只有当我将鼠标悬停在它们上面时，JButtons才会出现。调整窗口大小时也会消失

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class ButtonVisibilityExample {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Button Visibility Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new FlowLayout());

        JButton button = new JButton("Hover Me");
        button.setVisible(false); // 初始时不可见

        button.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseEntered(MouseEvent e) {
                button.setVisible(true); // 鼠标悬停时可见
            }

            @Override
            public void mouseExited(MouseEvent e) {
                button.setVisible(false); // 鼠标离开时不可见
            }
        });

        frame.add(button);
        frame.pack();
        frame.setVisible(true);
    }
}