Well I think this is desired behavior. But what if you grab a slider (so that the mouse button is held down), and drift out of the window? I thought some versions of Windows didn't handle this well, the slider would jump back home or something unless you kept pointing at it. The way it should work: as long as you are holding the button down, it should not matter what the x-coordinate of the mouse is if it's a y-slider, and vice-versa.