Java Swing - 如何在JScrolledPane中使用ViewPort设置添加JPanel

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;

public class Main {
  public static void main(String[] args) {
    JFrame f = new JFrame();
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setLayout(new BorderLayout());
    f.add(new TestPane());
    f.pack();
    f.setVisible(true);
  }
}
class TestPane extends JPanel {
  public TestPane() {
    setLayout(new BorderLayout());
    JPanel top = new JPanel(new GridBagLayout());
    top.setOpaque(false);
    top.add(new JLabel("OK"));
    JScrollPane sp = new JScrollPane();
    sp.setOpaque(false);
    sp.getViewport().setOpaque(false);
    sp.setViewportView(top);
    add(sp);
  }
  @Override
  public Dimension getPreferredSize() {
    return new Dimension(200, 200);
  }
  @Override
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D) g.create();
    g2d.setColor(Color.RED);
    g2d.drawLine(0, 0, getWidth(), getHeight());
    g2d.drawLine(getWidth(), 0, 0, getHeight());
    g2d.dispose();
  }
}