fix(media): Fix region checking in livestreams

lib/media/region_observer.js

@@ -27,13 +27,24 @@ shaka.media.RegionObserver = class extends shaka.util.FakeEventTarget {
    * destroy the timeline.
    *
    * @param {!shaka.media.RegionTimeline} timeline
+   * @param {boolean} startsPastZero
    */
-  constructor(timeline) {
+  constructor(timeline, startsPastZero) {
     super();
 
     /** @private {shaka.media.RegionTimeline} */
     this.timeline_ = timeline;
 
+    /**
+     * Whether the asset is expected to start at a time beyond 0 seconds.
+     * For example, if the asset is a live stream.
+     * If true, we will not start polling for regions until the playhead has
+     * moved past 0 seconds, to avoid bad behaviors where the current time is
+     * briefly 0 before we have enough data to play.
+     * @private {boolean}
+     */
+    this.startsPastZero_ = startsPastZero;
+
     /**
      * A mapping between a region and where we previously were relative to it.
      * When the value here differs from what we calculate, it means we moved and
@@ -121,6 +132,13 @@ shaka.media.RegionObserver = class extends shaka.util.FakeEventTarget {
   /** @override */
   poll(positionInSeconds, wasSeeking) {
     const RegionObserver = shaka.media.RegionObserver;
+    if (this.startsPastZero_ && positionInSeconds == 0) {
+      // Don't start checking regions until the timeline has begun moving.
+      return;
+    }
+    // Now that we have seen the playhead go past 0, it's okay if it goes
+    // back there (e.g. seeking back to the start).
+    this.startsPastZero_ = false;
 
     for (const region of this.timeline_.regions()) {
       const previousPosition = this.oldPosition_.get(region);

lib/player.js

@@ -2188,7 +2188,8 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     }
 
     this.playhead_ = this.createPlayhead(has.startTime);
-    this.playheadObservers_ = this.createPlayheadObserversForMSE_();
+    this.playheadObservers_ =
+        this.createPlayheadObserversForMSE_(has.startTimeOfLoad);
 
     // We need to start the buffer management code near the end because it will
     // set the initial buffering state and that depends on other components
@@ -2930,17 +2931,21 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
    * Create the observers for MSE playback. These observers are responsible for
    * notifying the app and player of specific events during MSE playback.
    *
+   * @param {number} startTimeOfLoad
    * @return {!shaka.media.PlayheadObserverManager}
    * @private
    */
-  createPlayheadObserversForMSE_() {
+  createPlayheadObserversForMSE_(startTimeOfLoad) {
     goog.asserts.assert(this.manifest_, 'Must have manifest');
     goog.asserts.assert(this.regionTimeline_, 'Must have region timeline');
     goog.asserts.assert(this.video_, 'Must have video element');
 
+    const startsPastZero = this.isLive() || startTimeOfLoad > 0;
+
     // Create the region observer. This will allow us to notify the app when we
     // move in and out of timeline regions.
-    const regionObserver = new shaka.media.RegionObserver(this.regionTimeline_);
+    const regionObserver = new shaka.media.RegionObserver(
+        this.regionTimeline_, startsPastZero);
 
     regionObserver.addEventListener('enter', (event) => {
       /** @type {shaka.extern.TimelineRegionInfo} */

test/media/region_observer_unit.js

@@ -31,7 +31,8 @@ describe('RegionObserver', () => {
     timeline = new shaka.media.RegionTimeline(
         shaka.test.Util.spyFunc(onSeekRange));
 
-    observer = new shaka.media.RegionObserver(timeline);
+    observer = new shaka.media.RegionObserver(
+        timeline, /* startsPastZero= */ false);
     observer.addEventListener('enter', (event) => {
       shaka.test.Util.spyFunc(onEnterRegion)(event['region'], event['seeking']);
     });