I don't have experience with Flutter, but based on quick glance they're using widgeting and, what I found quite important - ability to develop GUI outside of the application. Something that I think libraries like egui are missing (and which is easily obtainable with Tauri/Dioxus).

Rebuilding whole app to ensure that some box doesn't get cut off ruins development experience, especially for big apps.

Kudos to you guys, I hope you'll make Zed extensible, so that instead of writing my own editor I can use yours ;-)