JInternalFrameをJFrameとして表示する

JInternalFrame internal = new JInternalFrame("@title@");
BasicInternalFrameUI ui = (BasicInternalFrameUI) internal.getUI();
Component title = ui.getNorthPane();
for (MouseMotionListener l: title.getListeners(MouseMotionListener.class)) {
  title.removeMouseMotionListener(l);
}
DragWindowListener dwl = new DragWindowListener();
title.addMouseListener(dwl);
title.addMouseMotionListener(dwl);
JPanel p = new JPanel(new BorderLayout());
p.add(new JScrollPane(new JTree()));
p.add(new JButton(new AbstractAction("close") {
  @Override public void actionPerformed(ActionEvent e) {
    Window w = SwingUtilities.windowForComponent((Component) e.getSource());
    // w.dispose();
    w.getToolkit().getSystemEventQueue().postEvent(
      new WindowEvent(w, WindowEvent.WINDOW_CLOSING));
  }
}), BorderLayout.SOUTH);
internal.getContentPane().add(p);
internal.setVisible(true);

KeyboardFocusManager focusManager = KeyboardFocusManager.getCurrentKeyboardFocusManager();
focusManager.addPropertyChangeListener(e -> {
  String prop = e.getPropertyName();
  if ("activeWindow".equals(prop)) {
    try {
      internal.setSelected(Objects.nonNull(e.getNewValue()));
    } catch (PropertyVetoException ex) {
      throw new IllegalStateException(ex);
    }
  }
});