Undo GUILayout.Window Drag

I have a GUILayout.Window dragging with their rects serialized in objects, but how can I make each drag undoable? I can't seem to find a way to detect the beginning of a drag that is enabled by GUI.DragWindow() as a place to call Undo.RegisterCompleteObjectUndo (model, "Window Drag");. The desired behavior is that there are several windows displayed in my EditorWindow and when any one of them is moved, that move can be undone.

 void OnGUI() {
     int index = 0;
     foreach(MyObject mo in model.myObjects) {
           mo.rect = GUILayout.Window(index++, mo.rect, doWindow, "My Window");
     }
 }
 
 void doWindow(int id)
 {
     GUI.DragWindow();
 }