This event is fired when the user holds the left mouse button down and drags the mouse pointer over the heading cells. The value of EVENT-DATA-1 represents the column whose heading is being dragged while EVENT-DATA-2 is always set to 1. The X and Y properties are automatically set in order to match EVENT-DATA-1 and EVENT-DATA-2. This event works with header cells. For standard cells see the MSG-BEGIN-DRAG event.