MoveMouse bug

The following code works as expected

NewMouseYLocation=100
NewMouseYLocation=NewMouseYLocation+20
MoveMouse 1,NewMouseYLocation

The following (seemingly identical code) doesnt

NewMouseYLocation=100
MoveMouse 1,NewMouseYLocation+20

What it does is move the mouse to (1,100) then add 20 (and update the mouse location again) and therefore you see a mouse flicker as it's in the wrong place temporarily.

For the second one, the value of NewMouseYLocation doesn't change. It always move 2d past that point. So when you move the mouse, you see flickering