Nobody has yet cited [NiGui](https://github.com/simonkrauter/NiGui), which, for me, is the easiest interface and supports both Windows and Linux. I don't know how support for macOS is going. However, it leaves something to be desired in some respects.
[UIng](https://github.com/neroist/uing) I considered using it until I found out that for Windows it will load the C++ runtime (libstdc++) and I was discouraged. To link with WinAPI you don't need to have C++. Sad. [wNim](https://github.com/khchen/wNim), unfortunately, although very complete, is only Windows. [nimx](https://github.com/yglukhov/nimx) lacks proper documentation. I even considered using [Raylib (raygui)](https://github.com/Guevara-chan/Raylib-Forever), but I never tested it.
