Subclassing

Hi, is there a way to process code while a window's being dragged without stopping the move?

I need to keep a foreign window (App B) in the same position relative to mine (App A). So when I move App A, App B needs to move at the same time. I'm using Subclassing, GetWindowRect, and SetWindowPos to do this. When I move App A, App B moves at the same time, but this code processing interrupts the movement of App A, so I have to click App A and start dragging again. :(

Thanks.

Does it really have to drag in real-time? Sounds interesting though. What sort of process is it interupting, a loop?

I need to drag in real-time so that the two windows appear connected. The subclassing code interrupts the drag, even if the user still has the left mouse down. Any idea on how to let the subclassing code execute without interrupting the user's drag?