CardLayoutで作成したJTabbedPane風コンポーネントのタブエリアに水平JScrollBarを表示する

class CardLayoutTabbedPane extends JPanel {
  private final CardLayout cardLayout = new CardLayout();
  private final JPanel tabPanel = new JPanel(new FlowLayout(FlowLayout.LEADING, 0, 0));
  private final JPanel contentsPanel = new JPanel(cardLayout);
  private final JButton hiddenTabs = new JButton("V");
  private final ButtonGroup group = new ButtonGroup();
  private final JScrollPane tabArea = new JScrollPane(tabPanel) {
    @Override public boolean isOptimizedDrawingEnabled() {
      return false; // JScrollBar is overlap
    }

    @Override public void updateUI() {
      super.updateUI();
      EventQueue.invokeLater(() -> {
        getVerticalScrollBar().setUI(new OverlappedScrollBarUI());
        getHorizontalScrollBar().setUI(new OverlappedScrollBarUI());
        setLayout(new OverlapScrollPaneLayout());
        setComponentZOrder(getVerticalScrollBar(), 0);
        setComponentZOrder(getHorizontalScrollBar(), 1);
        setComponentZOrder(getViewport(), 2);
      });
      setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_NEVER);
      setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
      getVerticalScrollBar().setOpaque(false);
      getHorizontalScrollBar().setOpaque(false);
      setBackground(Color.DARK_GRAY);
      setViewportBorder(BorderFactory.createEmptyBorder());
      setBorder(BorderFactory.createEmptyBorder());
    }

    @Override public Dimension getPreferredSize() {
      Dimension d = super.getPreferredSize();
      d.height = 18 + 6;
      return d;
    }
  };

  protected CardLayoutTabbedPane() {
    super(new BorderLayout());
    setBorder(BorderFactory.createEmptyBorder(1, 1, 1, 1));
    setBackground(new Color(16, 16, 16));
    tabPanel.setInheritsPopupMenu(true);
    hiddenTabs.setFont(hiddenTabs.getFont().deriveFont(8f));
    hiddenTabs.setBorder(BorderFactory.createEmptyBorder(2, 8, 2, 8));
    hiddenTabs.setOpaque(false);
    hiddenTabs.setFocusable(false);
    hiddenTabs.setContentAreaFilled(false);
    JPanel header = new JPanel(new BorderLayout());
    header.add(new JLayer<>(tabArea, new HorizontalScrollLayerUI()));
    header.add(hiddenTabs, BorderLayout.EAST);
    add(header, BorderLayout.NORTH);
    add(contentsPanel);
  }

  protected JComponent createTabComponent(String title, Icon icon) {
    JToggleButton tab = new TabButton();
    tab.setInheritsPopupMenu(true);
    group.add(tab);
    tab.addMouseListener(new MouseAdapter() {
      @Override public void mousePressed(MouseEvent e) {
        if (SwingUtilities.isLeftMouseButton(e)) {
          ((AbstractButton) e.getComponent()).setSelected(true);
          cardLayout.show(contentsPanel, title);
        }
      }
    });
    EventQueue.invokeLater(() -> tab.setSelected(true));

    JLabel label = new JLabel(title, icon, SwingConstants.LEADING);
    label.setForeground(Color.WHITE);
    label.setIcon(icon);
    label.setOpaque(false);

    JButton close = new JButton(new CloseTabIcon(new Color(0xB0_B0_B0))) {
      @Override public Dimension getPreferredSize() {
        return new Dimension(12, 12);
      }
    };
    close.addActionListener(e -> {
      tabPanel.remove(tab);
      contentsPanel.remove(comp);
      boolean oneOrMore = tabPanel.getComponentCount() > 1;
      if (oneOrMore) {
        tabPanel.revalidate();
        TabButton b = (TabButton) tabPanel.getComponent(0);
        b.setSelected(true);
        cardLayout.first(contentsPanel);
      }
      tabPanel.revalidate();
    });
    close.setBorder(BorderFactory.createEmptyBorder());
    close.setFocusable(false);
    close.setOpaque(false);
    // close.setFocusPainted(false);
    close.setContentAreaFilled(false);
    close.setPressedIcon(new CloseTabIcon(new Color(0xFE_FE_FE)));
    close.setRolloverIcon(new CloseTabIcon(new Color(0xA0_A0_A0)));

    tab.add(label);
    tab.add(close, BorderLayout.EAST);
    return tab;
  }

  public void addTab(String title, Icon icon, Component comp) {
    JComponent tab = createTabComponent(title, icon);
    tabPanel.add(tab);
    contentsPanel.add(comp, title);
    cardLayout.show(contentsPanel, title);
    EventQueue.invokeLater(() -> tabPanel.scrollRectToVisible(tab.getBounds()));
  }

  public JScrollPane getTabArea() {
    return tabArea;
  }

  @Override public void doLayout() {
    BoundedRangeModel m = tabArea.getHorizontalScrollBar().getModel();
    hiddenTabs.setVisible(m.getMaximum() - m.getExtent() > 0);
    super.doLayout();
  }
}