Hello @RobotRoss, we're using one issue report per unofficially supported game title, and this issue needs to be evaluated on a per-game basis, so I've gone ahead and transferred this issue report to https://github.com/ValveSoftware/Proton/issues/7317#issuecomment-2850871227.
System Info
OS: Fedora 42
Desktop: Gnome 48 (Wayland)
GPU: AMD RX 6900XT (Mesa 25.0.4)
Steam Version: 1745876290
Proton: Experimental (bleeding-edge, 10, build 18333526)
Issue
When running games via Proton 10 (Proton 9 does not have this bug), the mouse will occasionally (at random) lose focus on the game window and float off onto another monitor, which in competitive games is a major problem. You can put the mouse back onto the game window, however the issue will reoccur, usually once it starts it happens quite regularly until the game is restarted.
I am told this issue does not affect single-monitor setups.
Games Affected
These are the games I've run into this on, or have heard from others that they have encountered it. There are likely others.