Version 3.4.8 of SteamOS, kernel 5.13.0-valve36-1-neptune, Mesa 22.2.0 (git-17e5312102). I've never seen anything like it. Captured via capture card and USB C hub, but behaves the same on the internal display. Apologies for the recording quality, I've not got much spare storage.
Obligatory flashing colors warning.
https://youtu.be/4lK0c9V5d68

Weird scaling problem (probably a gamescope issue, since that dev option seems to fix it)? What update branch are you on?

I think it's something to do with when there's no compositing being done, based on when it happened. Main branch everything.

That's weird, your youtube video says stable?

Ahhh crap, apologies, I'm a doof, I got my terminology mixed up. Stable everything.

Try updating to preview or main if possible and tell me the results (you can always roll back)