In 2020, I started working on a (C++) game engine. Since the only decent open-source UI option was Dear ImGui (which was obviously a bad choice for consumer-facing UIs), I ended up rolling my own retained-mode UI library on top of SDL. Now, it's fully-featured enough that I rarely have to touch it. There's even a major company using it for embedded products.
I don't get why every language's community doesn't just do the same thing: roll an idiomatic UI lib on top of SDL. It was tough, but I was able to do it as a single person (who was also building an entire game engine at the same time) over the course of a couple years.
How's the accessibility?
I haven't worked on screen reader support, yet. Support for alternative text input is built into SDL. UI size scaling is a feature I plan on adding eventually.
For most serious applications, accessibility isn't a second thought, it's a requirement and it's very hard to implement correctly.
So the solution is to build applications around less of a common base? I don't follow the logic, with respect to Zed. I get what you mean if there's a first-party UI solution in your language (e.g. Swift), but in that case you don't need an open-source UI library.
The solution, if you want a production ready GUI, is to use a GUI toolkit which already has decent accessibility support.
There aren't that many of those: .NET, AppKit/UIKit, SwiftUI, Qt, GTK, the web, wxWidgets (which is really just GTK/AppKit/.NET), probably a couple others. So you either use the native language of one of those toolkits, or you use bindings from your language to those toolkits.
I'm not trying to troll here, asking seriously: are there any immediate-mode GUI APIs with good a11y?
SDL is not perfect, e.g. you can't get pinch/zoom events on MacOS. IMO, using the OS APIs yourself is better.