QXcbWindow ignores stays-on-top or -below

#include <QGuiApplication>
#include <QWindow>

int main(int argc, char *argv[])
{
    QGuiApplication a(argc, argv);
    QWindow window;
    window.show();
    window.setFlags(window.flags() | Qt::WindowStaysOnTopHint);

    return a.exec();
}