feat(mining): cache remote media windows during card creation

- Reuse one temporary download for timing review, audio, and screenshots
- Expire cached windows after inactivity and clean them up on exit

src/core/services/media-timing-preview.test.ts

@@ -23,6 +23,21 @@ describe('buildMediaTimingPreviewArgs', () => {
     assert.equal(args.at(-1), '/video/show.mkv');
   });
 
+  test('keeps source timestamps for cached remote windows', () => {
+    const args = buildMediaTimingPreviewArgs('/tmp/review.sock', {
+      mediaPath: '/tmp/window.mkv',
+      absoluteTimestamps: true,
+    });
+
+    assert.ok(args.includes('--rebase-start-time=no'));
+    assert.equal(
+      buildMediaTimingPreviewArgs('/tmp/review.sock', { mediaPath: '/video/show.mkv' }).includes(
+        '--rebase-start-time=no',
+      ),
+      false,
+    );
+  });
+
   test('separates an option-like media path without adding optional audio arguments', () => {
     const args = buildMediaTimingPreviewArgs('/tmp/review.sock', {
       mediaPath: '--fullscreen',

src/core/services/media-timing-preview.ts

@@ -14,6 +14,8 @@ export interface MediaTimingPreviewStartOptions {
   executablePath?: string;
   audioTrackId?: number;
   volume?: number;
+  /** The file keeps source timestamps (a cached remote window); seek with the original times. */
+  absoluteTimestamps?: boolean;
 }
 
 type PreviewProcess = Pick<ChildProcess, 'kill' | 'once'>;
@@ -51,6 +53,9 @@ export function buildMediaTimingPreviewArgs(
   if (typeof options.volume === 'number' && Number.isFinite(options.volume)) {
     args.push(`--volume=${Math.max(0, options.volume)}`);
   }
+  if (options.absoluteTimestamps) {
+    args.push('--rebase-start-time=no');
+  }
   args.push('--', options.mediaPath);
   return args;
 }

src/core/services/media-timing-waveform.test.ts

@@ -35,6 +35,29 @@ test('speech waveform maps the selected FFmpeg stream and visible range', () =>
   assert.match(args[args.indexOf('-af') + 1] ?? '', /c0=FC/);
 });
 
+test('speech waveform seeks cached windows by source timestamps', () => {
+  const args = buildSpeechWaveformArgs(
+    {
+      mediaPath: { path: '/tmp/window.mkv', absoluteTimestamps: true, singleResolvedStream: true },
+      startTime: 8,
+      endTime: 15,
+    },
+    'downmix',
+  );
+
+  assert.deepEqual(args.slice(args.indexOf('-ss'), args.indexOf('-t') + 2), [
+    '-ss',
+    '8',
+    '-seek_timestamp',
+    '1',
+    '-i',
+    '/tmp/window.mkv',
+    '-t',
+    '7',
+  ]);
+  assert.equal(args.includes('-map'), false);
+});
+
 test('waveform peaks are normalized without flattening quieter sections', () => {
   const peaks = computeWaveformPeaks(pcm([0, 1_000, -2_000, 4_000, -8_000, 16_000]), 3);
 

src/core/services/media-timing-waveform.ts

@@ -1,4 +1,5 @@
 import { spawn } from 'node:child_process';
+import { normalizeMediaInput, type MediaInput } from '../../media-input';
 
 const WAVEFORM_SAMPLE_RATE = 8_000;
 const WAVEFORM_POINT_COUNT = 480;
@@ -9,7 +10,7 @@ const CENTER_CHANNEL_FILTER = `pan=mono|c0=FC,${SPEECH_FILTER}`;
 const DOWNMIX_FILTER = `aformat=channel_layouts=mono,${SPEECH_FILTER}`;
 
 export interface SpeechWaveformOptions {
-  mediaPath: string;
+  mediaPath: MediaInput;
   startTime: number;
   endTime: number;
   audioStreamIndex?: number;
@@ -22,6 +23,7 @@ export function buildSpeechWaveformArgs(
   mode: 'center' | 'downmix',
 ): string[] {
   const duration = options.endTime - options.startTime;
+  const input = normalizeMediaInput(options.mediaPath);
   const args = [
     '-hide_banner',
     '-nostdin',
@@ -29,8 +31,9 @@ export function buildSpeechWaveformArgs(
     'error',
     '-ss',
     String(options.startTime),
+    ...input.inputArgs,
     '-i',
-    options.mediaPath,
+    input.path,
     '-t',
     String(duration),
   ];

src/core/services/remote-media-window-cache.test.ts

@@ -0,0 +1,253 @@
+import assert from 'node:assert/strict';
+import * as fs from 'node:fs';
+import * as os from 'node:os';
+import * as path from 'node:path';
+import test from 'node:test';
+import {
+  buildRemoteMediaWindowArgs,
+  RemoteMediaWindowCache,
+  REMOTE_MEDIA_WINDOW_MAX_SECONDS,
+  type RemoteMediaWindowCacheOptions,
+} from './remote-media-window-cache';
+
+const SOURCE = {
+  path: 'https://jellyfin.example/Videos/abc/stream?static=true',
+  audioStreamIndex: 2,
+};
+
+type ExecFileStub = NonNullable<RemoteMediaWindowCacheOptions['execFile']>;
+
+function createStub(options: { fail?: boolean; empty?: boolean; defer?: boolean } = {}) {
+  const calls: string[][] = [];
+  const pendingCallbacks: Array<() => void> = [];
+  const execFile: ExecFileStub = (_file, args, _options, callback) => {
+    calls.push([...args]);
+    const finish = (): void => {
+      const outputPath = args.at(-1);
+      assert.ok(outputPath);
+      if (options.fail) {
+        callback(Object.assign(new Error('boom'), { code: 1 }));
+        return;
+      }
+      if (!options.empty) {
+        fs.writeFileSync(outputPath, 'mkv', 'utf8');
+      }
+      callback(null);
+    };
+    if (options.defer) {
+      pendingCallbacks.push(finish);
+    } else {
+      queueMicrotask(finish);
+    }
+  };
+  return {
+    calls,
+    execFile,
+    flush: () => {
+      for (const finish of pendingCallbacks.splice(0)) finish();
+    },
+  };
+}
+
+async function withCache(
+  stubOptions: Parameters<typeof createStub>[0],
+  cacheOptions: Omit<RemoteMediaWindowCacheOptions, 'execFile' | 'tempDir'>,
+  run: (cache: RemoteMediaWindowCache, stub: ReturnType<typeof createStub>) => Promise<void>,
+): Promise<void> {
+  const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-media-window-test-'));
+  const stub = createStub(stubOptions);
+  const cache = new RemoteMediaWindowCache({
+    tempDir,
+    execFile: stub.execFile,
+    idleTtlMs: 0,
+    logDebug: () => undefined,
+    ...cacheOptions,
+  });
+  try {
+    await run(cache, stub);
+  } finally {
+    cache.cleanup();
+    fs.rmSync(tempDir, { recursive: true, force: true });
+  }
+}
+
+function argValue(args: string[], flag: string): string | undefined {
+  const index = args.indexOf(flag);
+  return index === -1 ? undefined : args[index + 1];
+}
+
+test('buildRemoteMediaWindowArgs stream-copies the window with source timestamps intact', () => {
+  const args = buildRemoteMediaWindowArgs(
+    { ...SOURCE, inputOptions: { reconnect: true, headers: { Referer: 'https://a.example/' } } },
+    { startTime: 22.75, endTime: 33 },
+    '/tmp/window.mkv',
+  );
+
+  const inputIndex = args.indexOf('-i');
+  assert.equal(args[inputIndex + 1], SOURCE.path);
+  assert.ok(args.indexOf('-reconnect') < inputIndex);
+  assert.ok(args.indexOf('-headers') < inputIndex);
+  assert.equal(argValue(args, '-ss'), '22.75');
+  assert.equal(argValue(args, '-t'), '10.25');
+  assert.ok(args.indexOf('-t') < inputIndex);
+  assert.deepEqual(args.slice(args.indexOf('-map'), args.indexOf('-map') + 4), [
+    '-map',
+    '0:v:0?',
+    '-map',
+    '0:2',
+  ]);
+  assert.equal(argValue(args, '-c'), 'copy');
+  assert.ok(args.includes('-copyts'));
+  assert.ok(args.includes('-start_at_zero'));
+  assert.equal(argValue(args, '-f'), 'matroska');
+  assert.equal(args.at(-1), '/tmp/window.mkv');
+});
+
+test('buildRemoteMediaWindowArgs keeps every audio stream when none is selected', () => {
+  const args = buildRemoteMediaWindowArgs(
+    { path: SOURCE.path, audioStreamIndex: null },
+    { startTime: 0, endTime: 5 },
+    '/tmp/window.mkv',
+  );
+
+  assert.equal(args[args.lastIndexOf('-map') + 1], '0:a');
+});
+
+test('acquire downloads once and reuses the window for covered ranges', async () => {
+  await withCache({}, {}, async (cache, stub) => {
+    const window = await cache.acquire(SOURCE, { startTime: 10, endTime: 14 });
+
+    assert.equal(stub.calls.length, 1);
+    assert.equal(argValue(stub.calls[0]!, '-ss'), '9.75');
+    assert.equal(argValue(stub.calls[0]!, '-t'), '5.25');
+    assert.equal(window.startTime, 9.75);
+    assert.equal(window.endTime, 15);
+    assert.equal(window.audioStreamIndex, 2);
+    assert.ok(fs.existsSync(window.path));
+    assert.deepEqual(window.media, {
+      path: window.path,
+      source: 'remote-window',
+      singleResolvedStream: true,
+      absoluteTimestamps: true,
+    });
+
+    assert.equal(await cache.acquire(SOURCE, { startTime: 11, endTime: 15 }), window);
+    assert.equal(await cache.lookup(SOURCE, { startTime: 12, endTime: 12 }), window);
+    assert.equal(
+      await cache.lookup(
+        { path: SOURCE.path, audioStreamIndex: null },
+        { startTime: 12, endTime: 13 },
+      ),
+      window,
+    );
+    assert.equal(stub.calls.length, 1);
+  });
+});
+
+test('lookup never downloads and misses on other ranges, sources, or audio streams', async () => {
+  await withCache({}, {}, async (cache, stub) => {
+    assert.equal(await cache.lookup(SOURCE, { startTime: 10, endTime: 14 }), null);
+    assert.equal(stub.calls.length, 0);
+
+    await cache.acquire(SOURCE, { startTime: 10, endTime: 14 });
+    assert.equal(await cache.lookup(SOURCE, { startTime: 14, endTime: 16 }), null);
+    assert.equal(
+      await cache.lookup(
+        { path: 'https://other.example/stream', audioStreamIndex: 2 },
+        {
+          startTime: 11,
+          endTime: 12,
+        },
+      ),
+      null,
+    );
+    assert.equal(
+      await cache.lookup(
+        { path: SOURCE.path, audioStreamIndex: 3 },
+        { startTime: 11, endTime: 12 },
+      ),
+      null,
+    );
+    assert.equal(stub.calls.length, 1);
+  });
+});
+
+test('acquire widens to the union of the old window and replaces the old file', async () => {
+  await withCache({}, {}, async (cache, stub) => {
+    const first = await cache.acquire(SOURCE, { startTime: 10, endTime: 14 });
+    const second = await cache.acquire(SOURCE, { startTime: 8, endTime: 12 });
+
+    assert.equal(stub.calls.length, 2);
+    assert.equal(argValue(stub.calls[1]!, '-ss'), '7.75');
+    assert.equal(second.startTime, 7.75);
+    assert.equal(second.endTime, 15);
+    assert.notEqual(second.path, first.path);
+    assert.equal(fs.existsSync(first.path), false);
+    assert.ok(fs.existsSync(second.path));
+    assert.equal(cache.currentWindow, second);
+  });
+});
+
+test('acquire shares an in-flight download between concurrent callers', async () => {
+  await withCache({ defer: true }, {}, async (cache, stub) => {
+    const first = cache.acquire(SOURCE, { startTime: 10, endTime: 14 });
+    await Promise.resolve();
+    const second = cache.acquire(SOURCE, { startTime: 11, endTime: 13 });
+    const lookup = cache.lookup(SOURCE, { startTime: 12, endTime: 12 });
+    await Promise.resolve();
+    assert.equal(stub.calls.length, 1);
+
+    stub.flush();
+    const [a, b, c] = await Promise.all([first, second, lookup]);
+    assert.equal(a, b);
+    assert.equal(a, c);
+    assert.equal(stub.calls.length, 1);
+  });
+});
+
+test('acquire rejects on ffmpeg failure, leaves no file, and can retry', async () => {
+  await withCache({ fail: true }, {}, async (cache, stub) => {
+    await assert.rejects(
+      cache.acquire(SOURCE, { startTime: 10, endTime: 14 }),
+      /FFmpeg media window failed: boom/,
+    );
+    assert.equal(cache.currentWindow, null);
+    assert.equal(await cache.lookup(SOURCE, { startTime: 10, endTime: 14 }), null);
+
+    await assert.rejects(cache.acquire(SOURCE, { startTime: 10, endTime: 14 }));
+    assert.equal(stub.calls.length, 2);
+  });
+  await withCache({ empty: true }, {}, async (cache) => {
+    await assert.rejects(
+      cache.acquire(SOURCE, { startTime: 10, endTime: 14 }),
+      /exited without creating a media window/,
+    );
+  });
+});
+
+test('acquire refuses invalid and oversized ranges without spawning ffmpeg', async () => {
+  await withCache({}, {}, async (cache, stub) => {
+    await assert.rejects(cache.acquire(SOURCE, { startTime: 10, endTime: 10 }), /invalid/);
+    await assert.rejects(cache.acquire(SOURCE, { startTime: -1, endTime: 10 }), /invalid/);
+    await assert.rejects(
+      cache.acquire(SOURCE, { startTime: 0, endTime: REMOTE_MEDIA_WINDOW_MAX_SECONDS + 1 }),
+      /too long/,
+    );
+    assert.equal(stub.calls.length, 0);
+  });
+});
+
+test('the window is deleted after the idle timeout and on cleanup', async () => {
+  await withCache({}, { idleTtlMs: 20 }, async (cache) => {
+    const window = await cache.acquire(SOURCE, { startTime: 10, endTime: 14 });
+    await new Promise((resolve) => setTimeout(resolve, 60));
+
+    assert.equal(cache.currentWindow, null);
+    assert.equal(fs.existsSync(window.path), false);
+
+    const again = await cache.acquire(SOURCE, { startTime: 10, endTime: 14 });
+    cache.cleanup();
+    assert.equal(fs.existsSync(again.path), false);
+    assert.equal(fs.existsSync(path.dirname(again.path)), false);
+  });
+});

