QSkinny
The (Q)Skinny library is a framework built on top of the Qt scene graph and very few core classes from Qt/Quick. It offers a set of lightweight controls, that can be used from C++ and/or QML.
Doing the implementation in C++ allows to make use of the "scene graph" classes. Building controls from scene graph nodes allows for a lighter implementation than found with stacking "heavier" objects like QObject or QQuickItem. Offering a full featured C++ API allows the application code to benefit from following the same strategies.
Being "skinny" also means a design that separates concerns between the API and logic of the controls themselves, the styling of these controls, and the delegated rendering of the controls to the screen.
The current selection of implemented controls is driven by the needs of specific projects and therefore may feel a bit random. Conceptually, though, any type of control fits into QSkinny, as long as it is usable from C++ (as opposed to only QML).
This is a screenshot of a sample automotive UI (see examples/automotive):
