Widgets which don't paint their entire bounds show black areas when rendered above a QGraphicsView with a QGLWidget viewport

#include <QtGui>

#include <QGLWidget>

#if (QT_VERSION >= QT_VERSION_CHECK(5, 0, 0))
#include <QtWidgets>
#endif

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

    QGraphicsView window;
    window.setScene(new QGraphicsScene());
    window.setViewport(new QGLWidget(QGLFormat(QGL::SampleBuffers)));
    window.setViewportUpdateMode(QGraphicsView::FullViewportUpdate);

    QWidget *emptyWidget = new QWidget(&window);

    window.resize(400, 400);
    window.show();

    return app.exec();
}