fix(spf): derive the live window from any resolved track during selection churn

liveWindowFromState keyed the live window on the *selected* timeline-bearing
track, so when an ABR/user switch briefly left the newly-selected rendition
unresolved (a shell, no segments), the window blinked to null. That flipped
seekToLiveEdge out of `live` and stalled the seekable-range writer. The live
window is a presentation-level property — all renditions are time-aligned and
share the anchor — so fall back to any resolved track of the timeline-bearing
type when the selected one isn't resolved yet, rather than returning null. A
deselected track's window may trail live by up to one reload during the switch
gap; the selected track's fresh window resumes the moment it resolves.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/spf/src/playback/primitives/live-window.ts

@@ -14,6 +14,7 @@
 import type { ReadonlySignal } from '../../core/signals/primitives';
 import { type LiveWindow, liveWindowFor } from '../../media/live-window';
 import type { MaybeResolvedPresentation } from '../../media/types';
+import { getTracksByType } from '../../media/utils/tracks';
 
 export interface LiveWindowState {
   presentation: ReadonlySignal<MaybeResolvedPresentation | undefined>;
@@ -31,7 +32,28 @@ export function liveTrackId(state: LiveWindowState): string | undefined {
 }
 
 export function liveWindowFromState(state: LiveWindowState): LiveWindow | null {
-  return liveWindowFor(state.presentation.get(), liveTrackId(state));
+  const presentation = state.presentation.get();
+
+  // Prefer the selected timeline-bearing track when it's resolved.
+  const selected = liveWindowFor(presentation, liveTrackId(state));
+  if (selected) return selected;
+
+  // The selected track isn't resolved yet (e.g. briefly mid ABR / user switch).
+  // The live window is a presentation-level property — all renditions are
+  // time-aligned and share the anchor — so fall back to any resolved track of the
+  // timeline-bearing type rather than letting the window blink to `null`. A null
+  // blink would flip `seekToLiveEdge` out of `live` and re-fire its one-time edge
+  // seek (yanking a viewer who has scrubbed back), and stall the seekable-range
+  // writer. A deselected track's window may trail live by up to one reload during
+  // the switch gap — acceptable, and the selected track's fresh window resumes the
+  // moment it resolves.
+  if (!presentation) return null;
+  const type = state.selectedVideoTrackId?.get() ? 'video' : 'audio';
+  for (const track of getTracksByType(presentation, type)) {
+    const window = liveWindowFor(presentation, track.id);
+    if (window) return window;
+  }
+  return null;
 }
 
 /**

packages/spf/src/playback/primitives/tests/live-window.test.ts

@@ -5,6 +5,7 @@ import {
   type AudioTrack,
   type MaybeResolvedPresentation,
   MEDIA_PLAYLIST_METADATA_KEY,
+  type PartiallyResolvedVideoTrack,
   type Presentation,
   type SelectionSet,
   type VideoSelectionSet,
@@ -98,6 +99,29 @@ describe('liveWindowFromState', () => {
     });
   });
 
+  it('falls back to a resolved track when the selected track is not yet resolved (mid-switch)', () => {
+    // ABR/user switch in flight: the selected rendition (v-2) is still a shell.
+    // The window is presentation-level, so it must come from the resolved sibling
+    // (v-1) rather than blinking to null.
+    const shell: PartiallyResolvedVideoTrack = {
+      type: 'video',
+      id: 'v-2',
+      url: 'https://example.com/v2.m3u8',
+      mimeType: 'video/mp4',
+      codecs: ['avc1.640028'],
+      bandwidth: 2_000_000,
+    };
+    const pres: Presentation = {
+      id: 'pres-1',
+      url: 'https://example.com/master.m3u8',
+      startTime: 0,
+      selectionSets: [
+        { id: 'v-set', type: 'video', switchingSets: [{ id: 'vss', type: 'video', tracks: [videoTrack(100), shell] }] },
+      ],
+    };
+    expect(liveWindowFromState(state({ presentation: pres, videoId: 'v-2' }))).toEqual({ start: 100, end: 110 });
+  });
+
   it('returns null when no track is selected', () => {
     const pres = presentation({ video: videoTrack(100) });
     expect(liveWindowFromState(state({ presentation: pres, videoId: undefined, audioId: undefined }))).toBeNull();