src/core/services/remote-media-window-cache.ts

@@ -0,0 +1,377 @@
+import { execFile as nodeExecFile, type ExecFileException } from 'child_process';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { createLogger } from '../../logger';
+import { normalizeMediaInput, type MediaInput, type MediaInputOptions } from '../../media-input';
+
+const log = createLogger('media-window');
+
+export const REMOTE_MEDIA_WINDOW_TIMEOUT_MS = 120_000;
+export const REMOTE_MEDIA_WINDOW_MAX_SECONDS = 180;
+const HEAD_SLACK_SECONDS = 0.25;
+const TAIL_SLACK_SECONDS = 1;
+const DEFAULT_IDLE_TTL_MS = 10 * 60_000;
+const COVERAGE_EPSILON_SECONDS = 0.01;
+
+export interface RemoteMediaWindowSource {
+  path: string;
+  inputOptions?: MediaInputOptions;
+  /** FFmpeg stream index to keep; `null`/undefined keeps every audio stream. */
+  audioStreamIndex?: number | null;
+}
+
+export interface RemoteMediaWindowRange {
+  startTime: number;
+  endTime: number;
+}
+
+export interface RemoteMediaWindow {
+  path: string;
+  startTime: number;
+  endTime: number;
+  sourcePath: string;
+  audioStreamIndex: number | null;
+  /** Input descriptor for FFmpeg reads; timestamps stay absolute so callers keep source times. */
+  media: MediaInput;
+}
+
+type WindowExecFile = (
+  file: string,
+  args: readonly string[],
+  options: { timeout: number },
+  callback: (error: ExecFileException | null) => void,
+) => void;
+
+export interface RemoteMediaWindowCacheOptions {
+  tempDir?: string;
+  execFile?: WindowExecFile;
+  idleTtlMs?: number;
+  logDebug?: (message: string) => void;
+}
+
+interface PendingFetch extends RemoteMediaWindowRange {
+  sourcePath: string;
+  audioStreamIndex: number | null;
+  promise: Promise<RemoteMediaWindow>;
+}
+
+export function isRemoteMediaWindowSourcePath(value: string): boolean {
+  return /^https?:\/\//i.test(value.trim());
+}
+
+function describeSourceForDebugLog(sourcePath: string): string {
+  try {
+    return `remote:${new URL(sourcePath).hostname.toLowerCase() || 'unknown'}`;
+  } catch {
+    return 'remote:unknown';
+  }
+}
+
+function isUsableRange(range: RemoteMediaWindowRange, allowEmpty: boolean): boolean {
+  return (
+    Number.isFinite(range.startTime) &&
+    Number.isFinite(range.endTime) &&
+    range.startTime >= 0 &&
+    (allowEmpty ? range.endTime >= range.startTime : range.endTime > range.startTime)
+  );
+}
+
+function audioStreamMatches(
+  windowIndex: number | null,
+  requested: number | null | undefined,
+): boolean {
+  return requested == null || windowIndex === requested;
+}
+
+function covers(
+  candidate: RemoteMediaWindowRange & { sourcePath: string; audioStreamIndex: number | null },
+  source: RemoteMediaWindowSource,
+  range: RemoteMediaWindowRange,
+): boolean {
+  return (
+    candidate.sourcePath === source.path &&
+    audioStreamMatches(candidate.audioStreamIndex, source.audioStreamIndex) &&
+    candidate.startTime <= range.startTime + COVERAGE_EPSILON_SECONDS &&
+    candidate.endTime >= range.endTime - COVERAGE_EPSILON_SECONDS
+  );
+}
+
+/**
+ * Stream-copies `[startTime, endTime]` of a remote source into a local Matroska file.
+ * `-copyts -start_at_zero` keeps the source timestamps, so later reads seek with the
+ * original times via `-seek_timestamp 1` (see `MediaInput.absoluteTimestamps`).
+ */
+export function buildRemoteMediaWindowArgs(
+  source: RemoteMediaWindowSource,
+  range: RemoteMediaWindowRange,
+  outputPath: string,
+): string[] {
+  const input = normalizeMediaInput({ path: source.path, inputOptions: source.inputOptions });
+  const audioMap =
+    typeof source.audioStreamIndex === 'number' && Number.isInteger(source.audioStreamIndex)
+      ? `0:${source.audioStreamIndex}`
+      : '0:a';
+  return [
+    '-hide_banner',
+    '-nostdin',
+    '-loglevel',
+    'error',
+    '-ss',
+    String(range.startTime),
+    '-t',
+    String(range.endTime - range.startTime),
+    ...input.inputArgs,
+    '-i',
+    input.path,
+    '-map',
+    '0:v:0?',
+    '-map',
+    audioMap,
+    '-c',
+    'copy',
+    '-sn',
+    '-dn',
+    '-copyts',
+    '-start_at_zero',
+    '-f',
+    'matroska',
+    '-y',
+    outputPath,
+  ];
+}
+
+/**
+ * Holds one downloaded window of the current remote stream so the timing review,
+ * audio extraction, and screenshot all read the same local bytes instead of each
+ * re-fetching the clip over HTTP. A new window replaces the old one; the file is
+ * deleted after `idleTtlMs` without use, on `clear()`, or on `cleanup()`.
+ */
+export class RemoteMediaWindowCache {
+  private readonly tempDir: string;
+  private readonly execFile: WindowExecFile;
+  private readonly idleTtlMs: number;
+  private readonly logDebug: (message: string) => void;
+  private current: RemoteMediaWindow | null = null;
+  private pending: PendingFetch | null = null;
+  private idleTimer: ReturnType<typeof setTimeout> | null = null;
+  private sequence = 0;
+
+  constructor(options: RemoteMediaWindowCacheOptions = {}) {
+    this.tempDir = options.tempDir ?? path.join(os.tmpdir(), 'subminer-media-windows');
+    this.execFile = options.execFile ?? nodeExecFile;
+    this.idleTtlMs = options.idleTtlMs ?? DEFAULT_IDLE_TTL_MS;
+    this.logDebug = options.logDebug ?? ((message) => log.debug(message));
+  }
+
+  get currentWindow(): RemoteMediaWindow | null {
+    return this.current;
+  }
+
+  /** Returns a ready or in-flight window covering the range; never starts a download. */
+  async lookup(
+    source: RemoteMediaWindowSource,
+    range: RemoteMediaWindowRange,
+  ): Promise<RemoteMediaWindow | null> {
+    if (!isUsableRange(range, true)) return null;
+    if (this.current && covers(this.current, source, range)) {
+      this.touch();
+      return this.current;
+    }
+    const pending = this.pending;
+    if (pending && covers(pending, source, range)) {
+      try {
+        const window = await pending.promise;
+        this.touch();
+        return window;
+      } catch {
+        return null;
+      }
+    }
+    return null;
+  }
+
+  /** Returns a window covering the range, downloading (and widening) one when needed. */
+  async acquire(
+    source: RemoteMediaWindowSource,
+    range: RemoteMediaWindowRange,
+  ): Promise<RemoteMediaWindow> {
+    if (!isUsableRange(range, false)) {
+      throw new Error('Media window range is invalid.');
+    }
+    if (range.endTime - range.startTime > REMOTE_MEDIA_WINDOW_MAX_SECONDS) {
+      throw new Error('Media window range is too long to download.');
+    }
+
+    for (;;) {
+      const hit = await this.lookup(source, range);
+      if (hit) return hit;
+      const pending = this.pending;
+      if (!pending) break;
+      // Another caller is already downloading; wait for it, then re-check coverage.
+      await pending.promise.catch(() => null);
+    }
+
+    return this.fetch(source, this.planFetchRange(source, range));
+  }
+
+  clear(): void {
+    this.cancelIdleTimer();
+    const current = this.current;
+    this.current = null;
+    if (current) this.removeFile(current.path);
+  }
+
+  cleanup(): void {
+    this.clear();
+    try {
+      fs.rmSync(this.tempDir, { recursive: true, force: true });
+    } catch (error) {
+      log.error('Failed to cleanup media window directory:', error);
+    }
+  }
+
