#include <QtWidgets/QApplication>
#include <QtWidgets/QWidget>

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

    QWidget tlw;
    tlw.resize(640, 480);
    tlw.setStyleSheet("QWidget { background-color: blue; }");

    QWidget child(&tlw);
    child.setWindowFlag(Qt::FramelessWindowHint, true);
    child.setGeometry(320, 240, 100, 100);
    child.setStyleSheet("QWidget { background-color: red; }");

    tlw.winId();
    child.winId();

    child.setAttribute(Qt::WA_TranslucentBackground, true);
    child.setAttribute(Qt::WA_NoSystemBackground, false);

    tlw.show();

    return app.exec();
}