In 2017 I started NiGui, which is far away from complete and not actively continued. Nowadays I think non-native widgets is the better approach. I could think of starting it over again, but would like to do it in a team to be able to support the different OSes.
To Araq's points I want to add: * touch screen support (scrolling with the finger in every app) * scalable UI (Ctrl+plus/minus in every app)
