为什么paintComponent绘图JMenu除了红圈之外?[副本]

我正在尝试用2创建一个程序 JPanel 使用 BorderLayout 。中间面板用于随机绘制矩形,而南面板用于按钮。

我在的左上角看到了一个奇怪的按钮图像 JFrame 每当我将鼠标光标悬停在“北”或“南”按钮上时。我做了一些研究,发现这可能是拥有透明背景的原因。我试着使用 super.paintComponent(g) 对于面板,但是先前绘制的其余矩形消失。我需要矩形保持在 面板 但不是左上角那张奇怪的照片。

我不知道我做错了什么,希望有人能帮助或提供一些线索来解决这个问题。

    public class TwoBRandomRec extends JFrame{

    private static final long serialVersionUID = 1L;

    public static void main(String[] args) {
        TwoBRandomRec rec = new TwoBRandomRec();

        rec.setSize(500,500);
        rec.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        rec.setVisible(true);
    }

    public TwoBRandomRec() {
        //Create the buttons
        JButton north = new JButton("North");
        JButton south = new JButton("South");
        DrawPanel drawPanel = new DrawPanel(500,500);

        JPanel southP = new JPanel();
        southP.add(south);
        southP.add(north);

        this.add(drawPanel, BorderLayout.CENTER);
        this.add(southP, BorderLayout.SOUTH);

        this.setTitle("TwoButtonRandomRec");
        this.pack();        
    }

    public class DrawPanel extends JPanel {

        private static final long serialVersionUID = 1L;

        private Random rand = new Random();
        private int x,y,xSize,ySize;
        private int height,width;

        public DrawPanel(int w,int h) {
            width = w;
            height = h;
        }
        public void RandomX(){
             x=rand.nextInt(width-1);
             xSize=rand.nextInt(width-x);
         }

         public void RandomY(){
             y=rand.nextInt(height-1);
             ySize=rand.nextInt(height-y);
         }

         public Color RandomColour(){
             rand.nextInt(height);
             return new Color(rand.nextInt(255),rand.nextInt(255),rand.nextInt(255));
         }

        @Override
        protected void paintComponent(Graphics g) {
            RandomX();
            RandomY();

            g.setColor(RandomColour());
            g.fillRect(x, y, xSize, ySize);
            try {
                Thread.sleep(50);

            } catch (InterruptedException e) {  
            }

            repaint();
        }
    }
}