In last years a lot projects have appeared, which were trying to compete in GUI toolkit market share. All of them had its scope limited, however they brought quite fresh look on declarative programming interface [0][1], superb 2D rendering pipeline [2][3], ease of embedding [4] and adaption to low-power devices [5] but none of them is "complete".
Personally, I am amazed of the implementation of rxi/lite [6], which is just an app (C with Lua) based on SDL2 library. It uses 2x less RAM than sublime on 1GB file, has lower latency, covers clipboard, keyboard, mouse, drag and drop, some context menus but does it feel native? No. Among many issues, it is easy to report an error with screen-reading software but it is quite common problem in all aforementioned.
Perhaps in future, something like LSP specification will see a daylight, which in outcome will helps us to provide universal, modular, limited to certain scope libraries that can be easily plugged in case of need. Maybe then we will came closer to "native" GUIs. In my experience, there are numerous developers who were trying hard to deliver "complete" GUI toolkit but have failed.
[0]: https://github.com/cycfi/elements
[1]: https://flutter.dev/
[2]: https://github.com/linebender/druid
[3]: https://github.com/servo/pathfinder
[4]: https://github.com/ocornut/imgui