+  private planFetchRange(
+    source: RemoteMediaWindowSource,
+    range: RemoteMediaWindowRange,
+  ): RemoteMediaWindowRange {
+    let startTime = Math.max(0, range.startTime - HEAD_SLACK_SECONDS);
+    let endTime = range.endTime + TAIL_SLACK_SECONDS;
+    const current = this.current;
+    if (
+      current &&
+      current.sourcePath === source.path &&
+      audioStreamMatches(current.audioStreamIndex, source.audioStreamIndex)
+    ) {
+      // Keep what was already downloaded when the review timeline grows in one direction.
+      const unionStart = Math.min(startTime, current.startTime);
+      const unionEnd = Math.max(endTime, current.endTime);
+      if (unionEnd - unionStart <= REMOTE_MEDIA_WINDOW_MAX_SECONDS) {
+        startTime = unionStart;
+        endTime = unionEnd;
+      }
+    }
+    return { startTime, endTime };
+  }
+
+  private fetch(
+    source: RemoteMediaWindowSource,
+    range: RemoteMediaWindowRange,
+  ): Promise<RemoteMediaWindow> {
+    fs.mkdirSync(this.tempDir, { recursive: true });
+    this.sequence += 1;
+    const outputPath = path.join(this.tempDir, `window_${Date.now()}_${this.sequence}.mkv`);
+    const audioStreamIndex =
+      typeof source.audioStreamIndex === 'number' ? source.audioStreamIndex : null;
+    const description = describeSourceForDebugLog(source.path);
+    const startedAt = Date.now();
+    this.logDebug(
+      `[media-window] fetch start ${description} start=${range.startTime} end=${range.endTime} audioStream=${audioStreamIndex ?? 'all'}`,
+    );
+
+    const promise = new Promise<RemoteMediaWindow>((resolve, reject) => {
+      this.execFile(
+        'ffmpeg',
+        buildRemoteMediaWindowArgs(source, range, outputPath),
+        { timeout: REMOTE_MEDIA_WINDOW_TIMEOUT_MS },
+        (error) => {
+          const elapsedMs = Math.max(0, Date.now() - startedAt);
+          const size = error ? 0 : this.fileSize(outputPath);
+          if (error || size === 0) {
+            this.removeFile(outputPath);
+            const reason = error
+              ? error.code === 'ENOENT'
+                ? 'FFmpeg not found. Install FFmpeg to enable media generation.'
+                : `FFmpeg media window failed: ${error.message}`
+              : 'FFmpeg exited without creating a media window.';
+            this.logDebug(`[media-window] fetch failed ${description} elapsedMs=${elapsedMs}`);
+            reject(new Error(reason));
+            return;
+          }
+          const window: RemoteMediaWindow = {
+            path: outputPath,
+            startTime: range.startTime,
+            endTime: range.endTime,
+            sourcePath: source.path,
+            audioStreamIndex,
+            media: {
+              path: outputPath,
+              source: 'remote-window',
+              singleResolvedStream: true,
+              absoluteTimestamps: true,
+            },
+          };
+          this.logDebug(
+            `[media-window] fetch complete ${description} elapsedMs=${elapsedMs} bytes=${size}`,
+          );
+          this.replaceCurrent(window);
+          resolve(window);
+        },
+      );
+    });
+
+    const pending: PendingFetch = {
+      sourcePath: source.path,
+      audioStreamIndex,
+      startTime: range.startTime,
+      endTime: range.endTime,
+      promise,
+    };
+    this.pending = pending;
+    promise
+      .catch(() => undefined)
+      .then(() => {
+        if (this.pending === pending) this.pending = null;
+      });
+    return promise;
+  }
+
+  private replaceCurrent(window: RemoteMediaWindow): void {
+    const previous = this.current;
+    this.current = window;
+    if (previous && previous.path !== window.path) this.removeFile(previous.path);
+    this.touch();
+  }
+
+  private touch(): void {
+    this.cancelIdleTimer();
+    if (this.idleTtlMs <= 0 || !this.current) return;
+    const timer = setTimeout(() => {
+      if (this.idleTimer === timer) this.idleTimer = null;
+      this.clear();
+    }, this.idleTtlMs);
+    timer.unref?.();
+    this.idleTimer = timer;
+  }
+
+  private cancelIdleTimer(): void {
+    if (this.idleTimer) clearTimeout(this.idleTimer);
+    this.idleTimer = null;
+  }
+
+  private fileSize(filePath: string): number {
+    try {
+      return fs.statSync(filePath).size;
+    } catch {
+      return 0;
+    }
+  }
+
+  private removeFile(filePath: string): void {
+    try {
+      fs.unlinkSync(filePath);
+    } catch (error) {
+      if ((error as NodeJS.ErrnoException).code !== 'ENOENT') {
+        log.debug(`Failed to remove media window ${filePath}:`, (error as Error).message);
+      }
+    }
+  }
+}
+
+let sharedCache: RemoteMediaWindowCache | null = null;
+
+/** Process-wide cache so the review modal and card media generation share one download. */
+export function getSharedRemoteMediaWindowCache(): RemoteMediaWindowCache {
+  sharedCache ??= new RemoteMediaWindowCache();
+  return sharedCache;
+}

src/main.ts

@@ -464,6 +464,8 @@ import { handleCliCommandRuntimeServiceWithContext } from './main/cli-runtime';
 import { createOverlayModalRuntimeService } from './main/overlay-runtime';
 import { createOverlayModalInputState } from './main/runtime/overlay-modal-input-state';
 import { MediaTimingPreviewSession } from './core/services/media-timing-preview';
+import { getSharedRemoteMediaWindowCache } from './core/services/remote-media-window-cache';
+import { resolveMediaGenerationInput } from './anki-integration/media-source';
 import { generateSpeechWaveform } from './core/services/media-timing-waveform';
 import {
   collectMediaTimingContextLines,
@@ -2896,6 +2898,21 @@ const mediaTimingReviewRuntime = createMediaTimingReviewRuntime({
     configService.getConfig().mpv.executablePath || process.env.SUBMINER_MPV_PATH?.trim() || '',
   createPreviewSession: () => new MediaTimingPreviewSession(),
   generateWaveform: (options) => generateSpeechWaveform(options),
+  resolveMediaSource: async () => {
+    const resolved = await resolveMediaGenerationInput(appState.mpvClient, 'audio', {
+      getCachedMediaPath: (currentVideoPath, kind) =>
+        getCachedYoutubeMediaPathForCurrentPlayback(currentVideoPath, kind),
+      remoteCacheMode: shouldRequireYoutubeMediaCacheForCurrentPlayback() ? 'required' : 'optional',
+    });
+    return resolved
+      ? {
+          path: resolved.path,
+          ...(resolved.inputOptions ? { inputOptions: resolved.inputOptions } : {}),
+          singleResolvedStream: resolved.singleResolvedStream,
+        }
+      : null;
+  },
+  acquireMediaWindow: (source, range) => getSharedRemoteMediaWindowCache().acquire(source, range),
   getSubtitleContextLines: (range) =>
     collectMediaTimingContextLines({
       cues: appState.activeParsedSubtitleCues,
@@ -3973,6 +3990,7 @@ const {
     cleanupInternalSubtitleTrackCache: () => cachedInternalSubtitleTrackExtractor.clear(),
     cleanupYoutubeSubtitleTempDirs: () => youtubeFlowRuntime.cleanupSubtitleTempDirs(),
     cleanupYoutubeMediaCache: () => youtubeMediaCache.cleanup(),
+    cleanupRemoteMediaWindows: () => getSharedRemoteMediaWindowCache().cleanup(),
     cleanupJellyfinSubtitleCache: () => cleanupJellyfinSubtitleCache(),
     stopDiscordPresenceService: () => {
       void appState.discordPresenceService?.stop();

