JPanel内容窗格混乱

import javax.swing.*;

public class JPanelDemo {
    public static void main(String[] args) {
        JFrame frame = new JFrame("JPanel Demo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);

        JPanel panel = new JPanel();
        panel.setLayout(new GridLayout(3, 3));

        for (int i = 1; i <= 9; i++) {
            JButton button = new JButton("Button " + i);
            panel.add(button);
        }

        frame.add(panel);
        frame.setVisible(true);
    }
}