JComboBoxのドロップダウンリストで使用するJScrollBarを変更する

JComboBox<String> combo2 = new JComboBox<String>(makeModel()) {
  @Override public void updateUI() {
    UIManager.put(KEY, BorderFactory.createLineBorder(Color.GRAY));
    UIManager.put("ScrollBar.width", 10);
    UIManager.put("ScrollBar.thumbHeight", 20);
    UIManager.put("ScrollBar.minimumThumbSize", new Dimension(30, 30));
    UIManager.put("ScrollBar.incrementButtonGap", 0);
    UIManager.put("ScrollBar.decrementButtonGap", 0);
    UIManager.put("ScrollBar.thumb", THUMB);
    UIManager.put("ScrollBar.track", BACKGROUND);

    UIManager.put("ComboBox.foreground", FOREGROUND);
    UIManager.put("ComboBox.background", BACKGROUND);
    UIManager.put("ComboBox.selectionForeground", SELECTION_FOREGROUND);
    UIManager.put("ComboBox.selectionBackground", BACKGROUND);
    UIManager.put("ComboBox.buttonDarkShadow", BACKGROUND);
    UIManager.put("ComboBox.buttonBackground", FOREGROUND);
    UIManager.put("ComboBox.buttonHighlight", FOREGROUND);
    UIManager.put("ComboBox.buttonShadow", FOREGROUND);

    super.updateUI();
    setUI(new BasicComboBoxUI() {
      @Override protected JButton createArrowButton() {
        JButton b = new JButton(new ArrowIcon(BACKGROUND, FOREGROUND));
        b.setContentAreaFilled(false);
        b.setFocusPainted(false);
        b.setBorder(BorderFactory.createEmptyBorder());
        return b;
      }

      @Override protected ComboPopup createPopup() {
        return new BasicComboPopup(comboBox) {
          @Override protected JScrollPane createScroller() {
            JScrollPane sp = new JScrollPane(list) {
              @Override public void updateUI() {
                super.updateUI();
                getVerticalScrollBar().setUI(new WithoutArrowButtonScrollBarUI());
                getHorizontalScrollBar().setUI(new WithoutArrowButtonScrollBarUI());
              }
            };
            sp.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED)
            sp.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER)
            sp.setHorizontalScrollBar(null);
            return sp;
          }
        };
      }
    });
    Object o = getAccessibleContext().getAccessibleChild(0);
    if (o instanceof JComponent) {
      JComponent c = (JComponent) o;
      c.setBorder(BorderFactory.createMatteBorder(0, 1, 1, 1, Color.GRAY));
      c.setForeground(FOREGROUND);
      c.setBackground(BACKGROUND);
    }
  }
};

// ...
class ZeroSizeButton extends JButton {
  @Override public Dimension getPreferredSize() {
    return new Dimension();
  }
}

class WithoutArrowButtonScrollBarUI extends BasicScrollBarUI {
  @Override protected JButton createDecreaseButton(int orientation) {
    return new ZeroSizeButton();
  }

  @Override protected JButton createIncreaseButton(int orientation) {
    return new ZeroSizeButton();
  }

  @Override protected void paintTrack(Graphics g, JComponent c, Rectangle r) {
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setPaint(trackColor);
    g2.fill(r);
    g2.dispose();
  }

  @Override protected void paintThumb(Graphics g, JComponent c, Rectangle r) {
    JScrollBar sb = (JScrollBar) c;
    if (!sb.isEnabled()) {
      return;
    }
    BoundedRangeModel m = sb.getModel();
    if (m.getMaximum() - m.getMinimum() - m.getExtent() > 0) {
      Graphics2D g2 = (Graphics2D) g.create();
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      Color color;
      if (isDragging) {
        color = thumbDarkShadowColor.brighter();
      } else if (isThumbRollover()) {
        color = thumbLightShadowColor.brighter();
      } else {
        color = thumbColor;
      }
      g2.setPaint(color);
      g2.fillRect(r.x + 1, r.y + 1, r.width - 2, r.height - 2);
      g2.dispose();
    }
  }
}