I'd never heard of it so I did some quick googling.

From https://github.com/JetBrains/ring-ui:

> This collection of UI components aims to provide all the necessary building blocks for web-based products built inside JetBrains, as well as third-party plugins developed for JetBrains' products.

From https://blog.jetbrains.com/blog/2018/09/25/ring-ui-1-0-is-re...:

> At JetBrains, we use Ring UI components for our web-based products like YouTrack, Hub, TeamCity, and Upsource.