I have two monitors both with a single panel at the bottom. When I disable one of them and start a game (may not just be Steam, but that's all I use), the mouse cursor icon draws maybe a centimeter above where the cursor position actually is. If I alt tab out and click back in, the icon and position match until I click once in the game, then they immediately diverge once more. Using one older version of Proton I got a slightly different behavior. The icon and position matched, but I could not click on anything on the bottom centimeter or so of the screen. I do not know if this is a kwin issue, a proton issue, plasma issue, or something else. I wouldn't even know where to start. My guess is that panel on the disabled monitor is still alive in some way and conflicting. This is on Wayland. I tested multiple proton versions and multiple games. Logging out and back in does not resolve the issue. Re-enabling the second monitor solves the mouse position problems. The only other detail I can think of is the disabled monitor is in portrait orientation to the right of the primary monitor. Please let me know what more information I can provide.
Oh a few more details I remembered: Setting the game to regular fullscreen or borderless window fullscreen makes no difference. Toggling between a hardware or software cursor in the game makes no difference. I'm not even sure if these matter or do anything in Linux tbh.