一个 Java 入门 GUI 小程序,就是不太明白为什么我会多一个假 button 块

上面的那个 button 按钮是假的,第一鼠标移动上去不变样子,第二点也点不了
点下面那个真的我代码安排好的,很正常.并且点了一次之后上面那个假的也会消失,程序正常运行,色块变色
程序代码如下:

package SimpleGui3;
import javax.swing.*;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.*;

public class SimpleGui3 {    
    public static void main(String[] args) {
        MyFrame gui = new MyFrame();
    }
}

class MyFrame extends JFrame implements ActionListener {
    JButton button;
    MyDrawPanel draw_panel;
    
    public MyFrame() {    
        this.setVisible(true);
        this.setSize(300, 300);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        
        button = new JButton("click to change colors");
        button.addActionListener(this);
        this.getContentPane().add(BorderLayout.SOUTH,button);
        
        draw_panel = new MyDrawPanel();
        this.getContentPane().add(BorderLayout.CENTER,draw_panel);
        
    }
    
    public void actionPerformed(ActionEvent event) {
        this.repaint();
    }
}
//无关紧要的部分↓
class MyDrawPanel extends JPanel {
    public void paintComponent(Graphics g) {
        int[] color_array = new int[3];
        for(int i=0;i<color_array.length;i++) {
            color_array[i] = (int)(Math.random() * 255);
        }
        Color c = new Color(color_array[0],color_array[1],color_array[2]);
        g.setColor(c);
        g.fillRect(20, 50, 100, 100);
    }
}

到底是咋回事儿啊,求大佬解答,谢谢..