fix: Fix media source duration when using sequence mode (shaka-projec…

…t#4848)

Tested with
https://pub-e06bd0d1422649108618e7121f2af8b5.r2.dev/finite.m3u8

lib/media/media_source_engine.js

@@ -658,6 +658,8 @@ shaka.media.MediaSourceEngine = class {
       // into extracting a timestamp for us to align text segments in sequence
       // mode.
 
+      const duration = this.mediaSource_.duration;
+
       // Timestamps can only be reliably extracted from video, not audio.
       // Packed audio formats do not have internal timestamps at all.
       // Prefer video for this when available.
@@ -701,7 +703,7 @@ shaka.media.MediaSourceEngine = class {
         // Clear the buffer.
         await this.enqueueOperation_(
             contentType,
-            () => this.remove_(contentType, 0, this.mediaSource_.duration));
+            () => this.remove_(contentType, 0, duration));
 
         // Finally, flush the buffer in case of choppy video start on HLS fMP4.
         if (contentType == ContentType.VIDEO) {
@@ -713,6 +715,10 @@ shaka.media.MediaSourceEngine = class {
 
       // Now switch to sequence mode and fall through to our normal operations.
       sourceBuffer.mode = SEQUENCE;
+
+      // When we change the buffer mode the duration is lost, so we need to set
+      // it explicitly.
+      await this.setDuration(duration);
     }
 
     if (reference && this.sequenceMode_ && contentType != ContentType.TEXT) {