JDesktopPaneでスクロールバーは出ないのでしょうか？

import java.awt.Dimension;
import java.awt.event.*;
import javax.swing.*;

public class DeskTopPaneTest extends JFrame{

   public static void main(String[] args) {

      DeskTopPaneTest f = new DeskTopPaneTest("DeskTopPaneTest");

      f.setSize(300,300);
      f.show();
   }

   public DeskTopPaneTest(String title) {
      super(title);
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      
      final JDesktopPane pane = new JDesktopPane();
      final JScrollPane scroll = new JScrollPane(pane);
      getContentPane().add(scroll);
      
      final JInternalFrame frame = new JInternalFrame("hoge");
      frame.addComponentListener(new ComponentAdapter() {
         public void componentMoved(ComponentEvent e) {
            int pw = pane.getWidth();
            int ph = pane.getHeight();
            
            int fxw = frame.getX() + frame.getWidth();
            int fyh = frame.getY() + frame.getHeight();
            
            if(pw < fxw){
               pw = fxw;
            }
            else if(pw > scroll.getWidth()){
               pw = scroll.getViewport().getWidth();
            }
            if(ph < fyh){
               ph = fyh;
            }
            else if(ph > scroll.getHeight()){
               ph = scroll.getViewport().getHeight();
            }
            
            pane.setPreferredSize(new Dimension(pw, ph));
            scroll.validate();
         }
      });
         
      pane.add(frame);
      
      frame.setSize(100,100);
      frame.show();
   }
}