Swing/FloatableMenuBar のバックアップ(No.1)

JMenuBar menuBar = new JMenuBar();
menuBar.add(makeMenu("JMenu 1"));

JMenu menu2 = makeMenu("JMenu 2");
menu2.addMenuListener(new MenuListener() {
  private boolean isFloating(JMenu menu) {
    Container c = SwingUtilities.getAncestorOfClass(JToolBar.class, menu);
    return c instanceof JToolBar &&
        !((BasicToolBarUI) ((JToolBar) c).getUI()).isFloating();
  }

  @Override public void menuSelected(MenuEvent e) {
    JMenu menu = (JMenu) e.getSource();
    if (menu.isTopLevelMenu() && isFloating(menu)) {
      Dimension d = menu.getPreferredSize();
      Point p = menu.getLocation();
      Component cp = getRootPane().getContentPane();
      Point pt = SwingUtilities.convertPoint(menu.getParent(), p, cp);
      pt.y += d.height * 2;
      if (!cp.getBounds().contains(pt)) {
        EventQueue.invokeLater(() -> {
          JPopupMenu popup = menu.getPopupMenu();
          Rectangle bounds = popup.getBounds();
          Point loc = menu.getLocationOnScreen();
          // int h = bounds.height + UIManager.getInt("Menu.menuPopupOffsetY");
          loc.y -= bounds.height + UIManager.getInt("Menu.menuPopupOffsetY");
          popup.setLocation(loc);
        });
      }
    }
  }

  @Override public void menuDeselected(MenuEvent e) {
    // Do nothing
  }

  @Override public void menuCanceled(MenuEvent e) {
    // Do nothing
  }
});
menuBar.add(menu2);

JToolBar toolBar = new JToolBar();
toolBar.setLayout(new BorderLayout());
toolBar.add(menuBar);

add(toolBar, BorderLayout.NORTH);