This isn't the right fix, but FWIW -- if I remove the `performedTrackSelection = true;` line that I quoted in comment 2, then this bug and bug 2028931 both give EXPECTED RESULTS.
Bug 2029014 Comment 4 Edit History
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This isn't the right fix, but FWIW -- if I remove [this line](https://searchfox.org/firefox-main/rev/98bf4b92d3f5d7a9855281df4bf333210bcfbbc4/dom/media/webvtt/TextTrackManager.cpp#351) (the `performedTrackSelection = true;` line that I quoted in comment 2), in a local build, then this bug and bug 2028931 both give EXPECTED RESULTS.