code: function () {
              var oMask = this.mask;
              if (!oMask) {
                  if (!m_oMaskTemplate) {
                      m_oMaskTemplate = document.createElement("div");
                      m_oMaskTemplate.className = "mask";
                      m_oMaskTemplate.innerHTML = "&#160;";
                  }
                  oMask = m_oMaskTemplate.cloneNode(true);
                  oMask.id = this.id + "_mask";
  
                  document.body.insertBefore(oMask, document.body.firstChild);
  
                  this.mask = oMask;
  
                  if (YAHOO.env.ua.gecko && this.platform == "mac") {
                      Dom.addClass(this.mask, "block-scrollbars");
                  }
  
                  // Stack mask based on the element zindex
                  this.stackMask();
              }
          }
docs: Builds the mask that is laid over the document when the Panel is
  configured to be modal.
  @method buildMask