Thanks! But, this only tells me how to check the window's current position. It doesn't tell me that a drag has started, or that a drag has stopped.

I want to take some action at the end of the drag operation, for example displaying a message, or even moving the window back to its original position.

I could check the window's position after every Gtk2 event, of course. But this only tells me that the window is moving. It doesn't tell me when the window has stopped moving.