SUMMARY

With the 6.1 update, fullscreen or borderless games run with gamescope under X11 cause severe flickering between the game and the desktop, rendering the game unplayable on my system. I am using the nvidia 550 drivers. Wayland does not have this problem, but is unsuitable for my needs due to its greatly restricted selection of 4:3 resolutions on my system and the need for a 4:3 desktop for my use case (scaling VGA mode 13h games that run in 320x200 4:3). The problem can be avoided by rolling back to kwin 6.0.5 but using outdated software is untenable in the long run.

STEPS TO REPRODUCE
1. Under an X11 session with nvidia, run a fullscreen low-resolution game (e.g. Descent with DXX-Rebirth) using "gamescope -w 320 -h 200 -F pixel -S stretch foo".
2. Observe flickering. It may not trigger immediately on login to a fresh session; a surefire way to trigger it is by using nvidia-settings to set a scaled 4:3 resolution.
3. Roll kwin and plasma-shell back to 6.0.5, restart, try again.
4. Flickering should be gone, and will return when kwin is updated again.

OBSERVED RESULT
Game flickers, bits of the desktop appear through the game visuals, game is unplayable.

EXPECTED RESULT
Game should play smoothly and crisply in a simulation of 320x200 VGA as would be rendered by a CRT or very old LCD monitor.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
(available in About System)
KDE Plasma Version: 6.1.1
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.2

ADDITIONAL INFORMATION
Kernel: 6.9.6-zen
Video card: nVidia GeForce RTX 2070 8GB
Video driver: nvidia-dkms proprietary 550.90.07
Motherboard; Gigabyte GA-AB350 Gaming 3
Processor: AMD Ryzen 7 3800X