"I really wish some one would just wrap Metal Graphics API." There are wrappers that allow OpenGL and Vulkan that are used for cross platform game programming.
I know for sure that ImGui supports OS X so it should be viable for that use case. It is obviously not a "native" UI but I would personally prefer somethings that has it's own renderer so that it looks the same across platforms. It is portable, slim, fast and work with games. It lacks some features that more application oriented UI:s such as Qt has. While Qt also renders directly on hardware it is rather bloated and for some platforms such as Raspberry Pi it is not free to use.
