Problems with transparent JFrame

Question

I have created JFrame, set transparent color (new Color(0, 0, 0, 0)) as it background and place JPanel on it. I have overrided paintComponent method of JPanel and draw some image with alpha channel on it. Also I show popup menu when user right click on my JPanel.

And I was faced with the fact that the image of the pop-up menu remains on the JFrame after it should to disappear (just now the picture of a JPanel covers it). As I understand it, JFrame is not cleared before drawing a new frame.

Example

public class MainWindow extends JFrame {

    private MyPanel widget = new MyPanel();
    private Action quitAction = new AbstractAction("Quit") {
        @Override
        public void actionPerformed(ActionEvent actionEvent) {
            setVisible(false);
            dispose();
        }
    };

    private MainWindow() {
        super();
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setUndecorated(true);
        setBackground(new Color(0, 0, 0, 0));
        widget.loadImage("image.png");
        setContentPane(widget);
        pack();
        setAlwaysOnTop(true);
    }

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(() -> {
            MainWindow window = new MainWindow();
            window.setVisible(true);
        });
    }

    class MyPanel extends JPanel implements MouseListener {
        BufferedImage image;

        MyPanel() {
            super();
            addMouseListener(this);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintCompontent(g);
            if (characterImage != null) {
                g.drawImage(characterImage, 0, 0, this);
            }
        }

        @Override
        public void mouseClicked(MouseEvent e) {
        }

        @Override
        public void mousePressed(MouseEvent e) {
            clickPos = e.getLocationOnScreen();
            if (SwingUtilities.isRightMouseButton(e)) {
                JPopupMenu popupMenu = new JPopupMenu();
                popupMenu.add(quitAction);
                popupMenu.show(this, e.getX(), e.getY());
            }
        }

        @Override
        public void mouseReleased(MouseEvent e) {
        }

        @Override
        public void mouseEntered(MouseEvent e) {
        }

        @Override
        public void mouseExited(MouseEvent e) {
        }

        boolean loadImage(String name) {
            BufferedImage backupImage = image;
            try {
                image = ImageIO.read(MainWindow.class.getResourceAsStream("/images/" + name));
                setPreferredSize(new Dimension(image.getWidth(), image.getHeight()));
            } catch (IOException ex) {
                image = backupImage;
                return false;
            }
            return true;
        }

    }

}

Answer 1

So, based loosely on your code, once I corrected it to work, I don't seem to have any issues

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JPopupMenu;
import javax.swing.SwingUtilities;

public class MainWindow extends JFrame {

    private MyPanel widget = new MyPanel();
    private Action quitAction = new AbstractAction("Quit") {
        @Override
        public void actionPerformed(ActionEvent actionEvent) {
            setVisible(false);
            dispose();
        }
    };

    private MainWindow() {
        super();
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setUndecorated(true);
        setBackground(new Color(0, 0, 0, 0));
        widget.loadImage("MT.png");
        setContentPane(widget);
        pack();
        setLocationRelativeTo(null);
        setAlwaysOnTop(true);
    }

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(() -> {
            MainWindow window = new MainWindow();
            window.setVisible(true);
        });
    }

    class MyPanel extends JPanel implements MouseListener {

        BufferedImage image;

        MyPanel() {
            super();
            addMouseListener(this);
            setOpaque(false);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.RED);
            g.drawRect(0, 0, getWidth() - 1, getHeight() - 1);
            if (image != null) {
                g.drawImage(image, 0, 0, this);
            }
        }

        @Override
        public void mouseClicked(MouseEvent e) {
        }

        @Override
        public void mousePressed(MouseEvent e) {
//            clickPos = e.getLocationOnScreen();
            if (SwingUtilities.isRightMouseButton(e)) {
                JPopupMenu popupMenu = new JPopupMenu();
                popupMenu.add(quitAction);
                popupMenu.show(this, e.getX(), e.getY());
            }
        }

        @Override
        public void mouseReleased(MouseEvent e) {
        }

        @Override
        public void mouseEntered(MouseEvent e) {
        }

        @Override
        public void mouseExited(MouseEvent e) {
        }

        boolean loadImage(String name) {
            BufferedImage backupImage = image;
            try {
                image = ImageIO.read(MainWindow.class.getResourceAsStream("/images/" + name));
                setPreferredSize(new Dimension(image.getWidth(), image.getHeight()));
            } catch (IOException ex) {
                image = backupImage;
                return false;
            }
            return true;
        }

    }

}

Answer 2

I have found issue in my code. For everything to work correctly I need to insert this code before picture drawing using g.drawImage in paintCompontent.

if (g instanceof Graphics2D) {
    Graphics2D g2d = (Graphics2D)g;
    g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC));
}

Also, the problem appears not in all cases. For example, under Windows it worked well as it.