improve sysmenu behavior

Zhao Yuhang

2024-05-07 357532958f2e806c69c96f018333a45e65e35201

 examples/mainwindow/mainwindow.h

@@ -30,6 +30,8 @@
protected:
    bool event(QEvent *event) override;

    void closeEvent(QCloseEvent *event) override;

private:
    void installWindowAgent();
    void loadStyleSheet(Theme theme);