code: public void setHeader(HasJqmHeader header) {
          removeHeader();
          this.header = header;
          if (this.header == null) return;
  
          addLogical(header.getHeaderStage());
  
          if (panel == null) {
          	getElement().insertBefore(header.getJqmHeader().getElement(), getElement().getFirstChild());
          } else {
          	getElement().insertAfter(header.getJqmHeader().getElement(), panel.getElement());
          }
      }
docs: Sets the header element, overriding an existing header if any.