QMenu are shown at the wrong location in a QGraphicsScene

#include <QApplication>
#include <QMenu>
#include <QMenuBar>
#include <QGraphicsScene>
#include <QGraphicsView>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QMenuBar* bar = new QMenuBar;
    // Populate :
    QMenu* a1 = bar->addMenu("a1");
    a1->addMenu("b1")->addAction("c1");
    a1->addMenu("b2")->addAction("c2");
    QMenu* a2 = bar->addMenu("a2");
    a2->addMenu("b1")->addAction("c1");
    a2->addMenu("b2")->addAction("c2");
    QGraphicsScene scene;
    QGraphicsProxyWidget* proxy = scene.addWidget(bar);

    QGraphicsView graphicsView;
    graphicsView.setScene(&scene);

    // To get a larger window :
    //graphicsView.setSceneRect(0, 0, 500, 500);
    //graphicsView.resize(512, 512);
    graphicsView.show();

    return app.exec();
}