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).
Sure. To be clear I'm not suggesting that we should switch to fvwm3 because it (might) fix all of our problems. I'm suggesting we should switch to it primarily because fvwm2 is EOL and won't be receiving any new fixes. And if upgrading fixes *some* of our problems along the way, so much the better.
Of course, if there's a WM out there that works perfectly, I think it's a good idea to use it, but historically there's been hesitation around that.