STR:

0. Set media.videocontrols.picture-in-picture.display-text-tracks.enabled to true in about:config
1. Visit https://www.youtube.com/watch?v=OMfDp3Uk4EE and turn on captions. Make sure you've reached a part of the video where captions are coming in often.
2. Put the video into Picture-in-Picture
3. Put the originating tab into the background, and while listening to the video, watch the PiP player window captions

ER:

The player window captions and subtitles should continue to update regardless of how much time the originating tab has been in the background.

AR:

After a few seconds (maybe 5 - 10), the captions and subtitles start updating farrrrr less frequently. The conversation will proceed, but an old caption from seconds ago will still be displayed. Interestingly, you can "jump start" the captions to update by hovering the originating tab with the mouse (you don't even need to select it).

Note that hovering a tab like that causes us to "warm" it (pre-emptively paint it) so that we can switch faster to it if the user clicks... so that painting might be involved in the jump-start effect.

Also, I don't think this is a general issue with our text track support here. I think YouTube is doing some kind of performance optimization after it notices that it's been sitting in the background for a while.

Also see bug 1598654, which likely has similar causes (that one doesn't deal with captions that don't update, but videos that switch to low-quality when their originating tabs are backgrounded).

Upon closer inspection, this seems to be a dupe of bug 1755748.