Maybe you need to add an "(in Rust)" to all these sentences? Sure there are C++ frameworks, but they probably wanted a pure Rust UI framework?

My 2 cents: it's nice to have smooth rendering in your editor, but I'm currently mostly using a Java-based IDE (IDEA family), and it's responsive enough for my taste. If I were to use the current prototype of their editor, I'm afraid the usage of fixed-width fonts all over the place (which I assume can be fixed, but may also be due to constraints of the UI framework?) would probably bother me more than the 120 FPS UI would impress me (assuming that I had a monitor that was fast enough, which I don't). On the plus side, it sounds like they support sub-pixel rendering - if that's the case, kudos to them!