TIL Microsoft Paint has a "feature" where the mouse will move when you press an arrow key, even if Paint isn't in the foreground.

this is why wayland will never be used /s

Apparently it's got a feature that allows moving the mouse pointer using arrow keys, and for some reason sometimes it doesn't stop listening when the window loses focus, so Paint was running in the background since I often use it and it was still doing its job of moving the mouse.