code: protected HtmlTree getTreeHeader() {
          String title = configuration.getText("doclet.Window_Class_Hierarchy");
          HtmlTree bodyTree = getBody(true, getWindowTitle(title));
          HtmlTree htmlTree = (configuration.allowTag(HtmlTag.HEADER))
                  ? HtmlTree.HEADER()
                  : bodyTree;
          addTop(htmlTree);
          addNavLinks(true, htmlTree);
          if (configuration.allowTag(HtmlTag.HEADER)) {
              bodyTree.addContent(htmlTree);
          }
          return bodyTree;
      }
docs: Get the tree header.
  
  @return a content tree for the tree header