This issue first appeared as item #1 in bug 6425606.

When an undecorated top-level window is maximized (via setExtendedState()), the window's icon and title bar appear briefly as the window is maximizing.  Once maximized, the window is again undecorated.  Not a huge deal, but a bit distracting and certainly not proper behavior.

This bug has been present since the setUndecorated() API was added in 1.4 - I can reproduce on XP using 1.4.2_12, 1.5.0, and 1.6b85.

I used the following test case to reproduce this bug w/ an AWT Frame and a Swing JFrame:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class MaximizeTests implements ActionListener {
    static Frame frame;
    static Button awtBtn;
    static JFrame jframe;
    static JButton jBtn;

    public static void main(String[] args) {
        MaximizeTests al = new MaximizeTests();
        frame = new Frame("AWT Frame");
        frame.setUndecorated(true);
        awtBtn = new Button("Toggle Maximize");
        awtBtn.addActionListener(al);
        frame.add(awtBtn, BorderLayout.SOUTH);
        frame.add(new Label("AWT Frame"), BorderLayout.NORTH);
        frame.setBounds(10, 10, 200, 200);

        jframe = new JFrame("Swing Frame");
        jframe.setUndecorated(true);
        jBtn = new JButton("Toggle Maximize");
        jBtn.addActionListener(al);
        jframe.getContentPane().add(jBtn, BorderLayout.SOUTH);
        jframe.getContentPane().add(new JLabel("Swing Frame"), BorderLayout.NORTH);
        jframe.setBounds(220, 10, 200, 200);

        frame.setVisible(true);
        jframe.setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {
        Frame src = null;
        if (e.getSource() == awtBtn) {
            src = frame;
        }
        else if (e.getSource() == jBtn) {
            src = jframe;
        }
        if (frame != null) {
            int state = src.getExtendedState();
            src.setExtendedState(state ^ Frame.MAXIMIZED_BOTH);
        }
    }
}