mousemove() appears to be asynchronous

ok, now i've stripped everything as far as i possibly can - only running code in this project is the following:

Local width = 400, height = 300, mode = 16
Global half_width = width / 2, half_height = height / 2
Graphics width, height, mode
MoveMouse(half_width, half_height)
While MouseX() = half_width And MouseY() = half_height
Wend
End

...and the project just ends immediately!