This event occurs when the user moves the mouse into a grid control header cell while holding the left button down. This event is generated only when the user first clicks in a header cell. If the user clicks in a header and then drags the mouse over a non-header cell, MSG-HEADING-DRAGGED is generated and the closest header cell is used. These rules allow the user to be a bit sloppy when dragging the mouse.
EVENT-DATA-1 contains the column number of the cell dragged-to, and EVENT-DATA-2 contains the row number (one of these will be 1). For convenience, the properties X and Y are set to match these values for the duration of the event.