Keep initPTS across the chunks of a progressive multi-moof segment

In progressive mode every chunk of a segment is remuxed with the
segment's timeOffset, so only the first chunk's decode time lines up
with it. A second moof starting further into the segment was treated as
a new initPTS and appended over the chunks before it, stalling playback.

Fixes #5697

src/remux/passthrough-remuxer.ts

@@ -49,6 +49,7 @@ class PassThroughRemuxer extends Logger implements Remuxer {
   private initPTS: TimestampOffset | null = null;
   private initTracks?: TrackSet;
   private lastEndTime: number | null = null;
+  private lastChunkKey: string | null = null;
   private isVideoContiguous: boolean = false;
   private videoOnlyRemux: boolean = false;
   private decryptdata: DecryptData | null = null;
@@ -74,6 +75,7 @@ class PassThroughRemuxer extends Logger implements Remuxer {
 
   public resetTimeStamp(defaultInitPTS: TimestampOffset | null) {
     this.lastEndTime = null;
+    this.lastChunkKey = null;
     const initPTS = this.initPTS;
     if (initPTS && defaultInitPTS) {
       if (
@@ -89,6 +91,7 @@ class PassThroughRemuxer extends Logger implements Remuxer {
   public resetNextTimestamp() {
     this.isVideoContiguous = false;
     this.lastEndTime = null;
+    this.lastChunkKey = null;
   }
 
   public resetInitSegment(
@@ -441,8 +444,14 @@ class PassThroughRemuxer extends Logger implements Remuxer {
 
     decodeTime = baseOffsetSamples.start / timescale;
 
+    // Every chunk of a progressive segment is remuxed with the segment's
+    // timeOffset, so only the first chunk's decode time lines up with it
+    const chunkKey = `${chunkMeta.level}_${chunkMeta.sn}_${chunkMeta.part}`;
+    const segmentStart = chunkKey !== this.lastChunkKey;
+    this.lastChunkKey = chunkKey;
+
     if (
-      (accurateTimeOffset || !initPTS) &&
+      ((accurateTimeOffset && segmentStart) || !initPTS) &&
       (isInvalidInitPts(initPTS, decodeTime, timeOffset, duration) ||
         timescale !== initPTS.timescale)
     ) {