JComboBoxのドロップダウンリストにヘッダ・フッタを追加する

class HeaderFooterComboPopup extends BasicComboPopup {
  protected transient JLabel header;
  protected transient JMenuItem footer;

  public HeaderFooterComboPopup(JComboBox combo) {
    super(combo);
  }

  @Override protected void configurePopup() {
    super.configurePopup();
    configureHeader();
    configureFooter();
    add(header, 0);
    add(footer);
  }

  protected void configureHeader() {
    header = new JLabel("History");
    header.setBorder(BorderFactory.createEmptyBorder(4, 5, 4, 0));
    header.setMaximumSize(new Dimension(Short.MAX_VALUE, 20));
    header.setAlignmentX(1f);
  }

  protected void configureFooter() {
    int modifiers = InputEvent.CTRL_DOWN_MASK | InputEvent.SHIFT_DOWN_MASK;
    footer = new JMenuItem("Show All Bookmarks");
    footer.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_B, modifiers));
    footer.addActionListener(e -> {
      Window w = SwingUtilities.getWindowAncestor(getInvoker());
      JOptionPane.showMessageDialog(w, "Bookmarks");
    });
  }
}