REGRESSION: unknown window is created when the height of menu list is longer tha

A DESCRIPTION OF THE REGRESSION :
Windows XP Professional SP1+
JDK VERSION:mustang b71(not on this entry form,please append it)

click the menubar , when the total height of menu items is longer than the window itself, an unknown window is created and displayed on the window's Taskbar.

REPRODUCIBLE TESTCASE OR STEPS TO REPRODUCE:
package testbug;

import java.awt.Dimension;

import javax.swing.*;

public class TestJMenubar extends JFrame {
    private JMenuBar menuBar = new JMenuBar();

    private JMenu menuFile = new JMenu();

    private JMenuItem menuFileExit = new JMenuItem();

    private JMenuItem jMenuItem1 = new JMenuItem();

    private JMenuItem jMenuItem2 = new JMenuItem();

    public TestJMenubar() {
        try {
            jbInit();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void jbInit() throws Exception {
        this.setJMenuBar( menuBar );
        this.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        this.getContentPane().setLayout( null );
        this.setSize( new Dimension(400, 50) );
        this.setTitle( "Menu" );
        menuBar.setAutoscrolls(true);
        menuFile.setText( "File" );
        menuFileExit.setText("Item");
        jMenuItem1.setText("long 1");
        jMenuItem2.setText("long 2");
        menuFile.add( menuFileExit );
        menuFile.add(jMenuItem1);
        menuFile.add(jMenuItem2);
        menuBar.add(menuFile);
    }

    public static void main(String[] argv){
            TestJMenubar bar = new TestJMenubar();
            bar.setVisible(true);
    }

}


RELEASE LAST WORKED:
5.0 Update 6

RELEASE TEST FAILS:
mustang-b70

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
no extra window please.. at least prevent it from being displayed on taskbar.
ACTUAL -
it seems a new window is created and displayed on taskbar without any caption...

Release Regression From : 5.0u6
The above release value was the last known release where this
bug was known to work. Since then there has been a regression.