code: function forwardWheelEvent(event)
      {
          var clone = document.createEvent("WheelEvent");
          clone.initWebKitWheelEvent(event.wheelDeltaX, event.wheelDeltaY,
                                     event.view,
                                     event.screenX, event.screenY,
                                     event.clientX, event.clientY,
                                     event.ctrlKey, event.altKey, event.shiftKey, event.metaKey);
          this._mainPanel.element.dispatchEvent(clone);
      }
docs: Forward mouse wheel events from the unscrollable gutter to the main panel.