If you try to show system menu of internal frame under Win LAF (push left mouse
button or press Ctrl-Space when internal frame has a keyboard focus) you can see
it painted three times and system menu became very high.

-------------------------Test.java----------------------------
import javax.swing.*;
import java.awt.*;
import java.beans.*;

public class Test extends JFrame {

    public Test() {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
        }
        JDesktopPane dp = new JDesktopPane();
        setContentPane(dp);
        JInternalFrame jif = new JInternalFrame("IFrame",true,true,true,true);
        dp.add(jif);
        jif.setBounds(20,20,120,100);
        jif.setVisible(true);
        setBounds(20,20,300,200);
        setVisible(true);
    }

              
    public static void main(String[] argv) {
        Test t = new Test();
    }
}
------------------------------------------------------------------