code: function(event) {
      if (event && /^_(?:click|mouse(?:over|out|down|up|move))$/.test(event.type)) {
        var srcElement = event.target;
        var fromElement = event.type === "_mouseover" && event.relatedTarget ? event.relatedTarget : undefined;
        var toElement = event.type === "_mouseout" && event.relatedTarget ? event.relatedTarget : undefined;
        var pos = _getElementPosition(srcElement);
        var screenLeft = _window.screenLeft || _window.screenX || 0;
        var screenTop = _window.screenTop || _window.screenY || 0;
        var scrollLeft = _document.body.scrollLeft + _document.documentElement.scrollLeft;
        var scrollTop = _document.body.scrollTop + _document.documentElement.scrollTop;
        var pageX = pos.left + (typeof event._stageX === "number" ? event._stageX : 0);
        var pageY = pos.top + (typeof event._stageY === "number" ? event._stageY : 0);
        var clientX = pageX - scrollLeft;
        var clientY = pageY - scrollTop;
        var screenX = screenLeft + clientX;
        var screenY = screenTop + clientY;
        var moveX = typeof event.movementX === "number" ? event.movementX : 0;
        var moveY = typeof event.movementY === "number" ? event.movementY : 0;
        delete event._stageX;
        delete event._stageY;
        _extend(event, {
          srcElement: srcElement,
          fromElement: fromElement,
          toElement: toElement,
          screenX: screenX,
          screenY: screenY,
          pageX: pageX,
          pageY: pageY,
          clientX: clientX,
          clientY: clientY,
          x: clientX,
          y: clientY,
          movementX: moveX,
          movementY: moveY,
          offsetX: 0,
          offsetY: 0,
          layerX: 0,
          layerY: 0
        });
      }
      return event;
    }
docs: Add element and position data to `MouseEvent` instances
  @private