The compositing is disabled when playing a fullscreen game. Enabling compositing with SHIT + ALT + F12 is not possible. I have been trying to find the kwin log files but with no luck.
This is expected: the games tell KWin to turn off the compositor and KWin does so. If you don't like this behavior, there is an option in the compositor settings to disable it.
But it never comes back on again.
(In reply to Magnus Jørgensen from comment #2) > But it never comes back on again. I assume you have an NVIDIA graphics card? We are experiencing issues on NVIDIA with restarting the compositor. Unfortunately an NVIDIA specific problem.