(GUI) A problem with a panel and a menu

I have to write a game in Java (For those of you who are interested, it's called Paradux).
Now, I have a panel and a menu in the game. The panel is responsible for drawing the game board.

This is the initial state of the game:
http://up351.siz.co....4io2gdnyygw.png

(The image was uploaded on an Israeli server so in case it doesn't work, let me know, and I'll upload it to a different server).

Now, as soon as I make a move in the game, the board is repainting itself, and this is where the problem occurs.

When I repaint the panel, using the repaint method, the entire window is changing to the following state:
http://up351.siz.co....amfzjtzmnkg.png

When I change the size of the window, it will get better, but if I make a move again, the display will be ruined again.

Here's the code of the main window, which includes the menu and the panel:

import javax.swing.*;

public class Main extends JFrame {
    private static final long serialVersionUID = 1L;
    
    GameManager manager;
    
    public static void main(String[] args) {
        new Main();
    }
    
    public void initVariables() {
        manager = new GameManager();
    }
    
    public void initFrame() {
        this.setSize(480, 480);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        JMenuBar menuBar = new JMenuBar();
        JMenu menu1 = new JMenu("File");
        menuBar.add(menu1);
        this.setJMenuBar(menuBar);
        
        this.add(manager.getDisplay());
        
        this.setVisible(true);
    }
    
    public Main() {
        super("Paradux by BaraCK Levy");
        initVariables();
        initFrame();
    }
}

If you need the rest of the classes, just let me know.
Inside the GameManager class, there's a member called display of the type Display which extends JPanel.

Another strange situation that involves it is that if I press the File menu, and then exit the menu, the entire "New" menu item becomes the display. :SSS

Here are images of the situation:
http://up351.siz.co....jkyu2c2mwzy.png
http://up351.siz.co....tytjohjymfy.png

When I click on a tiny square on the game board on the upper left, the big game board is effected, a move is made, and the first problem occurs.

What's going on? It's like the panel is becoming the menu or something... Help me please. ;D