JScrollBarのノブを常に表示する

// UIManager.put("ScrollBar.alwaysShowThumb", Boolean.TRUE);
JScrollPane scroll = new JScrollPane(new JTree());
scroll.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);

String key = "ScrollBar.alwaysShowThumb";
JCheckBox check = new JCheckBox(key) {
  @Override public void updateUI() {
    super.updateUI();
    EventQueue.invokeLater(() ->
        setSelected(UIManager.getLookAndFeelDefaults().getBoolean(key)));
  }
};
check.addActionListener(e -> {
  UIManager.getLookAndFeelDefaults().put(key, check.isSelected());
  SwingUtilities.updateComponentTreeUI(scroll);
});