JScrollBarのKnobの最小サイズをJScrollPaneのサイズ相対に変更する

JScrollPane scroll = new JScrollPane(textArea);
scroll.setVerticalScrollBar(new JScrollBar(Adjustable.VERTICAL) {
  @Override public void updateUI() {
    super.updateUI();
    setUI(new MetalScrollBarUI() {
      @Override protected Dimension getMinimumThumbSize() {
        Dimension d = super.getMinimumThumbSize();
        Rectangle r = SwingUtilities.calculateInnerArea(scroll, null);
        d.height = Math.max(d.height, r.height / 12);
        return d;
      }
    });
    putClientProperty("JScrollBar.fastWheelScrolling", Boolean.TRUE);
  }
});