src/main/runtime/app-lifecycle-actions.test.ts

@@ -46,12 +46,13 @@ test('on will quit cleanup handler runs all cleanup steps', () => {
     cleanupInternalSubtitleTrackCache: () => calls.push('cleanup-internal-subtitles'),
     cleanupYoutubeSubtitleTempDirs: () => calls.push('cleanup-youtube-subtitles'),
     cleanupYoutubeMediaCache: () => calls.push('cleanup-youtube-media'),
+    cleanupRemoteMediaWindows: () => calls.push('cleanup-remote-media-windows'),
     cleanupJellyfinSubtitleCache: () => calls.push('cleanup-jellyfin-subtitles'),
     stopDiscordPresenceService: () => calls.push('stop-discord-presence'),
   });
 
   cleanup();
-  assert.equal(calls.length, 35);
+  assert.equal(calls.length, 36);
   assert.equal(calls[0], 'destroy-tray');
   assert.equal(calls[calls.length - 1], 'stop-discord-presence');
   assert.ok(calls.includes('cleanup-jellyfin-subtitles'));
@@ -60,6 +61,7 @@ test('on will quit cleanup handler runs all cleanup steps', () => {
   assert.ok(calls.includes('clear-linux-mpv-fullscreen-overlay-refresh-timeouts'));
   assert.ok(calls.includes('cleanup-youtube-subtitles'));
   assert.ok(calls.includes('cleanup-youtube-media'));
+  assert.ok(calls.includes('cleanup-remote-media-windows'));
   assert.ok(calls.indexOf('flush-mpv-log') < calls.indexOf('destroy-socket'));
 });
 
@@ -102,6 +104,7 @@ test('on will quit cleanup handler cleans jellyfin subtitle cache when stopping
     cleanupInternalSubtitleTrackCache: () => calls.push('cleanup-internal-subtitles'),
     cleanupYoutubeSubtitleTempDirs: () => calls.push('cleanup-youtube-subtitles'),
     cleanupYoutubeMediaCache: () => calls.push('cleanup-youtube-media'),
+    cleanupRemoteMediaWindows: () => calls.push('cleanup-remote-media-windows'),
     cleanupJellyfinSubtitleCache: () => calls.push('cleanup-jellyfin-subtitles'),
     stopDiscordPresenceService: () => calls.push('stop-discord-presence'),
   });

src/main/runtime/app-lifecycle-actions.ts

@@ -32,6 +32,7 @@ export function createOnWillQuitCleanupHandler(deps: {
   cleanupInternalSubtitleTrackCache: () => void;
   cleanupYoutubeSubtitleTempDirs: () => void;
   cleanupYoutubeMediaCache: () => void;
+  cleanupRemoteMediaWindows: () => void;
   cleanupJellyfinSubtitleCache: () => void;
   stopDiscordPresenceService: () => void;
 }) {
@@ -76,6 +77,7 @@ export function createOnWillQuitCleanupHandler(deps: {
     }
     deps.cleanupYoutubeSubtitleTempDirs();
     deps.cleanupYoutubeMediaCache();
+    deps.cleanupRemoteMediaWindows();
     deps.stopDiscordPresenceService();
     return Promise.resolve(stopSyncAutoScheduler);
   };

src/main/runtime/app-lifecycle-main-cleanup.test.ts

@@ -75,6 +75,7 @@ test('cleanup deps builder returns handlers that guard optional runtime objects'
     cleanupInternalSubtitleTrackCache: () => calls.push('cleanup-internal-subtitles'),
     cleanupYoutubeSubtitleTempDirs: () => calls.push('cleanup-youtube-subtitles'),
     cleanupYoutubeMediaCache: () => calls.push('cleanup-youtube-media'),
+    cleanupRemoteMediaWindows: () => calls.push('cleanup-remote-media-windows'),
     cleanupJellyfinSubtitleCache: () => calls.push('cleanup-jellyfin-subtitles'),
     stopDiscordPresenceService: () => calls.push('stop-discord-presence'),
   });
@@ -157,6 +158,7 @@ test('cleanup deps builder skips destroyed yomitan window', () => {
     cleanupInternalSubtitleTrackCache: () => {},
     cleanupYoutubeSubtitleTempDirs: () => {},
     cleanupYoutubeMediaCache: () => {},
+    cleanupRemoteMediaWindows: () => {},
     cleanupJellyfinSubtitleCache: () => {},
     stopDiscordPresenceService: () => {},
   });
@@ -210,6 +212,7 @@ test('cleanup deps builder skips global shortcut cleanup before app ready', () =
     cleanupInternalSubtitleTrackCache: () => {},
     cleanupYoutubeSubtitleTempDirs: () => {},
     cleanupYoutubeMediaCache: () => {},
+    cleanupRemoteMediaWindows: () => {},
     cleanupJellyfinSubtitleCache: () => {},
     stopDiscordPresenceService: () => {},
   });

src/main/runtime/app-lifecycle-main-cleanup.ts

@@ -61,6 +61,7 @@ export function createBuildOnWillQuitCleanupDepsHandler(deps: {
   cleanupInternalSubtitleTrackCache: () => void;
   cleanupYoutubeSubtitleTempDirs: () => void;
   cleanupYoutubeMediaCache: () => void;
+  cleanupRemoteMediaWindows: () => void;
   cleanupJellyfinSubtitleCache: () => void;
   stopDiscordPresenceService: () => void;
 }) {
@@ -148,6 +149,7 @@ export function createBuildOnWillQuitCleanupDepsHandler(deps: {
     cleanupInternalSubtitleTrackCache: () => deps.cleanupInternalSubtitleTrackCache(),
     cleanupYoutubeSubtitleTempDirs: () => deps.cleanupYoutubeSubtitleTempDirs(),
     cleanupYoutubeMediaCache: () => deps.cleanupYoutubeMediaCache(),
+    cleanupRemoteMediaWindows: () => deps.cleanupRemoteMediaWindows(),
     cleanupJellyfinSubtitleCache: () => deps.cleanupJellyfinSubtitleCache(),
     stopDiscordPresenceService: () => deps.stopDiscordPresenceService(),
   });

src/main/runtime/composers/startup-lifecycle-composer.test.ts

@@ -52,6 +52,7 @@ test('composeStartupLifecycleHandlers returns callable startup lifecycle handler
       cleanupInternalSubtitleTrackCache: () => {},
       cleanupYoutubeSubtitleTempDirs: () => {},
       cleanupYoutubeMediaCache: () => {},
+      cleanupRemoteMediaWindows: () => {},
       cleanupJellyfinSubtitleCache: () => {},
       stopDiscordPresenceService: () => {},
     },

src/main/runtime/media-timing-review.test.ts

@@ -1,6 +1,15 @@
 import assert from 'node:assert/strict';
 import { describe, test } from 'node:test';
 import type { MediaTimingReviewOpenPayload } from '../../types/anki';
+import type { SpeechWaveformOptions } from '../../core/services/media-timing-waveform';
+import type {
+  RemoteMediaWindow,
+  RemoteMediaWindowRange,
+  RemoteMediaWindowSource,
+} from '../../core/services/remote-media-window-cache';
+import type { MediaTimingPreviewSession } from '../../core/services/media-timing-preview';
+
+type MediaTimingPreviewSessionLike = Pick<MediaTimingPreviewSession, 'start'>;
 import {
   buildMediaTimingReviewPayload,
   collectMediaTimingContextLines,
@@ -177,12 +186,7 @@ test('media timing review pauses playback, resolves exact timing, and restores p
 });
 
 test('media timing review analyzes the visible range on the selected audio stream', async () => {
-  const waveformCalls: Array<{
-    mediaPath: string;
-    startTime: number;
-    endTime: number;
-    audioStreamIndex?: number;
-  }> = [];
+  const waveformCalls: SpeechWaveformOptions[] = [];
   let runtime: ReturnType<typeof createMediaTimingReviewRuntime>;
   runtime = createMediaTimingReviewRuntime({
     getMpvClient: () => ({
@@ -239,6 +243,300 @@ test('media timing review analyzes the visible range on the selected audio strea
   ]);
 });
 
