QToolButton popup menu not showing up if initially empty

    QVBoxLayout* layout = new QVBoxLayout;

    QToolButton* btn = new QToolButton;
    btn->setPopupMode(QToolButton::InstantPopup);

    QMenu* menu = new QMenu;
    connect(menu, &QMenu::aboutToShow,
            this, [this, menu]()
    {
        QTimer::singleShot(0, this, [menu](){
             menu->addAction(QString("Entry %1").arg(menu->actions().size()));
             qDebug() << "visible:" << menu->isVisible();
         });
    });

    // if the menu already has an entry it will show
    // menu->addAction("Entry 0");

    btn->setMenu(menu);

    layout->addWidget(btn);

    QWidget* w = new QWidget;
    w->setLayout(layout);
    setCentralWidget(w);