[mindviews]

While lacking in lots of built-in UI widgets, I use Cinder like a UI library. https://libcinder.org/ Perhaps "graphics library" would be a better descriptor and explains why I didn't find it on the list. But with mouse/touch, images, audio, video, etc. support, "graphics library" seems too narrow of a description. Maybe it's better to think of as a GUI library without a lot of commitments to the usual set of GUI widgets.

[pier25]

I used Cinder on a couple of projects and it's great for what it is. It would be a solid start for making your own UI library, but that is a considerable task.

[TehCorwiz]

Cinder looks neat. I'm curious though, the only packages it offers are for Windows or OS X. Do you know if it supports Linux at all?

[mindviews]

I've used it on Linux (Raspberry Pi for my case) just fine. Guides here if you need it: https://www.libcinder.org/docs/guides/linux-notes/index.html

[daeken]

It does, yeah. They don't distribute pre-built linux packages, but most distros do.