The commit of that patch only mentions audio tracks, but the code changes setTracks for both video and audio. One option might be to try to restore the old code path for video, and leave audio as is, since I see no mention of testing video—tests probably pass because they look for new color, not absence of old color (guessing). But solving it that way might be a bit messy from looking at it. Might be simpler if we can find what's wrong with the new path for video. Clearly, something is still attached to the old dom track, as it's basically sending both old and new track at once it seems. Hoping others more familiar with MTG can spot it.
Bug 1608118 Comment 9 Edit History
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The commit of the patch in comment 8 only mentions audio tracks, but the code changes setTracks for both video and audio. One option might be to try to restore the old code path for video, and leave audio as is, since I see no mention of testing video—tests probably pass because they look for new color, not absence of old color (guessing). But solving it that way might be a bit messy from looking at it. Might be simpler if we can find what's wrong with the new path for video. Clearly, something is still attached to the old dom track, as it's basically sending both old and new track at once it seems. Hoping others more familiar with MTG can spot it.