onMouseUp = function(event)
  {
    var x = (document.all) ? window.event.clientX : event.pageX;
    var y = (document.all) ? window.event.clientY : event.pageY;
    
    Event.stopObserving(document, "mousemove", onMouseMove);
    Event.stopObserving(document, "mouseip", onMouseUp);
    
    if (m_callback) { m_callback(m_element, x, y); }
  }
Callback. Wird beim Loslassen der Maus aufgerufen