Linux: focus follows mouse prevents games from starting & Steam UI breaks in general

[{RW} Statik`]

Hello,
Just posting this here in hopes it may help someone else.

I have always used the "focus follows mouse" whenever possible, on all OSes. It has never caused me any issues until I recently switched to Debian. Currently appears on Debian 12 w/ Cinnamon, some games refuse to load (not to mention it breaks the Steam UI, but this is well known).

So far, I've observed the game Overload freezes on a black screen, and American Truck Simulator wont start at all. I'm sure it has affected other games, so I have to go back through my library to see if this was the cause of other games failing that I eventually gave up on...

EDIT: It's not so simple. After testing yesterday, these games ran fine and I played for a while. Today, they refused to start until I also disabled the Steam overlay. I still can't run either game with the "focus follows mouse" setting, but suddenly also have to disable the Steam overlay... Very strange.