code: function(evt) {
          if (this.mouseDragStart != null) {
              var deltaY = this.mouseDragStart.y - evt.xy.y;
              var offsets = OpenLayers.Util.pagePosition(this.zoombarDiv);
              if ((evt.clientY - offsets[1]) > 0 && 
                  (evt.clientY - offsets[1]) < parseInt(this.zoombarDiv.style.height) - 2) {
                  var newTop = parseInt(this.slider.style.top) - deltaY;
                  this.slider.style.top = newTop+"px";
                  this.mouseDragStart = evt.xy.clone();
              }
              // set cumulative displacement
              this.deltaY = this.zoomStart.y - evt.xy.y;
              OpenLayers.Event.stop(evt);
          }
      }
docs: /*
  Method: zoomBarDrag
  This is what happens when a click has occurred, and the client is
  dragging.  Here we must ensure that the slider doesn't go beyond the
  bottom/top of the zoombar div, as well as moving the slider to its new
  visual location
  
  Parameters:
  evt - {<OpenLayers.Event>}