Swing/VariableRoundedBorderOnComboBox のバックアップ(No.1)

JComboBox<String> combo1 = new JComboBox<String>(model) {
  private transient PopupMenuListener listener;

  @Override public void updateUI() {
    setRenderer(null);
    removePopupMenuListener(listener);
    super.updateUI();
    setBorder(new RoundedCornerBorder());
    setRenderer(new RoundedCornerListCellRenderer());
    setUI(new BasicComboBoxUI() {
      // ...
    }
    listener = new HeavyWeightContainerListener();
    addPopupMenuListener(listener);
    Object o = getAccessibleContext().getAccessibleChild(0);
    if (o instanceof JComponent) {
      JComponent c = (JComponent) o;
      c.setBorder(new BottomRoundedCornerBorder());
      c.setBackground(Color.WHITE);
    }
  }
};
// ...
class HeavyWeightContainerListener implements PopupMenuListener {
  @Override public void popupMenuWillBecomeVisible(PopupMenuEvent e) {
    EventQueue.invokeLater(() -> {
      JComboBox<?> combo = (JComboBox<?>) e.getSource();
      combo.setBorder(new TopRoundedCornerBorder());
      Accessible a = combo.getUI().getAccessibleChild(combo, 0);
      if (a instanceof JPopupMenu) {
        Window w = SwingUtilities.getWindowAncestor((Component) a);
        if (w != null && w.getType() == Window.Type.POPUP) {
          // Popup$HeavyWeightWindow
          w.setBackground(new Color(0x0, true));
        }
      }
    });
  }

  @Override public void popupMenuWillBecomeInvisible(PopupMenuEvent e) {
    JComboBox<?> combo = (JComboBox<?>) e.getSource();
    combo.setBorder(new RoundedCornerBorder());
  }

  @Override public void popupMenuCanceled(PopupMenuEvent e) {
    /* not needed */
  }
}