I haven't found good platform accessibility API bindings for C#, so right now it just has narration support, adjustable text size/contrast, and full keyboard navigation. I'm hoping that later on in my project's development process I'll have the budget to hire someone to try and fully integrate with the platform APIs - it's designed so it will be possible by pushing updates to the retained model.
The way it works is that it has an immediate mode API (see https://github.com/sq/Libraries/blob/0ca01d949e3df5fabb1440d... for a simple example of the API) and then under the hood it uses a lightweight retained-mode graph, which means that when it's more convenient you can just write more traditional classes and components. In practice most of the UI I've written for my main project is a mix of both models, like for example an editor popup window uses IMGUI-style API while the items in a virtual listbox are represented by a small custom component.
It does have a full imgui-style layout engine under the hood instead of doing retained-mode layout, so it's able to fully re-generate layout from scratch every frame, and to minimize page tearing there is a system where components can request a second relayout pass (typically used for things like text autosize).
Here's a more complex mixed-mode example from one of my development tools: https://gist.github.com/kg/6a6ba42d5019b546858a2b18751de019
Almost all of the on-screen elements in this footage are either immediate-mode or retained-mode UI using the library: https://www.youtube.com/watch?v=ey3FtFWxbhA
