This turned out to be way more difficult than I thought. I just want to be able to move a window by dragging it from any point within the window. The obvious problem is that I don't know where the mouse is when it leaves the window. I was thinking that I could make an invisible window that spans the entire screen and use its WM_MOUSEMOVE to determine the mouse position, but that seems a little stupid. Maybe I'm going about this the wrong way.