docs: public void setBelow(Component belowContent) {
  
          int i = m_mainPanel.getComponentIndex(m_mainPanel);
          Component oldBelow = m_mainPanel.getComponent(i + 1);
          if (oldBelow == null) {
              m_mainPanel.addComponent(belowContent);
          } else {
              m_mainPanel.replaceComponent(oldBelow, belowContent);
          }
      }