Here's my feedback as someone coming from web development who desperately needs this:
As far as I can tell, nesting only works when the inside and outside of the container are both "size container to fit contents" as in the demo. Everything else I've done throws null errors. Documentation on nesting would be great.
As far as I can tell, flexible nested items just shrink GOs to zero no matter what the size or behaviors of their parent is. Seems like a bug to me.
I would really love to see an demo with an expanding container in a scrollable. I think 99% of responsive UIs need to dynamically display reactive series of components inside of a scrolled window.
If you're able to get these things working and documented, I'd leave a glowing review.