Has anybody made an attempt at making a library like this, but with cross platform user input, and support for accessibility? From personal experience, if you can output triangles and text, writing a UI library like this is maybe 2-3 days of work. The fun starts when you consider that younger people are touch-first.
The accessibility is the hardest part. My custom library (in C# atop a weird rendering stack) has partial narration support and full touch/gamepad/mouse/keyboard navigation, but getting all the way to integrating with native screen readers is basically impossible at this point - from investigating it, it'd probably take me at least 3 months to get it working at all, and it wouldn't be portable.
One thing you have to do for reasonable accessibility is maintain a retained model behind the scenes even if you have an immediate mode API, so that's what I did. The immediate mode API does a bunch of caching in order to construct and maintain an appropriate retained mode tree across frames, which makes it possible to cleanly handle things like focus, selection and narration for invisible controls, etc. You also have to bake accessibility into the API from the start, for example by making certain every single widget has a description or a reasonable approximation of one, and by making sure there is an approximation of roles for every widget too.
A simple 'read a text description of the focused/clicked control' also doesn't get you far enough for narration - for example if there's a slider or textarea, you don't want to read the description and then the new value every time it changes, your narration has to be 'smart' and know to only read the description initially.
Have you seen libAgar (https://libagar.org)? Cross platform support is certainly there, covering everything from windows XP (and earlier) to *bsd and SGI IRIX. I'm not sure what all having support for accessibility requires as I've never had to worry about it, but am curious if 1, agar has what's needed, and 2, what exactly is required of a GUI library for accessibility. Screen reader support? (Are there SR standards for desktop applications)? Dynamic scaling? High contrast?
(For embedded and/or touch first UI, LVGL is pretty nice, but probably lacking any semblance of accessibility features apart from keyboard navigation, but you could hook that yourself).
