We started out with pjax (which is awesome) but ended up rolling our own framework called Stacker which lets us have complete control over the UI.
Any plans to open source Stacker?
Not for the time being. Stacker is specifically developed for this unique UI style that we're using for Next. We don't open source our UI elements.
You know who did open source their UI elements? Twitter, with Bootstrap. And you know how awesome that was for everyone? Pretty awesome!
While I agree that Bootstrap is awesome (I use it myself), it's worth pointing out that what Twitter open sourced was not the magic of their core product, but rather the templates that they use to create internal applications. If you look at twitter.com, you don't see any of the same elements that are included in Bootstrap.
Twitter open sourcing bootstrap just made the whole web better. Open source projects especially. Projects like https://github.com/dcramer/sentry benefitted a lot from bootstrap.
Making UI open source helps everyone.
It's probably not that hard to reverse engineer what they are doing with Stacker.