That looks pretty interesting. Even though I usually like native-widget-type GUI libraries, this one looks pretty special. Also I like that all rendering is actually done with low level GPU calls and doesn't depend on other common toolkits like SDL.

If it is flexible enough to support non-obvious not builtin functionality will show.

It uses quite a lot of cpu cycles in idle though. That seems to be a huge drawback of immediate mode GUIs. At least for mostly static interfaces.