fix: adjustShuttle reads live rate with 0.25 step, fullscreen targets documentElement

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/lib/transport/playback.test.ts

@@ -19,6 +19,7 @@ import {
   isFullscreenActive,
   setPlaybackRate,
   getPlaybackRate,
+  adjustShuttle,
 } from '$lib/transport/playback';
 
 function createMockVideoElement(): HTMLVideoElement {
@@ -85,9 +86,11 @@ describe('playback — fullscreen helpers', () => {
     expect(isFullscreenActive()).toBe(false);
   });
 
-  it('toggleFullscreen calls requestFullscreen when not fullscreen', () => {
+  it('toggleFullscreen calls requestFullscreen on document.documentElement', () => {
+    const spy = vi.fn().mockResolvedValue(undefined);
+    document.documentElement.requestFullscreen = spy;
     toggleFullscreen();
-    expect(mockVideo.requestFullscreen).toHaveBeenCalled();
+    expect(spy).toHaveBeenCalled();
   });
 
   it('isFullscreenActive returns true when fullscreen element exists', () => {
@@ -127,3 +130,59 @@ describe('playback — playback rate helpers', () => {
     expect(getPlaybackRate()).toBe(1);
   });
 });
+
+describe('playback — adjustShuttle', () => {
+  let mockVideo: HTMLVideoElement;
+
+  beforeEach(() => {
+    mockVideo = createMockVideoElement();
+    setVideoElement(mockVideo);
+    mockVideo.playbackRate = 1;
+    Object.defineProperty(mockVideo, 'paused', {
+      value: true,
+      writable: true,
+      configurable: true,
+    });
+    mockVideo.play = vi.fn().mockResolvedValue(undefined);
+  });
+
+  it('increases rate by 0.25 when dir is 1', () => {
+    const result = adjustShuttle(1);
+    expect(result).toBe(1.25);
+    expect(mockVideo.playbackRate).toBe(1.25);
+  });
+
+  it('decreases rate by 0.25 when dir is -1', () => {
+    const result = adjustShuttle(-1);
+    expect(result).toBe(0.75);
+    expect(mockVideo.playbackRate).toBe(0.75);
+  });
+
+  it('reads current rate from video element, not external state', () => {
+    mockVideo.playbackRate = 0.75;
+    const result = adjustShuttle(1);
+    expect(result).toBe(1);
+  });
+
+  it('clamps to minimum 0.25', () => {
+    mockVideo.playbackRate = 0.25;
+    const result = adjustShuttle(-1);
+    expect(result).toBe(0.25);
+  });
+
+  it('clamps to maximum 4', () => {
+    mockVideo.playbackRate = 4;
+    const result = adjustShuttle(1);
+    expect(result).toBe(4);
+  });
+
+  it('starts playback if paused', () => {
+    adjustShuttle(1);
+    expect(mockVideo.play).toHaveBeenCalled();
+  });
+
+  it('returns 1 when no video element', () => {
+    setVideoElement(null);
+    expect(adjustShuttle(1)).toBe(1);
+  });
+});