--- a/dom/media/mediasource/TrackBuffersManager.cpp
+++ b/dom/media/mediasource/TrackBuffersManager.cpp
@@ -2457,27 +2457,16 @@ TrackBuffersManager::GetSample(TrackInfo
                                MediaResult& aResult)
 {
   MOZ_ASSERT(OnTaskQueue());
   auto& trackData = GetTracksData(aTrack);
   const TrackBuffer& track = GetTrackBuffer(aTrack);
 
   aResult = NS_ERROR_DOM_MEDIA_WAITING_FOR_DATA;
 
-  if (!track.Length()) {
-    aResult = NS_ERROR_DOM_MEDIA_END_OF_STREAM;
-    return nullptr;
-  }
-
-  if (trackData.mNextGetSampleIndex.isNothing() &&
-      trackData.mNextSampleTimecode == TimeUnit()) {
-    // First demux, get first sample.
-    trackData.mNextGetSampleIndex = Some(0u);
-  }
-
   if (trackData.mNextGetSampleIndex.isSome()) {
     if (trackData.mNextGetSampleIndex.ref() >= track.Length()) {
       aResult = NS_ERROR_DOM_MEDIA_END_OF_STREAM;
       return nullptr;
     }
     const MediaRawData* sample =
       GetSample(aTrack,
                 trackData.mNextGetSampleIndex.ref(),
@@ -2514,51 +2503,42 @@ TrackBuffersManager::GetSample(TrackInfo
       // Next sample isn't available yet. Use estimates.
       trackData.mNextSampleTimecode = nextSampleTimecode;
       trackData.mNextSampleTime = nextSampleTime;
     }
     aResult = NS_OK;
     return p.forget();
   }
 
-  if (trackData.mNextSampleTimecode >
-      track.LastElement()->mTimecode + track.LastElement()->mDuration) {
-    // The next element is past our last sample. We're done.
-    trackData.mNextGetSampleIndex = Some(uint32_t(track.Length()));
-    aResult = NS_ERROR_DOM_MEDIA_END_OF_STREAM;
+  aResult = SetNextGetSampleIndexIfNeeded(aTrack, aFuzz);
+
+  if (NS_FAILED(aResult)) {
     return nullptr;
   }
 
-  // Our previous index has been overwritten, attempt to find the new one.
-  int32_t pos = FindCurrentPosition(aTrack, aFuzz);
-  if (pos < 0) {
-    MSE_DEBUG("Couldn't find sample (pts:%" PRId64 " dts:%" PRId64 ")",
-              trackData.mNextSampleTime.ToMicroseconds(),
-              trackData.mNextSampleTimecode.ToMicroseconds());
-    return nullptr;
-  }
-
-  const RefPtr<MediaRawData>& sample = track[pos];
+  MOZ_RELEASE_ASSERT(trackData.mNextGetSampleIndex.isSome() &&
+                     trackData.mNextGetSampleIndex.ref() < track.Length());
+  const RefPtr<MediaRawData>& sample =
+    track[trackData.mNextGetSampleIndex.ref()];
   RefPtr<MediaRawData> p = sample->Clone();
   if (!p) {
     // OOM
     aResult = MediaResult(NS_ERROR_OUT_OF_MEMORY, __func__);
     return nullptr;
   }
 
   // Find the previous keyframe to calculate the evictable amount.
-  int32_t i = pos;
+  uint32_t i = trackData.mNextGetSampleIndex.ref();
   for (; !track[i]->mKeyframe; i--) {
   }
   UpdateEvictionIndex(trackData, i);
 
-  trackData.mNextGetSampleIndex = Some(uint32_t(pos)+1);
+  trackData.mNextGetSampleIndex.ref()++;
   trackData.mNextSampleTimecode = sample->mTimecode + sample->mDuration;
   trackData.mNextSampleTime = sample->GetEndTime();
-  aResult = NS_OK;
   return p.forget();
 }
 
 int32_t
 TrackBuffersManager::FindCurrentPosition(TrackInfo::TrackType aTrack,
                                          const TimeUnit& aFuzz) const
 {
   MOZ_ASSERT(OnTaskQueue());
@@ -2685,16 +2665,19 @@ TrackBuffersManager::SetNextGetSampleInd
     // The next element is past our last sample. We're done.
     trackData.mNextGetSampleIndex = Some(uint32_t(track.Length()));
     return NS_ERROR_DOM_MEDIA_END_OF_STREAM;
   }
 
   int32_t pos = FindCurrentPosition(aTrack, aFuzz);
   if (pos < 0) {
     // Not found, must wait for more data.
+    MSE_DEBUG("Couldn't find sample (pts:%" PRId64 " dts:%" PRId64 ")",
+              trackData.mNextSampleTime.ToMicroseconds(),
+              trackData.mNextSampleTimecode.ToMicroseconds());
     return NS_ERROR_DOM_MEDIA_WAITING_FOR_DATA;
   }
   trackData.mNextGetSampleIndex = Some(uint32_t(pos));
   return NS_OK;
 }
 
 void
 TrackBuffersManager::TrackData::AddSizeOfResources(MediaSourceDecoder::ResourceSizes* aSizes) const