如何修复窗口失去焦点并重新获得焦点时KeyAdapter不再工作

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

public class FocusKeyAdapterExample {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Focus KeyAdapter Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 200);

        JPanel panel = new JPanel();
        frame.add(panel);

        // 创建一个 KeyAdapter
        KeyAdapter keyAdapter = new KeyAdapter() {
            @Override
            public void keyPressed(KeyEvent e) {
                System.out.println("Key pressed: " + e.getKeyChar());
            }
        };

        // 添加 FocusListener 来监听窗口焦点的变化
        frame.addFocusListener(new FocusAdapter() {
            @Override
            public void focusGained(FocusEvent e) {
                // 当窗口重新获得焦点时，重新注册 KeyAdapter
                panel.addKeyListener(keyAdapter);
            }

            @Override
            public void focusLost(FocusEvent e) {
                // 当窗口失去焦点时，移除 KeyAdapter
                panel.removeKeyListener(keyAdapter);
            }
        });

        // 初始注册 KeyAdapter
        panel.addKeyListener(keyAdapter);

        frame.setVisible(true);
    }
}