fix: calculate EXT-X-START TIME-OFFSET from actual playlist end for live streams (#1599)

For live streams with negative TIME-OFFSET in EXT-X-START, the offset
was incorrectly calculated from seekableEnd, which already has
liveEdgeDelay subtracted. This caused playback to start further behind
the live edge than intended.

Per HLS spec, negative TIME-OFFSET should be relative to the end of the
last Media Segment. This fix adds liveEdgeDelay back to seekableEnd
before applying the offset, making VHS behavior consistent with native
HLS implementations (e.g., Safari).

Example: With liveEdgeDelay=18 and TIME-OFFSET=-18:
- Before: startPoint = seekableEnd - 18 (36s behind actual live edge)
- After: startPoint = seekableEnd (18s behind, matching Safari)

Author: LorDisturbia

src/playlist-controller.js

@@ -1221,7 +1221,15 @@ export class PlaylistController extends videojs.EventTarget {
         const offset = media.start.timeOffset;
 
         if (offset < 0) {
-          startPoint = Math.max(seekableEnd + offset, seekable.start(0));
+          // Per HLS spec, negative TIME-OFFSET is from the end of the last Media Segment.
+          // For live streams, seekableEnd already has liveEdgeDelay subtracted,
+          // so we need to add it back to get the actual playlist end.
+          // Clamp to seekableEnd to avoid seeking into the unsafe live edge zone.
+          const main = this.mainPlaylistLoader_.main;
+          const liveEdgeDelay = Vhs.Playlist.liveEdgeDelay(main, media);
+          const actualPlaylistEnd = seekableEnd + liveEdgeDelay;
+
+          startPoint = Math.max(Math.min(actualPlaylistEnd + offset, seekableEnd), seekable.start(0));
         } else {
           startPoint = Math.min(seekableEnd, offset);
         }

test/manifests/startLiveNegative.m3u8

@@ -0,0 +1,15 @@
+#EXTM3U
+#EXT-X-START:TIME-OFFSET=-30,PRECISE=YES
+#EXT-X-TARGETDURATION:10
+#EXTINF:10,
+media-00001.ts
+#EXTINF:10,
+media-00002.ts
+#EXTINF:10,
+media-00003.ts
+#EXTINF:10,
+media-00004.ts
+#EXTINF:10,
+media-00005.ts
+#EXTINF:10,
+media-00006.ts

test/videojs-http-streaming.test.js

@@ -485,6 +485,30 @@ QUnit.test('seeks to negative offset point', function(assert) {
   assert.strictEqual(currentTime, 35, 'seeked to negative offset');
 });
 
+QUnit.test('seeks to negative offset on live stream from actual playlist end', function(assert) {
+  let currentTime = 0;
+
+  this.player.autoplay(true);
+  this.player.on('seeking', () => {
+    currentTime = this.player.currentTime();
+  });
+  this.player.src({
+    src: 'startLiveNegative.m3u8',
+    type: 'application/vnd.apple.mpegurl'
+  });
+
+  this.clock.tick(1);
+
+  openMediaSource(this.player, this.clock);
+  this.player.tech_.trigger('play');
+  this.standardXHRResponse(this.requests.shift());
+  this.clock.tick(1);
+
+  // 6 segments * 10s = 60s total, liveEdgeDelay = 30, TIME-OFFSET = -30
+  // Per HLS spec, offset is from playlist end (60s), so startPoint = 60 - 30 = 30
+  assert.strictEqual(currentTime, 30, 'seeked to negative offset from actual playlist end on live stream');
+});
+
 QUnit.test(
   'duration is set when the source opens after the playlist is loaded',
   function(assert) {