My mouse escaped the window, so I changed the launch options:

gamescope --force-grab-cursor -w (monitorwidth) -h (monitorheight) %command%

I kinda wish gamescope had a good option to skip that w,h, but no one’s told me about one yet.

I wonder if it’s to make sure people haven’t modified the client to run cheats, give themselves level 999 weapons and such.