import changes from `dev' branch of rmottola/Arctic-Fox:

- Bug 1229987: P2. Drain decoder when encountering gap. r=cpearce This allows for all buffered frames to be playable. (7ac0b2ddec)
- Bug 1229987: P3. Update mochitests and add new one verifying behavior. r=cpearce (f4a231e9c8)
- Bug 1229987: P4. Stop pre-rolling when encountering WAITING_FOR_DATA. r=cpearce (721df00361)
- Bug 1205470: [MSE] Remove assertion. r=cpearce (857f4bfcd0)
- fix comment (2e6e5f800c)
- Bug 1226707: P2 Ensure we won't operate on a decoder that failed to initialize. r=cpearce (fa86fc1bb3)
- Bug 1226707: P3. Only create the type of decoder we will need. r=cpearce (49a3d28e6c)
- Bug 1229987: P5. Drop frames during internal seeking early. r=cpearce (9aeb23d3e6)
- Bug 1197075: P4. Reject skip promise on cancellation or shutdown. r=edwin (b3d7af2cdd)
- remove ifdef (3524eedb5a)
- Bug 1229028 - remove #ifdef NS_BUILD_REFCNT_LOGGING methods from nsXULPrototypeNode; r=bz (5b87939fd6)
- Bug 1214295 - Fix up entry points for ClickWithInputSource. r=bz (d7fda37d34)
- Bug 1217307 - Remove some unnecessary null checks in rest of dom/xul/. r=njn (87486950c6)
- Bug 1186811 (part 1) - Replace nsBaseHashtable::EnumerateRead() calls in dom/storage/ with iterators. r=baku. (0c35317169)
- Bug 1186811 (part 2) - Replace nsBaseHashtable::EnumerateRead() calls in dom/storage/ with iterators. r=baku. (4a367a23e8)
- Bug 1186811 (part 3) - Replace nsBaseHashtable::EnumerateRead() calls in dom/storage/ with iterators. r=baku. (e9861ab1ab)
- Bug 1186811 (part 4) - Replace nsBaseHashtable::EnumerateRead() calls in dom/storage/ with iterators. r=baku. (34037caa14)
- Bug 1189426 - Don't assert false in DOMStorageDBThread::InsertDBOp when we failed to open the database. r=smaug (b40612f549)
- Bug 1192194 - Make DOMStorageDBThead::mDBReady atomic. r=smaug (ce34bfd1a6)
- Bug 1208897 - Fix an initialization order bug in DOMStorageDBThread; r=baku (7efba6dea3)
- Bug 1209349 - Audit the callers of the two-argument OriginAttributes. r=janv (5bdc041a87)
- Bug 1189998, Part 1 - Consolidate Push client interfaces. r=mt,dragana (bd0eec93cb)
- Bug 1222619 - about:serviceworkers should show the correct cache entries, r=bkelly (ee9b3f1740)
- Bug 1189998, Part 2 - Migrate Push service callers. r=mt (392d6aa21d)
- Bug 1230672 part 1 - Make '-moz-column-fill:auto' work also when 'overflow' isn't 'visible'. r=bz (c10d71c206)
- Bug 1230672 part 2 - Add reftest with -moz-column-fill:auto and overflow:hidden. (8062b9e222)
- Bug 1227162 - [css-grid][css-align] Make the scrolled pseudo frame inherit more CSS Align / Grid properties. r=dholbert (2f6e04f3f8)
- Bug 1164783 - Removing trailing spaces. r=dbaron (980a19fb5d)
- Bug 1230207 - Add support for display:grid/flex layout on <fieldset> elements. r=bz,dholbert (9e1ba66f34)
- bug 1230025 remove declaration of non-existant NS_NewThumbFrame r=dholbert (8b1f7cba54)
- Bug 1230672 part 3 - Add support for multicol layout on <fieldset> elements. r=bz (475f8b329f)
- Bug 1232271 - initialize local scalar mSolidColor with default value - NS_RGBA(0, 0, 0, 0). r=matt.woodrow (7b8cd28617)
- Bug 1144534 - If we have tiling, don't reduce layer resolution for large transforms. r=mattwoodrow (96785125bb)
- Bug 1221842 - Don't reallocate DLBI geometry objects if nothing has changed. r=roc (69cbb41714)
- Bug 1232635 - since presContext is always a valid pointer remove the useless null check that also confused Coverity. r=bz (8426c6a2ff)

dom/media/MediaDecoderStateMachine.cpp

@@ -783,6 +783,17 @@ MediaDecoderStateMachine::OnNotDecoded(MediaData::Type aType,
                self->WaitRequestRef(aRejection.mType).Complete();
              }));
 
+    // We are out of data to decode and will enter buffering mode soon.
+    // We want to play the frames we have already decoded, so we stop pre-rolling
+    // and ensure that loadeddata is fired as required.
+    if (isAudio) {
+      StopPrerollingAudio();
+    } else {
+      StopPrerollingVideo();
+    }
+    if (mState == DECODER_STATE_BUFFERING || mState == DECODER_STATE_DECODING) {
+        MaybeFinishDecodeFirstFrame();
+    }
     return;
   }
 
@@ -1480,7 +1491,7 @@ MediaDecoderStateMachine::Seek(SeekTarget aTarget)
     return MediaDecoder::SeekPromise::CreateAndReject(/* aIgnored = */ true, __func__);
   }
 
-  NS_ASSERTION(mState > DECODER_STATE_DECODING_METADATA,
+  MOZ_ASSERT(mState > DECODER_STATE_DECODING_METADATA,
                "We should have got duration already");
 
   if (mState < DECODER_STATE_DECODING ||