#include <QtWidgets/QtWidgets>

int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
  // Just to make the behavior more identical. It is not a part of the problem.
  QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
  QApplication a(argc, argv);
  QMainWindow w;

  w.setMinimumWidth(300);

  w.setWindowTitle(QStringLiteral("Qt Version: ") + qVersion());

  auto SomeMenu = new QMenu("Menu Action 2");

  QMenu *fileMenu = w.menuBar()->addMenu("File");

  w.setGeometry(-3000, 100, 300, 300);

  fileMenu->addAction("Action 1");
  fileMenu->addMenu(SomeMenu);
  fileMenu->addAction("Action 3");

  QObject::connect(SomeMenu, &QMenu::aboutToShow, SomeMenu,
          [=]() { SomeMenu->clear();
    SomeMenu->addAction("SubItem 1");
    SomeMenu->addAction("SubItem 2");
  });

  w.show();
  return a.exec();
}