Windows/Vista style: Scrollbar disappears from QComboBox embedded into QGraphicsView when using QGLWidget as viewport

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QGraphicsView graphics_view;
    graphics_view.setFrameStyle(0);
    graphics_view.setViewport(new QGLWidget(QGLFormat(QGL::SampleBuffers)));
    graphics_view.setViewportUpdateMode(QGraphicsView::FullViewportUpdate);

    QGraphicsScene *scene = new QGraphicsScene;
    graphics_view.setScene(scene);

    QWidget window;
    QGraphicsProxyWidget* proxy = graphics_view.scene()->addWidget(&window);
    proxy->setPos(0, 0);
    proxy->setCacheMode(QGraphicsItem::NoCache);

    QComboBox* combo = new QComboBox(&window);
    for (int i = 0; i < 20; ++i)
    {
        combo->addItem(QString::number(i));
    }

    QHBoxLayout* layout = new QHBoxLayout(&window);
    layout->addWidget(combo);
    window.setLayout(layout);

    graphics_view.show();

    return app.exec();
}