code: function() {
              if (this.mask) {
                  var panelZ = Dom.getStyle(this.element, "zIndex");
                  if (!YAHOO.lang.isUndefined(panelZ) && !isNaN(panelZ)) {
                      Dom.setStyle(this.mask, "zIndex", panelZ - 1);
                  }
              }
          }
docs: Sets the zindex of the mask, if it exists, based on the zindex of
  the Panel element. The zindex of the mask is set to be one less
  than the Panel element's zindex.
  
  <p>NOTE: This method will not bump up the zindex of the Panel
  to ensure that the mask has a non-negative zindex. If you require the
  mask zindex to be 0 or higher, the zindex of the Panel
  should be set to a value higher than 0, before this method is called.
  </p>
  @method stackMask