mpv round-trips. When
+ * `sourceKey` or `cuesRevision` differs from the last full snapshot's, the renderer fetches a
+ * new `SubtitleSidebarSnapshot`.
+ */
+export interface SubtitleSidebarPlaybackState {
   sourceKey: string | null;
-  cues: SubtitleCue[];
-  currentTimeSec?: number | null;
+  /** Bumps whenever main swaps in a different parsed cue array. */
+  cuesRevision: number;
+  currentTimeSec: number | null;
   currentSubtitle: {
     text: string;
     startTime: number | null;
     endTime: number | null;
   };
+}
+
+export interface SubtitleSidebarSnapshot extends SubtitleSidebarPlaybackState {
+  cues: SubtitleCue[];
   config: SubtitleSidebarSnapshotConfig;
 }