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