fix(HLS): don't do sequence mode workaround unless there's a text str…

…eam (#5079)

The VTT cue timing fix in
#4217 seems to have
caused a regression for videos without VTT streams, where the first
segment would sometimes not play smoothly. It also appears to fix the
gap jumping seen in the "Art of Motion (HLS, TS)" demo. This was
partially fixed for fMP4 in
#4553 but didn't fix
the issue for MPEG-2 TS.

Fixes #4975.

lib/media/media_source_engine.js

@@ -408,6 +408,13 @@ shaka.media.MediaSourceEngine = class {
               'expected \'open\'');
         }
 
+        if (sequenceMode && !streamsByType.has(ContentType.TEXT)) {
+          // There's no text stream, so we can set sequence mode early instead
+          // of setting it after the first segment is appended in appendBuffer_.
+          sourceBuffer.mode =
+              shaka.media.MediaSourceEngine.SourceBufferMode_.SEQUENCE;
+        }
+
         this.eventManager_.listen(
             sourceBuffer, 'error',
             () => this.onError_(contentType));

test/media/media_source_engine_unit.js

@@ -639,6 +639,7 @@ describe('MediaSourceEngine', () => {
       };
       const initObject = new Map();
       initObject.set(ContentType.VIDEO, fakeVideoStream);
+      initObject.set(ContentType.TEXT, fakeTextStream);
 
       await mediaSourceEngine.init(initObject, /* sequenceMode= */ true);