We have UIS:GetMouseLocation() as well as mouse.X/mouse.Y to tell the coordinates of the mouse while it's within game window bounds, however it does not tell us if it leaves them.
There's the events UIS:WindowFocusReleased and UIS:WindowFocused, but those only work if the player clicks while the mouse is off the screen.
Is there something to tell that the mouse has left the screen, but the player has not yet clicked off the screen?