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.
[0] https://www.construct.net/en/blogs/ashleys-blog-2/simple-sof...