Looks like winit: https://docs.rs/winit/0.20.0-alpha3/winit/

This is based on the cargo manifest: https://github.com/cloudhead/rx/blob/master/Cargo.toml

If you're interested in this space, there's also a lot of effort going into druid: https://github.com/xi-editor/druid

Edit: actually just see this sibling comment: https://news.ycombinator.com/item?id=21115680

None is the answer: the context management (window, input etc.) is done by glfw, but the UI is very minimal and doesn't use any UI toolkit.