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

JTabbedPane tabs = new JTabbedPane() {
  @Override public void updateUI() {
    super.updateUI();
    if (getUI() instanceof MetalTabbedPaneUI) {
      setUI(new MetalTabbedPaneUI() {
        @Override protected void navigateSelectedTab(int direction) {
          super.navigateSelectedTab(direction);
          focusIndex = getFocusIndex();
        }
      });
    }
  }
};
tabs.addChangeListener(e -> {
  // System.out.println("Tab selected");
  focusIndex = tabs.getSelectedIndex();
  // focusIndex = -1;
  tabs.repaint();
});
tabs.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT);
String help1 = "SPACE: selectTabWithFocus";
String help2 = "LEFT: navigateLeft";
String help3 = "RIGHT: navigateRight";
JTextArea textArea = new JTextArea(String.join("\n", help1, help2, help3));
textArea.setEditable(false);
tabs.addTab("help", new JScrollPane(textArea));
IntStream.range(0, 10).forEach(i -> tabs.addTab("title" + i, new JLabel("JLabel" + i)));

InputMap im = tabs.getInputMap(WHEN_FOCUSED);
im.put(KeyStroke.getKeyStroke(KeyEvent.VK_SPACE, 0), "selectTabWithFocus");

String key = "TabbedPane.selectionFollowsFocus";
JCheckBox check = new JCheckBox(key, UIManager.getBoolean(key)) {
  @Override public void updateUI() {
    super.updateUI();
    boolean b = UIManager.getLookAndFeelDefaults().getBoolean(key);
    setSelected(b);
    UIManager.put(key, b);
  }
};
check.setFocusable(false);
check.addActionListener(e -> {
  boolean b = ((JCheckBox) e.getSource()).isSelected();
  UIManager.put(key, b);
  SwingUtilities.updateComponentTreeUI(tabs);
});

add(new JLayer<>(tabs, new LayerUI<JTabbedPane>() {
  @Override public void paint(Graphics g, JComponent c) {
    super.paint(g, c);
    if (c instanceof JLayer) {
      JLayer<?> layer = (JLayer<?>) c;
      JTabbedPane tabbedPane = (JTabbedPane) layer.getView();
      if (focusIndex >= 0 && focusIndex != tabbedPane.getSelectedIndex()) {
        Rectangle r = tabbedPane.getBoundsAt(focusIndex);
        Graphics2D g2 = (Graphics2D) g.create();
        g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, .5f));
        g2.setPaint(Color.RED);
        g2.fill(r);
        g2.dispose();
      }
    }
  }
}));