JScrollBarのArrowButtonを非表示にする

UIManager.put("ScrollBar.width", 10);
UIManager.put("ScrollBar.thumbHeight", 20); // GTK, SynthLookAndFeel
UIManager.put("ScrollBar.minimumThumbSize", new Dimension(30, 30));
UIManager.put("ScrollBar.incrementButtonGap", 0);
UIManager.put("ScrollBar.decrementButtonGap", 0);

JScrollPane scroll = new JScrollPane(new JTextArea(txt)) {
  @Override public void updateUI() {
    super.updateUI();
    getVerticalScrollBar().setUI(new ArrowButtonlessScrollBarUI());
    getHorizontalScrollBar().setUI(new ArrowButtonlessScrollBarUI());
  }
};
// ...

class ZeroSizeButton extends JButton {
  private static final Dimension ZERO_SIZE = new Dimension();

  @Override public Dimension getPreferredSize() {
    return ZERO_SIZE;
  }
}

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

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

  // @Override protected Dimension getMinimumThumbSize() {
  //   // return new Dimension(20, 20);
  //   return UIManager.getDimension("ScrollBar.minimumThumbSize");
  // }

  // ...
}