allow qml demo choose special effect

Yuhang Zhao

2023-12-28 68c8cc026233e9694579ce38b8daa25008477d18

 src/widgets/widgetwindowagent.cpp

@@ -80,6 +80,9 @@
        if (!d->context->setTitleBar(w)) {
            return;
        }
#ifdef Q_OS_MAC
        setSystemButtonArea(nullptr);
#endif
        Q_EMIT titleBarChanged(w);
    }