Hidden QOpenGLWidget does not render()

void GLWidget::drawOffscreen()
{
    //the context should be valid. make sure it is current for painting
    makeCurrent();
    if (!m_isInitialized)
    {
        initializeGL();
        resizeGL(width(), height());
    }
    if (!m_fbo || m_fbo->width() != width() || m_fbo->height() != height())
    {
        //allocate additional? FBO for rendering or resize it if widget size changed
        delete m_fbo;
        QOpenGLFramebufferObjectFormat format;
        format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
        m_fbo = new QOpenGLFramebufferObject(width(), height(), format);
        resizeGL(width(), height());
    }

    //#1 DOES NOT WORK: bind FBO and render() widget
    m_fbo->bind();
    QOpenGLPaintDevice fboPaintDev(width(), height());
    QPainter painter(&fboPaintDev);
    painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing);
    render(&painter);
    painter.end();
    //You could now grab the content of the framebuffer we've rendered to
    QImage image1 = m_fbo->toImage();
    image1.save(QString("fb1.png"));
    m_fbo->release();
    //#1 --------------------------------------------------------------

    //#2 WORKS: bind FBO and render stuff with paintGL() call
    m_fbo->bind();
    paintGL();
    //You could now grab the content of the framebuffer we've rendered to
    QImage image2 = m_fbo->toImage();
    image2.save(QString("fb2.png"));
    m_fbo->release();
    //#2 --------------------------------------------------------------

    //bind default framebuffer again. not sure if this necessary and isn't supposed to use defaultFramebuffer()...
    m_fbo->bindDefault();
    doneCurrent();
}

void GLWidget::paintGL()
{
    //When doing mixed QPainter/OpenGL rendering make sure to use a QOpenGLPaintDevice, otherwise only OpenGL content is visible!
    //I'm not sure why, because according to the docs (http://doc.qt.io/qt-5/topics-graphics.html) this is supposed to be the same...
    QOpenGLPaintDevice fboPaintDev(width(), height());
    QPainter painter(&fboPaintDev);
    painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing);
    //This is what you'd use (and what would work) if the widget was visible
    //QPainter painter;
    //painter.begin(this);

    //now start OpenGL painting
    painter.beginNativePainting();
    glClearColor(0.5f, 0.0f, 0.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    ...
    painter.endNativePainting();
    //draw non-OpenGL stuff with QPainter
    painter.drawText(20, 40, "Foo");
    ...
    painter.end();
}

MainWindow::MainWindow()
{
    scene = new QGraphicsScene;
    hiddenView = new QGraphicsView(scene);
    hiddenGLWidget = new QOpenGLWidget;
    hiddenView->setViewport(hiddenGLWidget);
    //hiddenView->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    //hiddenView->show();
}

void MainWindow::screenshot()
{
    //DOES NOT WORK: try regular grab functions
    QPixmap pixmap1 = hiddenView->grab(); //image with scrollbars, no OpenGL content
    pixmap1.save("bla1.png");
    QPixmap pixmap2 = hiddenGLWidget->grab(); //produces an empty image
    pixmap2.save("bla2.png");
    //try grabbing only the QOpenGLWidget framebuffer
    QImage image1 = hiddenGLWidget->grabFramebuffer(); //null image
    image1.save("bla3.png");

    //WORKS: render via FBO
    hiddenGLWidget->makeCurrent();
    QOpenGLFramebufferObjectFormat format;
    format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
    QOpenGLFramebufferObject * fbo = new QOpenGLFramebufferObject(hiddenView->width(), hiddenView->height(), format);
    fbo->bind();
    QOpenGLPaintDevice fboPaintDev(hiddenView->width(), hiddenView->height());
    QPainter painter(&fboPaintDev);
    painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing);
    hiddenView->render(&painter); //WORKS and captures mixed OpenGL and non-OpenGL QGraphicsitems
    //hiddenView->repaint(); //does not work
    //hiddenView->scene()->render(&painter); //does not work
    //hiddenGLWidget->paintGL(); //might work. can not call, protected
    //hiddenGLWidget->render(&painter); //does not work
    //hiddenGLWidget->repaint(); //does not work
    painter.end();
    QImage image2 = fbo->toImage();
    image2.save("bla4.png");
    fbo->release();
    delete fbo;
}