Web combobox list is opened to wrong position if MainWindow is frameless

WebView opens combobox list in wrong position when parent is QMainWindow Form and it's frameless. There's no (visible) issue if QMainWindow is not form.

Please see and test with attached application. Example is using Qt account download section drop/comboboxes.

Workaround is to setGeometry to MainWindow after show.

    MainWindow w;
    w.setWindowFlags(Qt::FramelessWindowHint);
    w.show();
// WORKAROUND is to set geometry to mainwindow after show
//    w.setGeometry(400,400,800,600);