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 about 2 seconds.

Then I remembered that I had some issue when I first installed Debian 12 two years, though I forgot which issues exactly, and that I solved them by switching from Wayland to X11.