+const REMOTE_STREAM_URL = 'https://jellyfin.example/Videos/abc/stream?static=true';
+
+function createWindowStub(options: { fail?: boolean } = {}) {
+  const calls: Array<{ source: RemoteMediaWindowSource; range: RemoteMediaWindowRange }> = [];
+  const acquireMediaWindow = async (
+    source: RemoteMediaWindowSource,
+    range: RemoteMediaWindowRange,
+  ): Promise<RemoteMediaWindow> => {
+    calls.push({ source, range });
+    if (options.fail) throw new Error('offline');
+    const windowPath = `/tmp/window-${range.startTime}-${range.endTime}.mkv`;
+    return {
+      path: windowPath,
+      startTime: range.startTime,
+      endTime: range.endTime,
+      sourcePath: source.path,
+      audioStreamIndex: source.audioStreamIndex ?? null,
+      media: {
+        path: windowPath,
+        source: 'remote-window',
+        singleResolvedStream: true,
+        absoluteTimestamps: true,
+      },
+    };
+  };
+  return { calls, acquireMediaWindow };
+}
+
+function createRemoteReviewRuntime(options: {
+  windowStub: ReturnType<typeof createWindowStub>;
+  waveformCalls: SpeechWaveformOptions[];
+  previewStarts: Array<Parameters<MediaTimingPreviewSessionLike['start']>[0]>;
+  previewPlays: Array<[string, number, number]>;
+  disposed: string[];
+  openModal: (
+    runtime: ReturnType<typeof createMediaTimingReviewRuntime>,
+    payload: MediaTimingReviewOpenPayload,
+  ) => Promise<void>;
+}) {
+  let runtime!: ReturnType<typeof createMediaTimingReviewRuntime>;
+  runtime = createMediaTimingReviewRuntime({
+    getMpvClient: () => ({
+      connected: true,
+      currentVideoPath: REMOTE_STREAM_URL,
+      currentAudioStreamIndex: 2,
+      requestProperty: async (name) =>
+        ({ pause: true, duration: 100, aid: 3, volume: 60 })[
+          name as 'pause' | 'duration' | 'aid' | 'volume'
+        ] ?? null,
+      send: () => undefined,
+    }),
+    getCurrentMediaPath: () => REMOTE_STREAM_URL,
+    getMpvExecutablePath: () => 'mpv',
+    resolveMediaSource: async () => ({
+      path: REMOTE_STREAM_URL,
+      inputOptions: { reconnect: true },
+    }),
+    acquireMediaWindow: options.windowStub.acquireMediaWindow,
+    generateWaveform: async (waveformOptions) => {
+      options.waveformCalls.push(waveformOptions);
+      return [0.1, 0.8, 0.2];
+    },
+    createPreviewSession: () => {
+      let mediaPath = '';
+      return {
+        start: async (startOptions) => {
+          mediaPath = startOptions.mediaPath;
+          options.previewStarts.push(startOptions);
+        },
+        play: async (startTime, endTime) => {
+          options.previewPlays.push([mediaPath, startTime, endTime]);
+        },
+        stop: async () => undefined,
+        dispose: () => {
+          options.disposed.push(mediaPath);
+        },
+      };
+    },
+    openModal: async (payload) => {
+      await options.openModal(runtime, payload);
+      return true;
+    },
+    showStatus: () => undefined,
+  });
+  return runtime;
+}
+
+test('media timing review downloads one window of a remote stream for the waveform and preview', async () => {
+  const windowStub = createWindowStub();
+  const waveformCalls: SpeechWaveformOptions[] = [];
+  const previewStarts: Array<Parameters<MediaTimingPreviewSessionLike['start']>[0]> = [];
+  const previewPlays: Array<[string, number, number]> = [];
+  const disposed: string[] = [];
+  const runtime = createRemoteReviewRuntime({
+    windowStub,
+    waveformCalls,
+    previewStarts,
+    previewPlays,
+    disposed,
+    openModal: async (active, payload) => {
+      const waveform = await active.getWaveform({
+        reviewId: payload.reviewId,
+        startTime: payload.timelineStartTime,
+        endTime: payload.timelineEndTime,
+      });
+      assert.deepEqual(waveform, { ok: true, peaks: [0.1, 0.8, 0.2] });
+      assert.deepEqual(
+        await active.previewRange({ reviewId: payload.reviewId, startTime: 9.5, endTime: 12.5 }),
+        { ok: true },
+      );
+      active.resolveReview({
+        reviewId: payload.reviewId,
+        decision: { action: 'confirm', startTime: 9.5, endTime: 12.5 },
+      });
+    },
+  });
+
+  const decision = await runtime.requestReview({
+    kind: 'word',
+    text: '字幕',
+    startTime: 10,
+    endTime: 12,
+    audioPadding: 0.5,
+    maxMediaDuration: 30,
+  });
+
+  assert.deepEqual(decision, { action: 'confirm', startTime: 9.5, endTime: 12.5 });
+  assert.deepEqual(windowStub.calls, [
+    {
+      source: { path: REMOTE_STREAM_URL, inputOptions: { reconnect: true }, audioStreamIndex: 2 },
+      range: { startTime: 7.5, endTime: 14.5 },
+    },
+  ]);
+  assert.deepEqual(waveformCalls, [
+    {
+      mediaPath: {
+        path: '/tmp/window-7.5-14.5.mkv',
+        source: 'remote-window',
+        singleResolvedStream: true,
+        absoluteTimestamps: true,
+      },
+      startTime: 7.5,
+      endTime: 14.5,
+    },
+  ]);
+  assert.deepEqual(previewStarts, [
+    {
+      mediaPath: '/tmp/window-7.5-14.5.mkv',
+      executablePath: 'mpv',
+      volume: 60,
+      absoluteTimestamps: true,
+    },
+  ]);
+  assert.deepEqual(previewPlays, [['/tmp/window-7.5-14.5.mkv', 9.5, 12.5]]);
+  assert.deepEqual(disposed, ['/tmp/window-7.5-14.5.mkv']);
+});
+
+test('media timing review restarts the preview on a wider window when the timeline grows', async () => {
+  const windowStub = createWindowStub();
+  const waveformCalls: SpeechWaveformOptions[] = [];
+  const previewStarts: Array<Parameters<MediaTimingPreviewSessionLike['start']>[0]> = [];
+  const previewPlays: Array<[string, number, number]> = [];
+  const disposed: string[] = [];
+  const runtime = createRemoteReviewRuntime({
+    windowStub,
+    waveformCalls,
+    previewStarts,
+    previewPlays,
+    disposed,
+    openModal: async (active, payload) => {
+      await active.previewRange({ reviewId: payload.reviewId, startTime: 9.5, endTime: 12.5 });
+      // The user revealed two more seconds before the clip.
+      await active.getWaveform({ reviewId: payload.reviewId, startTime: 5.5, endTime: 14.5 });
+      await active.previewRange({ reviewId: payload.reviewId, startTime: 6, endTime: 12.5 });
+      active.resolveReview({ reviewId: payload.reviewId, decision: { action: 'use-original' } });
+    },
+  });
+
+  await runtime.requestReview({
+    kind: 'sentence',
+    text: '字幕',
+    startTime: 10,
+    endTime: 12,
+    audioPadding: 0.5,
+    maxMediaDuration: 30,
+  });
+
+  assert.deepEqual(
+    windowStub.calls.map((call) => call.range),
+    [
+      { startTime: 7.5, endTime: 14.5 },
+      { startTime: 5.5, endTime: 14.5 },
+    ],
+  );
+  assert.deepEqual(
+    previewStarts.map((start) => start.mediaPath),
+    ['/tmp/window-7.5-14.5.mkv', '/tmp/window-5.5-14.5.mkv'],
+  );
+  assert.deepEqual(previewPlays, [
+    ['/tmp/window-7.5-14.5.mkv', 9.5, 12.5],
+    ['/tmp/window-5.5-14.5.mkv', 6, 12.5],
+  ]);
+  assert.deepEqual(disposed, ['/tmp/window-7.5-14.5.mkv', '/tmp/window-5.5-14.5.mkv']);
+  assert.equal(waveformCalls[0]?.startTime, 5.5);
+});
+
+test('media timing review falls back to the remote stream after one failed window download', async () => {
+  const windowStub = createWindowStub({ fail: true });
+  const waveformCalls: SpeechWaveformOptions[] = [];
+  const previewStarts: Array<Parameters<MediaTimingPreviewSessionLike['start']>[0]> = [];
+  const previewPlays: Array<[string, number, number]> = [];
+  const disposed: string[] = [];
+  const runtime = createRemoteReviewRuntime({
+    windowStub,
+    waveformCalls,
+    previewStarts,
+    previewPlays,
+    disposed,
+    openModal: async (active, payload) => {
+      await active.getWaveform({
+        reviewId: payload.reviewId,
+        startTime: payload.timelineStartTime,
+        endTime: payload.timelineEndTime,
+      });
+      await active.previewRange({ reviewId: payload.reviewId, startTime: 9.5, endTime: 12.5 });
+      active.resolveReview({ reviewId: payload.reviewId, decision: { action: 'use-original' } });
+    },
+  });
+
+  await runtime.requestReview({
+    kind: 'word',
+    text: '字幕',
+    startTime: 10,
+    endTime: 12,
+    audioPadding: 0.5,
+    maxMediaDuration: 30,
+  });
+
+  assert.equal(windowStub.calls.length, 1);
+  assert.deepEqual(waveformCalls, [
+    {
+      mediaPath: { path: REMOTE_STREAM_URL, inputOptions: { reconnect: true } },
+      startTime: 7.5,
+      endTime: 14.5,
+      audioStreamIndex: 2,
+    },
+  ]);
+  assert.deepEqual(previewStarts, [
+    { mediaPath: REMOTE_STREAM_URL, executablePath: 'mpv', volume: 60, audioTrackId: 3 },
+  ]);
+  assert.deepEqual(previewPlays, [[REMOTE_STREAM_URL, 9.5, 12.5]]);
+});
+
+test('media timing review never downloads windows for local media', async () => {
+  const windowStub = createWindowStub();
+  let runtime!: ReturnType<typeof createMediaTimingReviewRuntime>;
+  runtime = createMediaTimingReviewRuntime({
+    getMpvClient: () => ({
+      connected: true,
+      currentVideoPath: '/video/show.mkv',
+      requestProperty: async (name) => (name === 'duration' ? 100 : name === 'pause' ? true : null),
+      send: () => undefined,
+    }),
+    getCurrentMediaPath: () => '/video/show.mkv',
+    getMpvExecutablePath: () => 'mpv',
+    resolveMediaSource: async () => ({ path: '/video/show.mkv' }),
+    acquireMediaWindow: windowStub.acquireMediaWindow,
+    generateWaveform: async () => [0.1, 0.8, 0.2],
+    createPreviewSession: () => ({
+      start: async () => undefined,
+      play: async () => undefined,
+      stop: async () => undefined,
+      dispose: () => undefined,
+    }),
+    openModal: async (payload) => {
+      await runtime.getWaveform({ reviewId: payload.reviewId, startTime: 7.5, endTime: 14.5 });
+      runtime.resolveReview({ reviewId: payload.reviewId, decision: { action: 'use-original' } });
+      return true;
+    },
+    showStatus: () => undefined,
+  });
+
+  await runtime.requestReview({
+    kind: 'sentence',
+    text: '字幕',
+    startTime: 10,
+    endTime: 12,
+    audioPadding: 0.5,
+    maxMediaDuration: 30,
+  });
+
+  assert.equal(windowStub.calls.length, 0);
+});
+
 test('media timing review rejects stale and out-of-range actions before allowing discard', async () => {
   const { runtime, payload, pendingDecision, previewCalls } = await startActiveMediaTimingReview({
     maxMediaDuration: 3,
@@ -384,11 +682,16 @@ test('media timing review restores playback when setup fails after pausing', asy
       send: ({ command }) => commands.push(command),
     }),
     getCurrentMediaPath: () => '/video/show.mkv',
-    getMpvExecutablePath: () => 'mpv',
-    generateWaveform: async () => [],
-    createPreviewSession: () => {
+    getMpvExecutablePath: () => {
       throw new Error('preview setup failed');
     },
+    generateWaveform: async () => [],
+    createPreviewSession: () => ({
+      start: async () => undefined,
+      play: async () => undefined,
+      stop: async () => undefined,
+      dispose: () => undefined,
+    }),
     openModal: async () => true,
     showStatus: () => undefined,
   });

src/main/runtime/media-timing-review.ts

@@ -11,6 +11,13 @@ import type {
   MediaTimingReviewWaveformResult,
 } from '../../types/anki';
 import type { SpeechWaveformOptions } from '../../core/services/media-timing-waveform';
+import {
+  isRemoteMediaWindowSourcePath,
+  type RemoteMediaWindow,
+  type RemoteMediaWindowRange,
+  type RemoteMediaWindowSource,
+} from '../../core/services/remote-media-window-cache';
+import type { MediaInput, MediaInputOptions } from '../../media-input';
 
 const INITIAL_TIMELINE_MARGIN_SECONDS = 2;
 const REVIEW_DECISION_TIMEOUT_MS = 5 * 60_000;
@@ -31,19 +38,36 @@ interface PreviewSession {
     executablePath?: string;
     audioTrackId?: number;
     volume?: number;
+    absoluteTimestamps?: boolean;
   }): Promise<void>;
   play(startTime: number, endTime: number): Promise<void>;
   stop(): Promise<void>;
   dispose(): void;
 }
 
