fix system menu

Zhao Yuhang

2023-12-09 3a5e5aea06e87710e24db0d7439ab81e3e3d301f

 examples/mainwindow/mainwindow.cpp

@@ -99,5 +99,6 @@
    setMenuWidget(windowBar);
    setCentralWidget(clockWidget);
    setWindowTitle("Example MainWindow");
    // setContentsMargins({0, 1, 0, 0});
    resize(640, 480);
}