Mouse focus isn't "fully grabbed" in some situations, for some people after alt+tab, when running borderless, etc.
Only way I can reproduce is to have some other process steal / mess with the window focus when RIK is launching. This causes the cursor not to be fully grabbed, and quick movements will move outside of the window. Will also register clicks outside the window. A click inside the window usually regrabs the input too, but it's a bit unreliable
Without SDL 2, seems the most feasible way is to force a re-focus of the window as the input is registered, so did that for now. Can be disabled with engine_force_refocus 0, in case it would cause some unforeseen issues for someone.