Previously mentioned at https://github.com/ValveSoftware/Proton/issues/168#issuecomment-440107101, possibly not a game-specific issue.
Not sure if this is same issue. https://github.com/ValveSoftware/Proton/issues/168 talks about an invisible cursor while mouse input still works, but in this case all mouse input seems to be lost. The menus are only usable with the keyboard, and when back in action it is no more possible to move the camera around using the mouse.
Thanks for clarifying. On second thought, what you've described could be a game-specific issue, so I've transferred this issue to https://github.com/ValveSoftware/Proton/issues/980#issuecomment-489347055 because we're using one issue report per unofficially supported game title.
Compatibility Report
System Information
I confirm:
Symptoms
Mouse control is lost after returning to the game menus / restarting after dying ingame. This does only happen if the graphics are running in flipping-mode.
Reproduction
Enable flipping/g-sync in the nvidia graphics driver. Ensure flipping is active (turn off 2nd monitor, watch indicator). Start random multiplayer game. Get overrun by zombies.