+interface ReviewMediaSource {
+  path: string;
+  inputOptions?: MediaInputOptions;
+  singleResolvedStream?: boolean;
+}
+
 interface ActiveReview {
   payload: MediaTimingReviewOpenPayload;
+  /** What the hidden mpv preview plays when no cached window is available. */
   mediaPath: string;
+  /** What the waveform reads when no cached window is available. */
+  waveformMedia: MediaInput;
   audioStreamIndex?: number;
+  /** Remote source to download windows of; null for local media or without a cache. */
+  windowSource: RemoteMediaWindowSource | null;
+  /** Latest window returned for this review; reused while it still covers the request. */
+  window: RemoteMediaWindow | null;
+  windowRequest: (RemoteMediaWindowRange & { promise: Promise<RemoteMediaWindow | null> }) | null;
+  windowFailed: boolean;
+  previewOptions: { executablePath?: string; audioTrackId?: number; volume?: number };
+  preview: { path: string; session: Promise<PreviewSession> } | null;
   mpvClient: ReviewMpvClient;
   restorePlayback: boolean;
-  preview: Promise<PreviewSession>;
   resolve: (decision: MediaTimingReviewDecision) => void;
 }
 
@@ -53,6 +77,13 @@ export interface MediaTimingReviewRuntimeDeps {
   getMpvExecutablePath: () => string;
   createPreviewSession: () => PreviewSession;
   generateWaveform: (options: SpeechWaveformOptions) => Promise<number[]>;
+  /** Resolves the FFmpeg-readable stream URL and headers behind the current media path. */
+  resolveMediaSource?: () => Promise<ReviewMediaSource | null>;
+  /** Downloads (or reuses) a local window of a remote source covering the range. */
+  acquireMediaWindow?: (
+    source: RemoteMediaWindowSource,
+    range: RemoteMediaWindowRange,
+  ) => Promise<RemoteMediaWindow>;
   getSubtitleContextLines?: (range: { startTime: number; endTime: number }) => {
     previous: MediaTimingReviewContextLine[];
     next: MediaTimingReviewContextLine[];
@@ -199,6 +230,84 @@ export function createMediaTimingReviewRuntime(deps: MediaTimingReviewRuntimeDep
     }
   }
 
+  function ensureWindow(
+    review: ActiveReview,
+    range: RemoteMediaWindowRange,
+  ): Promise<RemoteMediaWindow | null> {
+    const { windowSource } = review;
+    if (!windowSource || review.windowFailed || !deps.acquireMediaWindow) {
+      return Promise.resolve(null);
+    }
+    const coversRange = (candidate: RemoteMediaWindowRange): boolean =>
+      candidate.startTime <= range.startTime && candidate.endTime >= range.endTime;
+    if (review.window && coversRange(review.window)) return Promise.resolve(review.window);
+    const inFlight = review.windowRequest;
+    if (inFlight && coversRange(inFlight)) return inFlight.promise;
+
+    const request = {
+      startTime: range.startTime,
+      endTime: range.endTime,
+      promise: Promise.resolve<RemoteMediaWindow | null>(null),
+    };
+    request.promise = deps
+      .acquireMediaWindow(windowSource, { startTime: range.startTime, endTime: range.endTime })
+      .then((window) => {
+        review.window = window;
+        return window;
+      })
+      .catch(() => {
+        // Fall back to the remote source for the rest of this review instead of retrying.
+        review.windowFailed = true;
+        return null;
+      })
+      .finally(() => {
+        if (review.windowRequest === request) review.windowRequest = null;
+      });
+    review.windowRequest = request;
+    return request.promise;
+  }
+
+  /**
+   * Returns the preview player for the range, restarting it when the range needs a
+   * different file (the first cached window, or a wider one after the timeline grew).
+   */
+  async function previewFor(
+    review: ActiveReview,
+    range: RemoteMediaWindowRange,
+  ): Promise<PreviewSession> {
+    const window = await ensureWindow(review, range);
+    if (active !== review) {
+      // The review ended during the download; do not start a player nobody will dispose.
+      throw new Error('This timing review is no longer active.');
+    }
+    const mediaPath = window?.path ?? review.mediaPath;
+    if (review.preview?.path === mediaPath) return review.preview.session;
+
+    const previous = review.preview;
+    const session = deps.createPreviewSession();
+    const { audioTrackId, ...previewOptions } = review.previewOptions;
+    const started = session
+      .start({
+        mediaPath,
+        ...previewOptions,
+        // A cached window keeps one audio stream, so mpv's track id from the source no longer applies.
+        ...(window
+          ? { absoluteTimestamps: true }
+          : audioTrackId !== undefined
+            ? { audioTrackId }
+            : {}),
+      })
+      .then(() => session)
+      .catch((error) => {
+        session.dispose();
+        throw error;
+      });
+    review.preview = { path: mediaPath, session: started };
+    void started.catch(() => {});
+    if (previous) void previous.session.then((old) => old.dispose()).catch(() => {});
+    return started;
+  }
+
   async function runReview(request: MediaTimingReviewRequest): Promise<MediaTimingReviewDecision> {
     const mpvClient = deps.getMpvClient();
     const mediaPath =
@@ -208,11 +317,12 @@ export function createMediaTimingReviewRuntime(deps: MediaTimingReviewRuntimeDep
       return { action: 'use-original' };
     }
 
-    const [pauseRaw, durationRaw, audioTrackRaw, volumeRaw] = await Promise.all([
+    const [pauseRaw, durationRaw, audioTrackRaw, volumeRaw, resolvedSource] = await Promise.all([
       mpvClient.requestProperty?.('pause').catch(() => null) ?? null,
       mpvClient.requestProperty?.('duration').catch(() => null) ?? null,
       mpvClient.requestProperty?.('aid').catch(() => null) ?? null,
       mpvClient.requestProperty?.('volume').catch(() => null) ?? null,
+      deps.resolveMediaSource?.().catch(() => null) ?? null,
     ]);
     const pauseState = booleanProperty(pauseRaw);
     mpvClient.send({ command: ['set_property', 'pause', 'yes'] });
@@ -232,38 +342,51 @@ export function createMediaTimingReviewRuntime(deps: MediaTimingReviewRuntimeDep
       mediaDuration: finiteNumber(durationRaw) ?? undefined,
       ...(contextLines ? { contextLines } : {}),
     });
-    const previewSession = deps.createPreviewSession();
-    const preview = previewSession
-      .start({
-        mediaPath,
-        executablePath: deps.getMpvExecutablePath(),
-        audioTrackId: finiteNumber(audioTrackRaw) ?? undefined,
-        volume: finiteNumber(volumeRaw) ?? undefined,
-      })
-      .then(() => previewSession)
-      .catch((error) => {
-        previewSession.dispose();
-        throw error;
-      });
-    void preview.catch(() => {});
+    const sourcePath = resolvedSource?.path.trim() || mediaPath;
+    const inputOptions = resolvedSource?.inputOptions;
+    const audioStreamIndex =
+      resolvedSource?.singleResolvedStream || mpvClient.currentAudioStreamIndex == null
+        ? undefined
+        : mpvClient.currentAudioStreamIndex;
+    const windowSource: RemoteMediaWindowSource | null =
+      deps.acquireMediaWindow && isRemoteMediaWindowSourcePath(sourcePath)
+        ? {
+            path: sourcePath,
+            ...(inputOptions ? { inputOptions } : {}),
+            audioStreamIndex: audioStreamIndex ?? null,
+          }
+        : null;
 
     let resolveDecision!: (decision: MediaTimingReviewDecision) => void;
     const decisionPromise = new Promise<MediaTimingReviewDecision>((resolve) => {
       resolveDecision = resolve;
     });
-    active = {
+    const review: ActiveReview = {
       payload,
       mediaPath,
-      ...(mpvClient.currentAudioStreamIndex !== null &&
-      mpvClient.currentAudioStreamIndex !== undefined
-        ? { audioStreamIndex: mpvClient.currentAudioStreamIndex }
-        : {}),
+      waveformMedia: inputOptions ? { path: sourcePath, inputOptions } : sourcePath,
+      ...(audioStreamIndex !== undefined ? { audioStreamIndex } : {}),
+      windowSource,
+      window: null,
+      windowRequest: null,
+      windowFailed: false,
+      previewOptions: {
+        executablePath: deps.getMpvExecutablePath(),
+        audioTrackId: finiteNumber(audioTrackRaw) ?? undefined,
+        volume: finiteNumber(volumeRaw) ?? undefined,
+      },
+      preview: null,
       mpvClient,
       restorePlayback: pendingPauseRestore === mpvClient,
-      preview,
       resolve: resolveDecision,
     };
+    active = review;
     pendingPauseRestore = null;
+    // Download the visible timeline once now; the waveform and preview both wait on it.
+    void previewFor(review, {
+      startTime: payload.timelineStartTime,
+      endTime: payload.timelineEndTime,
+    }).catch(() => {});
 
