JComboBoxのドロップダウンリストを2段に変更する

ComboBoxModel<String> model = makeModel();
int rowCount = (model.getSize() + 1) / 2;
JComboBox<String> combo = new JComboBox<String>(model) {
  @Override public Dimension getPreferredSize() {
    Insets i = getInsets();
    Dimension d = super.getPreferredSize();
    int w = Math.max(100, d.width);
    int h = d.height;
    int buttonWidth = 20; // ???
    return new Dimension(
        buttonWidth + w + i.left + i.right, h + i.top + i.bottom);
  }

  @Override public void updateUI() {
    super.updateUI();
    setMaximumRowCount(rowCount);
    setPrototypeDisplayValue("12345");
    Accessible o = getAccessibleContext().getAccessibleChild(0);
    if (o instanceof ComboPopup) {
      JList<?> list = ((ComboPopup) o).getList();
      list.setLayoutOrientation(JList.VERTICAL_WRAP);
      list.setVisibleRowCount(rowCount);
      Border b0 = list.getBorder();
      Border b1 = new ColumnRulesBorder();
      list.setBorder(BorderFactory.createCompoundBorder(b0, b1));
      list.setFixedCellWidth((getPreferredSize().width - 2) / 2);
    }
  }
};