I have the same issue, started to happen to me lately, maybe after I updated into Monterey? maybe a lil bit after that(started to happen like 1-2 days ago). Going full screen on Safari while watching video on Youtube, it freezes and no response at all, I have managed to get green screen once; after that whenever it freezes like that on fullscreen on Safari&Youtube, I just press and hold to forceshutdown macbook and reopen it. The issue did not happen on Chrome for me, on the other hand, I switch to Chrome to watch fullscreen Youtube videos last a couple days. Need a new update to fix that issue imo, asap pls :x