#include <QApplication>
#include <QWindow>
#include <QTimer>

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

    QWindow w1;
    w1.setGeometry(100, 100, 640, 400);
    w1.setTitle("Window 1");

    auto w2 = new QWindow;
    w2->setGeometry(150, 150, 400, 400);
    w2->setTransientParent(&w1);
    w2->setTitle("Window 2");

    auto w3 = new QWindow;
    w3->setGeometry(200, 200, 400, 400);
    w3->setTransientParent(w2);
    w3->setTitle("Window 3");

    w1.show();
    w2->show();
    w3->show();

    QTimer::singleShot(5000, w2, &QObject::deleteLater);

    return app.exec();
}