I've been writing a cross-platform UI toolkit, currently macOS, Windows, and Linux. Eventually I'd like to compile for wasm/Canvas, android, and iOS. C++ was sort of designed for this kind of thing, it is performant, and it interfaces with the OS well. Eventually I'd like to have at least Python, Swift, and Rust bindings, because I really like using Swift, but none of the UI parts are available outside macOS.
It's probably not ready for use yet, but give it six months or so: https://github.com/eightbrains/uitk