I've always lamented that QML and QtWidgets are separate rather than integrated. It would be awesome if there was a declarative way to specify much of the same information that you have to specify programmatically with QtWidgets (e.g., the cumbersome process of defining nested sizers), and then you could write code to handle the actual "business logic". But instead we have two totally separate UI frameworks where you can either use the nice native QtWidgets or you can have a nice declarative UI definiton, but not both.

It exists since 2018 aha https://www.kdab.com/declarative-widgets/