     const opened = await deps.openModal(payload).catch(() => false);
     if (!opened) {
@@ -317,7 +440,10 @@ export function createMediaTimingReviewRuntime(deps: MediaTimingReviewRuntimeDep
       return { ok: false, message: 'The selected preview range is invalid.' };
     }
     try {
-      const previewSession = await current.preview;
+      const previewSession = await previewFor(current, request);
+      if (active !== current) {
+        return { ok: false, message: 'This timing review is no longer active.' };
+      }
       await previewSession.play(request.startTime, request.endTime);
       return { ok: true };
     } catch (error) {
@@ -347,11 +473,15 @@ export function createMediaTimingReviewRuntime(deps: MediaTimingReviewRuntimeDep
     }
 
     try {
+      const window = await ensureWindow(current, request);
+      if (active !== current) {
+        return { ok: false, message: 'This timing review is no longer active.' };
+      }
       const peaks = await deps.generateWaveform({
-        mediaPath: current.mediaPath,
+        mediaPath: window?.media ?? current.waveformMedia,
         startTime: request.startTime,
         endTime: request.endTime,
-        ...(current.audioStreamIndex !== undefined
+        ...(!window && current.audioStreamIndex !== undefined
           ? { audioStreamIndex: current.audioStreamIndex }
           : {}),
       });
@@ -370,8 +500,8 @@ export function createMediaTimingReviewRuntime(deps: MediaTimingReviewRuntimeDep
       return { ok: false, message: 'This timing review is no longer active.' };
     }
     try {
-      const previewSession = await current.preview;
-      await previewSession.stop();
+      const previewSession = current.preview ? await current.preview.session : null;
+      await previewSession?.stop();
       return { ok: true };
     } catch (error) {
       return {
@@ -403,7 +533,7 @@ export function createMediaTimingReviewRuntime(deps: MediaTimingReviewRuntimeDep
     const current = active;
     active = null;
     if (!current) return;
-    void current.preview.then((session) => session.dispose()).catch(() => {});
+    void current.preview?.session.then((session) => session.dispose()).catch(() => {});
     if (current.restorePlayback && current.mpvClient.connected) {
       current.mpvClient.send({ command: ['set_property', 'pause', 'no'] });
     }

src/media-generator.test.ts

@@ -10,6 +10,9 @@ import {
   MediaGenerator,
   type MediaGeneratorOptions,
 } from './media-generator';
+import { RemoteMediaWindowCache } from './core/services/remote-media-window-cache';
+
+const REMOTE_STREAM_URL = 'https://jellyfin.example/Videos/abc/stream?static=true';
 
 async function withStubbedFfmpeg(
   run: (generator: MediaGenerator, argsPath: string) => Promise<void>,
@@ -21,6 +24,7 @@ async function withStubbedFfmpeg(
   const root = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-media-generator-test-'));
   const binDir = path.join(root, 'bin');
   const tempDir = path.join(root, 'media');
+  const windowsDir = path.join(root, 'windows');
   const argsPath = path.join(root, 'ffmpeg-args.txt');
   fs.mkdirSync(binDir, { recursive: true });
   const ffmpegStubPath = path.join(binDir, 'ffmpeg-stub.cjs');
@@ -34,7 +38,7 @@ async function withStubbedFfmpeg(
       "  console.log(' V..... libaom-av1');",
       '  process.exit(0);',
       '}',
-      "fs.writeFileSync(process.env.SUBMINER_TEST_FFMPEG_ARGS, JSON.stringify(args), 'utf8');",
+      "fs.appendFileSync(process.env.SUBMINER_TEST_FFMPEG_ARGS, JSON.stringify(args) + '\\n', 'utf8');",
       'const outputPath = args.at(-1);',
       "if (process.env.SUBMINER_TEST_FFMPEG_SKIP_OUTPUT !== '1') {",
       "  fs.writeFileSync(outputPath, 'avif', 'utf8');",
@@ -61,12 +65,18 @@ async function withStubbedFfmpeg(
   } else {
     delete process.env.SUBMINER_TEST_FFMPEG_SKIP_OUTPUT;
   }
-  const generator = new MediaGenerator(tempDir, options);
+  // Each test gets its own window cache so remote inputs never leak windows between tests.
+  const remoteMediaWindows = new RemoteMediaWindowCache({ tempDir: windowsDir, idleTtlMs: 0 });
+  const generator = new MediaGenerator(tempDir, {
+    remoteMediaWindows,
+    ...options,
+  });
 
   try {
     await run(generator, argsPath);
   } finally {
     generator.cleanup();
+    remoteMediaWindows.cleanup();
     process.env.PATH = originalPath;
     if (originalArgsPath === undefined) {
       delete process.env.SUBMINER_TEST_FFMPEG_ARGS;
@@ -82,8 +92,17 @@ async function withStubbedFfmpeg(
   }
 }
 
+function readAllFfmpegArgs(argsPath: string): string[][] {
+  return fs
+    .readFileSync(argsPath, 'utf8')
+    .split('\n')
+    .filter((line) => line.trim().length > 0)
+    .map((line) => JSON.parse(line) as string[]);
+}
+
+/** Arguments of the most recent ffmpeg invocation. */
 function readFfmpegArgs(argsPath: string): string[] {
-  return JSON.parse(fs.readFileSync(argsPath, 'utf8')) as string[];
+  return readAllFfmpegArgs(argsPath).at(-1) ?? [];
 }
 
 test('buildAnimatedImageVideoFilter holds lead-in until the next frame after the audio boundary', () => {
@@ -272,41 +291,131 @@ test('generateAudio recreates missing temp directory before invoking ffmpeg', as
 });
 
 test('generateAudio adds remote input options before the ffmpeg input', async () => {
-  await withStubbedFfmpeg(async (generator, argsPath) => {
-    await generator.generateAudio(
-      {
-        path: 'https://rr1---sn.example.googlevideo.com/videoplayback?mime=audio%2Fwebm',
-        inputOptions: {
-          reconnect: true,
-          userAgent: 'Mozilla/5.0',
-          headers: {
-            Referer: 'https://www.youtube.com/',
-            Origin: 'https://www.youtube.com',
+  await withStubbedFfmpeg(
+    async (generator, argsPath) => {
+      await generator.generateAudio(
+        {
+          path: 'https://rr1---sn.example.googlevideo.com/videoplayback?mime=audio%2Fwebm',
+          inputOptions: {
+            reconnect: true,
+            userAgent: 'Mozilla/5.0',
+            headers: {
+              Referer: 'https://www.youtube.com/',
+              Origin: 'https://www.youtube.com',
+            },
           },
         },
-      },
+        10,
+        12,
+      );
+
+      const args = readFfmpegArgs(argsPath);
+      const inputIndex = args.indexOf('-i');
+      assert.ok(inputIndex > 0);
+      assert.ok(args.indexOf('-reconnect') > -1);
+      assert.ok(args.indexOf('-reconnect') < inputIndex);
+      assert.equal(args[args.indexOf('-reconnect') + 1], '1');
+      assert.equal(args[args.indexOf('-reconnect_streamed') + 1], '1');
+      assert.equal(args[args.indexOf('-reconnect_on_network_error') + 1], '1');
+      assert.equal(args[args.indexOf('-reconnect_on_http_error') + 1], '403,5xx');
+      assert.equal(args[args.indexOf('-reconnect_delay_max') + 1], '5');
+      assert.equal(args[args.indexOf('-user_agent') + 1], 'Mozilla/5.0');
+      assert.equal(
+        args[args.indexOf('-headers') + 1],
+        'Referer: https://www.youtube.com/\r\nOrigin: https://www.youtube.com\r\n',
+      );
+    },
+    { remoteMediaWindows: null },
+  );
+});
+
+test('generateAudio downloads a remote window once and extracts from it with absolute seeks', async () => {
+  await withStubbedFfmpeg(async (generator, argsPath) => {
+    await generator.generateAudio(
+      { path: REMOTE_STREAM_URL, inputOptions: { reconnect: true } },
       10,
       12,
+      0.5,
+      2,
     );
 
-    const args = readFfmpegArgs(argsPath);
-    const inputIndex = args.indexOf('-i');
-    assert.ok(inputIndex > 0);
-    assert.ok(args.indexOf('-reconnect') > -1);
-    assert.ok(args.indexOf('-reconnect') < inputIndex);
-    assert.equal(args[args.indexOf('-reconnect') + 1], '1');
-    assert.equal(args[args.indexOf('-reconnect_streamed') + 1], '1');
-    assert.equal(args[args.indexOf('-reconnect_on_network_error') + 1], '1');
-    assert.equal(args[args.indexOf('-reconnect_on_http_error') + 1], '403,5xx');
-    assert.equal(args[args.indexOf('-reconnect_delay_max') + 1], '5');
-    assert.equal(args[args.indexOf('-user_agent') + 1], 'Mozilla/5.0');
-    assert.equal(
-      args[args.indexOf('-headers') + 1],
-      'Referer: https://www.youtube.com/\r\nOrigin: https://www.youtube.com\r\n',
-    );
+    const calls = readAllFfmpegArgs(argsPath);
+    assert.equal(calls.length, 2);
+    const [fetchArgs, audioArgs] = calls as [string[], string[]];
+    assert.equal(fetchArgs[fetchArgs.indexOf('-i') + 1], REMOTE_STREAM_URL);
+    assert.ok(fetchArgs.indexOf('-reconnect') < fetchArgs.indexOf('-i'));
+    assert.equal(fetchArgs[fetchArgs.indexOf('-ss') + 1], '9.25');
+    assert.equal(fetchArgs[fetchArgs.lastIndexOf('-map') + 1], '0:2');
+    assert.ok(fetchArgs.includes('-copyts'));
+
+    const windowPath = audioArgs[audioArgs.indexOf('-i') + 1];
+    assert.ok(windowPath?.endsWith('.mkv'));
+    assert.notEqual(windowPath, REMOTE_STREAM_URL);
+    assert.equal(audioArgs[audioArgs.indexOf('-ss') + 1], '9.5');
+    assert.equal(audioArgs[audioArgs.indexOf('-seek_timestamp') + 1], '1');
+    assert.ok(audioArgs.indexOf('-seek_timestamp') < audioArgs.indexOf('-i'));
+    assert.equal(audioArgs.includes('-reconnect'), false);
+    assert.equal(audioArgs.includes('-map'), false);
+    assert.equal(audioArgs.includes('-probesize'), false);
+    assert.ok(audioArgs.includes('loudnorm=I=-23:TP=-2:LRA=11'));
   });
 });
 
