fix(CEA): reset PTS on new init segment (#6671)

Fixes
#6606 (comment)

lib/media/closed_caption_parser.js

@@ -26,8 +26,10 @@ shaka.media.IClosedCaptionParser = class {
    * Initialize the caption parser. This should be called whenever new init
    * segment arrives.
    * @param {BufferSource} initSegment
+   * @param {boolean=} adaptation True if we just automatically switched active
+   *   variant(s).
    */
-  init(initSegment) {}
+  init(initSegment, adaptation = false) {}
 
   /**
    * Parses embedded CEA closed captions and interacts with the underlying
@@ -89,12 +91,16 @@ shaka.media.ClosedCaptionParser = class {
   /**
    * @override
    */
-  init(initSegment) {
+  init(initSegment, adaptation = false) {
     shaka.log.debug('Passing new init segment to CEA parser');
-    // Reset underlying decoder when new init segment arrives
-    // to clear stored pts values.
-    this.reset();
-    this.ceaParser_.init(initSegment);
+    if (!adaptation) {
+      // Reset underlying decoder when new init segment arrives
+      // to clear stored pts values.
+      // This is necessary when a new Period comes in DASH or a discontinuity
+      // in HLS.
+      this.reset();
+      this.ceaParser_.init(initSegment);
+    }
   }
 
   /**

lib/media/media_source_engine.js

@@ -964,7 +964,7 @@ shaka.media.MediaSourceEngine = class {
       // If it is the init segment for closed captions, initialize the closed
       // caption parser.
       if (!reference) {
-        this.captionParser_.init(data);
+        this.captionParser_.init(data, adaptation);
       } else {
         const closedCaptions = this.captionParser_.parseFrom(data);
         if (closedCaptions.length) {