Fwiw if we decide we need to change the CI WM, I'd suggest using OpenBox. It seems to generally do a better job than fvwm and any other WM tested (see https://test.winehq.org/data/patterns.html).
I'm not testing fvwm3 there but as far as I can see it still suffers from the same issue fvwm2, where it might steal input focus from our windows when WM_TAKE_FOCUS is used (https://github.com/fvwmorg/fvwm/pull/101).