JComboBox の GlassPane 上でのレンダリング

        JFrame frame = new JFrame();
        frame.setSize(200, 200);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.getRootPane().getLayeredPane();
                
        JPanel panel = new JPanel(new GridBagLayout());
        panel.setOpaque(false); // これで表示はされるが・・・
        JComboBox comboBox = new JComboBox(new String[]{"A", "B", "C"});
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridy = 0;
        panel.add(comboBox, gbc);
        gbc.gridy = 1;
        panel.add(new JLabel("Hello World"),gbc);
        frame.getRootPane().setGlassPane(panel);
        panel.setVisible(true);
 

import java.awt.GridLayout;
import java.text.MessageFormat;

import javax.swing.JApplet;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JLayeredPane;
import javax.swing.JPanel;
import javax.swing.WindowConstants;

public class LayeredPaneSample extends JApplet {

	private JLayeredPane layeredPane = new JLayeredPane();
	
	public void init() {
		setContentPane(layeredPane);
		layeredPane.setLayout(null);

		JPanel panel1 = new JPanel();
		panel1.setLayout(new GridLayout(5, 1));
		for (int i = 0; i < 5; i++) {
			panel1.add(new JLabel(MessageFormat.format("Line{0, number, #}:　これはテストです", i)));
		}
		layeredPane.setLayer(panel1, JLayeredPane.DEFAULT_LAYER);
		layeredPane.add(panel1);
		panel1.setBounds(0, 0, 200, 100);

		JComboBox comboBox = new JComboBox(new String[]{"A", "B", "C"});
		layeredPane.setLayer(comboBox, JLayeredPane.POPUP_LAYER);
		layeredPane.add(comboBox);
		comboBox.setBounds(10, 10, 100, 103);
    }

	public static void main(String[] args) {
		final JFrame frame = new JFrame();
		JApplet applet = new LayeredPaneSample();

		applet.init();

		frame.setContentPane(applet.getContentPane());
		frame.setBounds(100, 100, 308, 199);
		frame.setVisible(true);
		frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
	}
}