From Xojo Documentation
This event fires continuously after the mouse button was pressed inside the Control. Mouse location is local to the control passed in to x, y.
|This event will not occur unless you return True in the MouseDown event first.|
The parameters x and y are local coordinates, i.e. they represent the position of the mouse relative to the upper-left corner of the Control.
As this event is fired continuously (hundreds of time per second), it is your responsibility to determine if the mouse has really moved.