import javax.swing.*;

public class TestScrollBarThumb implements Runnable {
    public static void main(String[] args) throws Exception {
        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        SwingUtilities.invokeLater(new TestScrollBarThumb());
    }

    @Override
    public void run() {
        JFrame frame = new JFrame(getClass().getSimpleName());
        frame.add(new JScrollPane(new JTextArea(5, 2000)));
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setSize(100, 100);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}