When the main window remains at the top, the child Windows are occluded

#include <QtGui>
#include <QtWidgets>

int main(int argc, char * argv[])
{
    QApplication app(argc, argv);

    QDialog* mw = new QDialog(0, Qt::WindowStaysOnTopHint | Qt::X11BypassWindowManagerHint);
    mw->setWindowTitle(QLatin1String("Main"));
    mw->resize(400, 300);
    mw->move(100, 100);
    mw->show();
    QWidget* d = new QWidget(mw, Qt::Tool);
    d->setWindowTitle(QLatin1String("Tool"));
    d->resize(200, 100);
    d->show();
    return app.exec();
}