The point of QWidgets is not to create a "modern app with fancy animations", that's what QML is better suited for. It can still be done, QWidgets supports themi

The point of QWidgets is not to create a "modern app with fancy animations", that's what QML is better suited for. It can still be done, QWidgets supports theming and all that, but that's not what it does best. QWidgets is for making well integrated, "boring" desktop apps. For nested menus, or any menu for that matter, use QMenu. If your OS is fully supported by Qt and the standard system behavior is to animate the menu, it will be animated in your app.