JPopupMenu ignores setOpaque(false) near screen edges

Question

Dear group
I try to paint a custom JPopupMenu with transparency, but this is ignored when the popup is opened so it overlaps near screen edges (bottom and right side). This is also the case if the popup overlaps the Microsoft Windows toolbar at the bottom.
I may be able to emulate JPopupMenus using custom components in a JLayeredPane, but can anyone explain the above behaviour?
Thanks and Best Regards,
Aksel, Denmark
Environment: j2sdk-se 1.6.0_10 and windows xp
Example:
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Rectangle;
import java.awt.Toolkit;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPopupMenu;
public class TransparencyPopupNearBorderTest {
public static void main(String[] args) {
final JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLocationRelativeTo(null);
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
frame.setBounds(new Rectangle(screenSize));
frame.setUndecorated(true);
frame.setPreferredSize(screenSize);
frame.addMouseListener(new MouseAdapter() {
@Override
public void mouseReleased(MouseEvent e) {
JPopupMenu popup = new JPopupMenu();
popup.setOpaque(false);
JLabel label = new JLabel("TRANSPARENT");
label.setFont(new Font("Helvetica", Font.PLAIN, 42));
popup.add(label);
popup.setLightWeightPopupEnabled(false);
popup.show(frame, e.getX(), e.getY());
frame.getContentPane().setBackground(Color.BLUE);
frame.pack();
frame.setVisible(true);
}

Answer 1

Don't forget to use the Code Formatting Tags so the posted code retains its original formatting. That is done by selecting the code and then clicking on the "Code" button above the question input area.
I don't know if transparency painting has changed in 1.6.0_10. Prior to that I believe transparency can only be achieved in lightweight components (ie. Swing does all the painting). JFrame, JWindow and JDialog are not lightweight because they use OS components.
In the case of a popup, it is lightweight when entirely contained within its parent frame. But a lightweight popup can not be painted outside the bounds of the frame so a JWindow (I believe) is used as the popup, which can't be transparent.