Failed to place widget over widget container containing QOpenGLWindow

You can download an example here: https://forum.qt.io/post/753086

I created a window that inherited from QOpenGLWindow:

```
class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
```

I'm trying to place a button over a container that contains a QOpenGLWindow:

```

        QVBoxLayout *rootLayout = new QVBoxLayout();
        setLayout(rootLayout);

        m_pOpenGLWindow = new OpenGLWindow();
        QWidget *container = QWidget::createWindowContainer(m_pOpenGLWindow, this);
        rootLayout->addWidget(container);

        m_pStartGameButton = new QPushButton(QString("Start Game"), this);
        // m_pStartGameButton->raise();
        m_pStartGameButton->move(0, 0);
        m_pStartGameButton->setWindowFlags(m_pStartGameButton->windowFlags() | Qt::WindowStaysOnTopHint);

```

But the button is behind the container.