JLayeredPane doesn't show canvas layer below a transparent JButton layer

Question

I have a JLayeredPane with two layers. The bottom layer is a canvas with bounds (0, 0, 600, 400) and the top layer is a JButton with bounds (500, 300, 40, 40). The JButton is completely tansparent. So in this JButton area the canvas should be visible. But instead there is a white rectangle. It doesn't matter for the issue what is drawn on the canvas or if it is a canvas at all.

Here is the code:

public class Main extends JFrame {
    
    private static final long serialVersionUID = 1L;
    
    private final Canvas canvas;
    private final JButton button;
    
    Main() {
        super("InvisibleButton");
        
        this.canvas = new Canvas();
        this.canvas.setBounds(0, 0, 600, 400);
        this.canvas.requestFocus();
        this.canvas.setIgnoreRepaint(true);
        this.canvas.setVisible(true);
        
        this.button = new JButton();
        this.button.setBounds(500, 300, 40, 40);
        
        // Doesn't work
        this.button.setOpaque(false);
        this.button.setBorderPainted(false);
        this.button.setFocusPainted(false);
        this.button.setContentAreaFilled(false);
                
        this.button.addMouseMotionListener(new MouseAdapter() {
            
            @Override
            public void mouseDragged(MouseEvent e) {
                System.out.println("Dragged");
            }
            
        });
        
        JLayeredPane jlp = new JLayeredPane();
        jlp.setBounds(0, 0, 600, 400);
        jlp.add(this.button);
        jlp.add(this.canvas);
        
        this.add(jlp);
        this.setLayout(null);
        this.setSize(600, 400);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setResizable(false);
        this.setVisible(true);
        
        Timer repainter = new Timer();
        repainter.schedule(new TimerTask() {

            @Override
            public void run() {
                Graphics2D g2d = (Graphics2D)canvas.getGraphics();
                
                Shape s = new Rectangle.Double(0, 0, 600, 400);
                g2d.setColor(Color.BLUE);
                g2d.fill(s);
                g2d.draw(s);
            }
            
        }, 0, 100);
    }
    
    public static void main(String[] args) {
        new Main();
    }

}

The drag event works great. But there is this white rectangle:1