fix: gap jumping when gap exists at start position (#5384)

Before playback starts the video element reports as seeking == true, but
because we haven't installed our seek listener yet, the gap controller
does not receive the seek event.

To work around this, we explicitly tell the gap controller when playback
starts so that it knows the startup position, and can synthesize a seek
event.

lib/media/gap_jumping_controller.js

@@ -58,6 +58,9 @@ shaka.media.GapJumpingController = class {
     /** @private {number} */
     this.prevReadyState_ = video.readyState;
 
+    /** @private {number} */
+    this.startTime_ = 0;
+
     /** @private {number} */
     this.gapsJumped_ = 0;
 
@@ -123,6 +126,18 @@ shaka.media.GapJumpingController = class {
     this.onPollGapJump_();
   }
 
+  /**
+   * Called when playback has started and the video element is
+   * listening for seeks.
+   *
+   * @param {number} startTime
+   */
+  onStarted(startTime) {
+    if (this.video_.seeking && !this.seekingEventReceived_) {
+      this.seekingEventReceived_ = true;
+      this.startTime_ = startTime;
+    }
+  }
 
   /** Called when a seek has started. */
   onSeeking() {
@@ -164,8 +179,8 @@ shaka.media.GapJumpingController = class {
     // while paused on a livestream.  We make an exception for time 0, since we
     // may be _required_ to seek on startup before play can begin, but only if
     // autoplay is enabled.
-    if (this.video_.paused && (this.video_.currentTime != 0 ||
-      (!this.video_.autoplay && this.video_.currentTime == 0))) {
+    if (this.video_.paused && (this.video_.currentTime != this.startTime_ ||
+      (!this.video_.autoplay && this.video_.currentTime == this.startTime_))) {
       return;
     }
 

lib/media/playhead.js

@@ -251,6 +251,7 @@ shaka.media.MediaSourcePlayhead = class {
     this.videoWrapper_ = new shaka.media.VideoWrapper(
         mediaElement,
         () => this.onSeeking_(),
+        (realStartTime) => this.onStarted_(realStartTime),
         () => this.getStartTime_(startTime));
 
     /** @type {shaka.util.Timer} */
@@ -390,6 +391,16 @@ shaka.media.MediaSourcePlayhead = class {
     }
   }
 
+  /**
+   * Called when the video element has started up and is listening for new seeks
+   *
+   * @param {number} startTime
+   * @private
+   */
+  onStarted_(startTime) {
+    this.gapController_.onStarted(startTime);
+  }
+
   /**
    * Handles when a seek happens on the video.
    *

lib/media/video_wrapper.js

@@ -27,15 +27,19 @@ shaka.media.VideoWrapper = class {
   /**
    * @param {!HTMLMediaElement} video
    * @param {function()} onSeek Called when the video seeks.
+   * @param {function(number)} onStarted Called when the video has started.
    * @param {function():number} getStartTime Calle to get the time to start at.
    */
-  constructor(video, onSeek, getStartTime) {
+  constructor(video, onSeek, onStarted, getStartTime) {
     /** @private {HTMLMediaElement} */
     this.video_ = video;
 
     /** @private {function()} */
     this.onSeek_ = onSeek;
 
+    /** @private {function(number)} */
+    this.onStarted_ = onStarted;
+
     /** @private {?number} */
     this.startTime_ = null;
 
@@ -173,6 +177,7 @@ shaka.media.VideoWrapper = class {
     this.started_ = true;
 
     this.eventManager_.listen(this.video_, 'seeking', () => this.onSeek_());
+    this.onStarted_(this.video_.currentTime);
   }
 };