So this has been happening for maybe a week and a half for me. Basically on live streams on YouTube when I go to fullscreen, it instead makes the video take up part of the screen, with the rest black (imagine Twitch theater mode but the chat area is just black).

I thought at first this was a bug in an extension I have which gives a Twitch style theater mode option, but I just tried disabling it and even running Firefox in safe mode. Same problem.

Just curious if anyone else has seen this?