I'm trying to reproduce your issues with my config, but I have no idea what I'm doing. Can you upload a sample I can try to work with?
Someone who plays Mono-based games reports they work fine with xmonad,k so that may not be the problem. Two others I'm aware of are:
(1) Your config does not have `ewmh` configured (this seems unlikely); or
(2) you're having `_NET_WORKAREA` issues.
If it's EWMH, simply configuring the `ewmh` combinator should fix it. But quite a lot of stuff required EWMH these days, so I can't imagine you haven't run into this before.
`_NET_WORKAREA` is complex. I'm told by other people that they have no problem with it (more correctly, without it), but I find I need to set it for some things (notably, KDE menus) to work properly. The difference may be that I have a status bar at the top of my screen.