Can reproduce this issue whenever there is a difference between window render resolution and display resolution—that is, even if gamescope's reported resolution matches the display, if the window gets scaled up at all, absolute mouse commands ("Move to Position" and "Mouse Region") are likely to not function as expected. In my case, the game resolution was several times smaller than the display, and I couldn't see the cursor at all when activating a mouse region.
Still not working in higher resolutions than the standard 1280x800, changed the resolution of an RTS game to 1600x1000 and 1920x1200 and the mouse region does not cover the whole screen.
Can confirm. With the release of the new Steam Controller and the new possibilities while being docked, this issue rises to a whole new level. One has to decide if the RTS is to be played docked or on the go, which is bad tbh.
Nothing extracted yet.
Steam deck OS: stable and main, client version: stable and beta.
Mouse region is offset if the game resolution is smaller than the display resolution.
Example:
Game Resolution settings at deck level: (default) (720p)
External display resolution actual: 1080p60
Results: Mouse region is mapped over the entire resolution of the display, but because the window only fills a subset of the display resolution the coordinates of center are offset down and to the right.
Mouse region is unusable in it current state in docked mode.