Earlier quoted context omitted.
The Steam main window did not open, although Steam itself did load in the background. I could work around this by disabling smooth scrolling on web view and some other GPU-related option (I forgot exactly). But then there was a strange glitch on every single game (both native and Proton-based). Periodically (e.g. every ~10 seconds on some 3D games, on every screen reload on some other) the screen turned black for abo…
What DE? And that's with all the latest components as above? I wouldn't use Debian stable for gaming purposes, since it falls behind very quickly. Debian testing / unstable is a better idea, and even then you'd want to install latest amdgpu firmware manually potentially.
I'll try updating the firmware and drivers manually if I have more issues in the future, thanks.