+test('generateScreenshot reuses a downloaded window but never downloads one itself', async () => {
+  await withStubbedFfmpeg(async (generator, argsPath) => {
+    await generator.generateScreenshot(REMOTE_STREAM_URL, 11, { format: 'jpg' });
+    let calls = readAllFfmpegArgs(argsPath);
+    assert.equal(calls.length, 1);
+    assert.equal(calls[0]![calls[0]!.indexOf('-i') + 1], REMOTE_STREAM_URL);
+
+    await generator.generateAudio(REMOTE_STREAM_URL, 10, 12);
+    await generator.generateScreenshot(REMOTE_STREAM_URL, 11, { format: 'jpg' });
+    await generator.generateScreenshot(REMOTE_STREAM_URL, 40, { format: 'jpg' });
+
+    calls = readAllFfmpegArgs(argsPath);
+    assert.equal(calls.length, 5);
+    const insideWindow = calls[3]!;
+    assert.ok(insideWindow[insideWindow.indexOf('-i') + 1]?.endsWith('.mkv'));
+    assert.equal(insideWindow[insideWindow.indexOf('-seek_timestamp') + 1], '1');
+    const outsideWindow = calls[4]!;
+    assert.equal(outsideWindow[outsideWindow.indexOf('-i') + 1], REMOTE_STREAM_URL);
+  });
+});
+
+test('generateAnimatedImage downloads the clip window before encoding', async () => {
+  await withStubbedFfmpeg(async (generator, argsPath) => {
+    await generator.generateAnimatedImage(REMOTE_STREAM_URL, 10, 12, 0, { fps: 10 });
+
+    const calls = readAllFfmpegArgs(argsPath).filter(
+      (args) => args[0] !== '-hide_banner' || args[1] !== '-encoders',
+    );
+    assert.equal(calls.length, 2);
+    assert.equal(calls[0]![calls[0]!.indexOf('-i') + 1], REMOTE_STREAM_URL);
+    assert.ok(calls[1]![calls[1]!.indexOf('-i') + 1]?.endsWith('.mkv'));
+    assert.equal(calls[1]![calls[1]!.indexOf('-seek_timestamp') + 1], '1');
+  });
+});
+
+test('generateAudio reads the remote source directly when the window download fails', async () => {
+  await withStubbedFfmpeg(
+    async (generator, argsPath) => {
+      await generator.generateAudio(REMOTE_STREAM_URL, 10, 12);
+
+      const args = readFfmpegArgs(argsPath);
+      assert.equal(args[args.indexOf('-i') + 1], REMOTE_STREAM_URL);
+      assert.equal(args.includes('-seek_timestamp'), false);
+    },
+    {
+      remoteMediaWindows: new RemoteMediaWindowCache({
+        execFile: (_file, _args, _options, callback) =>
+          queueMicrotask(() => callback(Object.assign(new Error('offline'), { code: 1 }))),
+        idleTtlMs: 0,
+        logDebug: () => undefined,
+      }),
+    },
+  );
+});
+
 test('generateAudio skips stale audio stream maps for single resolved streams', async () => {
   await withStubbedFfmpeg(async (generator, argsPath) => {
     await generator.generateAudio(
@@ -320,8 +429,9 @@ test('generateAudio skips stale audio stream maps for single resolved streams',
       22,
     );
 
-    const args = readFfmpegArgs(argsPath);
-    assert.equal(args.includes('-map'), false);
+    const [fetchArgs, audioArgs] = readAllFfmpegArgs(argsPath) as [string[], string[]];
+    assert.equal(fetchArgs[fetchArgs.lastIndexOf('-map') + 1], '0:a');
+    assert.equal(audioArgs.includes('-map'), false);
   });
 });
 

src/media-generator.ts

@@ -22,6 +22,12 @@ import * as path from 'path';
 import * as os from 'os';
 import { createLogger } from './logger';
 import { normalizeMediaInput, type MediaInput } from './media-input';
+import {
+  getSharedRemoteMediaWindowCache,
+  isRemoteMediaWindowSourcePath,
+  type RemoteMediaWindowCache,
+  type RemoteMediaWindowRange,
+} from './core/services/remote-media-window-cache';
 
 const log = createLogger('media');
 const AUDIO_NORMALIZATION_FILTER = 'loudnorm=I=-23:TP=-2:LRA=11';
@@ -86,6 +92,11 @@ export interface MediaGeneratorOptions {
   logDebug?: (message: string) => void;
   now?: () => number;
   execFile?: MediaGeneratorExecFile;
+  /**
+   * Local window cache for http(s) sources. Defaults to the process-wide cache shared
+   * with the timing review; pass `null` to always read remote sources directly.
+   */
+  remoteMediaWindows?: RemoteMediaWindowCache | null;
 }
 
 function sanitizeDebugToken(value: string, fallback: string): string {
@@ -232,6 +243,54 @@ export class MediaGenerator {
     }, delayMs);
   }
 
+  /**
+   * Swaps an http(s) input for the locally cached window that covers `range`, so the
+   * clip is downloaded once instead of per FFmpeg run. `acquire` downloads on a miss;
+   * `lookup` only reuses a window that another step already fetched. Any failure falls
+   * back to reading the remote source directly.
+   */
+  private async resolveRemoteWindowInput(
+    input: MediaInput,
+    range: RemoteMediaWindowRange,
+    audioStreamIndex: number | null | undefined,
+    mode: 'acquire' | 'lookup',
+  ): Promise<MediaInput> {
+    const cache =
+      this.options.remoteMediaWindows === undefined
+        ? getSharedRemoteMediaWindowCache()
+        : this.options.remoteMediaWindows;
+    const sourcePath = typeof input === 'string' ? input : input.path;
+    if (!cache || !isRemoteMediaWindowSourcePath(sourcePath)) {
+      return input;
+    }
+    const source = {
+      path: sourcePath,
+      ...(typeof input === 'object' && input.inputOptions
+        ? { inputOptions: input.inputOptions }
+        : {}),
+      audioStreamIndex:
+        typeof input === 'object' && input.singleResolvedStream ? null : (audioStreamIndex ?? null),
+    };
+    const description = describeMediaInputForDebugLog(input);
+    try {
+      const window =
+        mode === 'acquire' ? await cache.acquire(source, range) : await cache.lookup(source, range);
+      if (!window) {
+        this.logMediaDebug(`window miss ${description} mode=${mode}`);
+        return input;
+      }
+      this.logMediaDebug(
+        `window hit ${description} mode=${mode} start=${window.startTime} end=${window.endTime}`,
+      );
+      return window.media;
+    } catch (error) {
+      this.logMediaDebug(
+        `window failed ${description} mode=${mode} reason=${sanitizeDebugToken((error as Error).message, 'error')}`,
+      );
+      return input;
+    }
+  }
+
   private ffmpegError(label: string, error: ExecFileException): Error {
     if (error.code === 'ENOENT') {
       return new Error('FFmpeg not found. Install FFmpeg to enable media generation.');
@@ -281,7 +340,13 @@ export class MediaGenerator {
     const safePadding = Number.isFinite(padding) ? Math.max(0, padding) : 0;
     const start = Math.max(0, startTime - safePadding);
     const duration = endTime - start + safePadding;
-    const mediaInput = normalizeMediaInput(videoPath);
+    const sourceInput = await this.resolveRemoteWindowInput(
+      videoPath,
+      { startTime: start, endTime: start + duration },
+      audioStreamIndex,
+      'acquire',
+    );
+    const mediaInput = normalizeMediaInput(sourceInput);
     const inputDescription = describeMediaInputForDebugLog(videoPath);
     const hasSelectedAudioStream =
       !mediaInput.singleResolvedStream &&
@@ -385,7 +450,15 @@ export class MediaGenerator {
       png: 'png',
       webp: 'webp',
     };
-    const mediaInput = normalizeMediaInput(videoPath);
+    // A single frame is cheap to fetch remotely, so only reuse a window another step downloaded.
+    const mediaInput = normalizeMediaInput(
+      await this.resolveRemoteWindowInput(
+        videoPath,
+        { startTime: timestamp, endTime: timestamp },
+        null,
+        'lookup',
+      ),
+    );
     const inputDescription = describeMediaInputForDebugLog(videoPath);
 
     const args: string[] = [
@@ -533,9 +606,17 @@ export class MediaGenerator {
       );
     }
 
+    const mediaInput = normalizeMediaInput(
+      await this.resolveRemoteWindowInput(
+        videoPath,
+        { startTime: start, endTime: start + duration },
+        null,
+        'acquire',
+      ),
+    );
+
     return new Promise((resolve, reject) => {
       const outputPath = this.createTempOutputPath('animation', 'avif');
-      const mediaInput = normalizeMediaInput(videoPath);
       const startedAt = this.nowMs();
 
       const encoderArgs: string[] = ['-c:v', av1Encoder];

src/media-input.ts

@@ -11,6 +11,12 @@ export type MediaInput =
       source?: string;
       inputOptions?: MediaInputOptions;
       singleResolvedStream?: boolean;
+      /**
+       * The file keeps the original media timestamps instead of starting at zero (a
+       * stream-copied window of a longer source). Seek with `-ss` against those
+       * absolute timestamps rather than relative to the file's own start time.
+       */
+      absoluteTimestamps?: boolean;
     };
 
 export type NormalizedMediaInput = {
@@ -89,6 +95,10 @@ export function normalizeMediaInput(input: MediaInput): NormalizedMediaInput {
     inputArgs.push('-headers', headers);
   }
 
+  if (input.absoluteTimestamps) {
+    inputArgs.push('-seek_timestamp', '1');
+  }
+
   return {
     path: input.path,
     inputArgs,