void MainWindow::showMenu()
{
    QMenu *menu = new QMenu(this);
    menu->setAttribute(Qt::WA_DeleteOnClose);
    menu->addAction(ui->actionStart);
    menu->popup(QCursor::pos());
}

void MainWindow::startTriggered()
{
    for (int i = 0; i <= 100; i += 1)
    {
        ui->progressBar->setValue(i);
        /* First call to processEvents() triggers QMenu destructor on Qt 6
         * QMenu is deleted but the object is still needed by code on stack!
         * When this function finishes and the code on stack uses now-deleted
         * QMenu object and memory corruption occurs.
         *
         * On Release builds the crash usually happens the second time the
         * action is triggered. When debugging Debug builds in Visual Studio
         * the exception is seen after first trigger.
         * 
         * This code works perfectly fine with Qt 5.
         */
        QCoreApplication::processEvents();
    }
}