Wrong menu position for QToolButton placed on a scene

#include <QApplication>
#include <QHBoxLayout>
#include <QMenu>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QToolButton>

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

    QToolButton* btn = new QToolButton;
    QMenu* m = new QMenu;
    m->addAction( "test" );
    btn->setMenu( m );
    btn->setPopupMode( QToolButton::InstantPopup );

    QWidget* const w = new QWidget;
    QHBoxLayout* const l = new QHBoxLayout;
    l->addWidget( btn );
    w->setLayout( l );

    QGraphicsScene scene;
    scene.addWidget( w );

    QGraphicsView graphicsView;
    graphicsView.setScene(&scene);

    graphicsView.show();

    return app.exec();
}