SUMMARY
 
While I play, I wanted to check some information online so I had a window of Firefox and the game on full-screen mode, and I used alt-tab to switch between them. After a few times I get strange window behavior that is pretty challenging to explain.
I tried to capture this internally, but for some reason those issues won't been seen, so I used my phone and uploaded to imgur (https://imgur.com/a/YA1aLTt).

STEPS TO REPRODUCE
0.  Use X11
1.  Open a steam game that runs full screen (I use proton, not sure if it is related though).
2.  Open Firefox.
3.  Alt-tab between 1 and 2 them a few times.

OBSERVED RESULT
https://imgur.com/a/YA1aLTt
Note that closing and reopening the windows that shows this issues seems to fix it until the next time that I glitch it with alt-tabbing.


EXPECTED RESULT

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.27.9
KDE Frameworks Version: 5.111.0
Qt Version: 5.15.11

ADDITIONAL INFORMATION
Distro: Gentoo
gcc: 13.2.1
glibc: 2.38-r7
kernel: 6.6.0-gentoo x86_64
nvidia-drivers: 545.29.02