Also with your custom UI framework, you will need to re-implement a text editor from scratch which is a pretty daunting task by the way [0]. Do you also have plans to open source the text editing / make text editing a part of the framework? Text editing (right there with accessibility) is the core reason why one would not go with a pure immediate mode GUI for any app. I guess you are baking immediate mode with some extras?
Rust on Mac with an open source UI - wow! If nothing else, you guys are ambitious as heck. Can't wait to see more. Also, loved the youtube video you linked to earlier in the comments - great to see an actual working demo instead of just an announcement.
It's been a few years since I used Flutter, but doesn't it still require you to use the related language Dart? I liked the idea of it, and I thought the tight integration between Flutter and Dart allowed them to do some cool things. But I wasn't a huge fan of the language itself. And I think it's a lot to ask prospective extension-writers to learn a new language.
Rust actually has a library with very good flutter integration, it does require using dart for the actual flutter part though, but it works well together.