Yeah it's not an impossibly complex task, which is why I'm working on it, one step at a time. But it's much more involved than it sounds at first.

Of course a simple "widget" library is relatively easy to make, provided you already have powerful drawing and text functions. Then you realize, integrating text entry and copy paste consistent with the operating system gets much more tricky if you're making all your own widgets from scratch. More complex rich markup is even harder though... getting text to flow around correctly while achieving the layout you want at the same time. I think CSS does a fairly good job here, but I think we can do better.

A lot of the work before you even get to this stage though lies in good cross platform drawing and text rendering. OpenGL support is a 'must', of course. Then, 2D and vector graphics need to be implemented. OpenVG is supposed to provide this, but is useless as a cross platform solution, because it's only supported on a handful of mobile only systems. So I'd either have to implement all of OpenVG through OpenGL (and I certainly don't look forward to implementing the mess that is SVG) to get HW acceleration, or make my own VG library.

But that doesn't even solve text rendering. Text rendering western fonts is relatively easy, but it gets much more complex when you need to support unicode cursive fonts like Arabic with HW acceleration consistently. Fortunately the actual shape generation is already solved in the GNU world, but caching fonts efficiently on the GPU and with smooth antialiasing is another matter. It would be nice to just use the operating systems text renderer, but not only would the results be inconsistent across platforms, but I don't think it's possible to do this on an OpenGL framebuffer on all platforms.

So once I have HW cached text rendering, and vector graphics, only then can I even start thinking about building a widget library on top of it.

So it's much more complex than it initially sounds when you have to essentially reinvent the entirety of what the OS GUI API should provide. And if you can't settle for lowest-common-denominator functionality provided by all OSes, then reinventing the wheel is your only remaining option.

I know how to do all this, it's just a lot of work. If I was working on it full time it would go fairly quickly, but it doesn't seem to be a very profitable task so that's not feasible right now.