code: function() {
          OpenLayers.Element.addClass(document.body, "olDragDown");
          this.documentEvents = true;
          OpenLayers.Event.observe(document, "mousemove", this._docMove);
          OpenLayers.Event.observe(document, "mouseup", this._docUp);
      }
docs: Method: addDocumentEvents
  Start observing document events when documentDrag is true and the mouse
  cursor leaves the map viewport while dragging.