From Xojo Documentation
The mouse button was released. Use the x and y parameters to determine if the mouse button was released within the control's boundaries.
|This event will not occur unless you return True in the MouseDown event. The return value is ignored.|
The parameters x and y are local coordinates, i.e. they represent the position of the mouse click relative to the upper-left corner or the Scrollbar. Mouse clicks that are released to the left or above a control are negative.