Window mouse position is sometimes weird.

[museun]

On windows, atleast, when there is a mouse area on the right side and then window loses focus by moving the mouse to the right and off of the screen and another window gains focus then moving back from the right into the window and a mouse event is done -- the right-most mouse area gets the event.

This is caused by mouse position not being tracked while the window isn't focused. what we should do is ignore mouse events after a WindowFocusGain until we get a new cursor position (e.g. a MouseMove).