It's perhaps not as custom as I made it sound. The code we have is a wrapper around the [virtual-dom](https://github.com/Matt-Esch/virtual-dom) library which does most of the low level work.

The "widget" framework is a series of simple ES6 classes that allow us to keep state around and delegate events as actions up to Ember controllers. We had a lot of pre-existing, battle tested code that was written using Ember idioms and was not a performance problem, so we kept that around and used the virtual dom stuff just for rendering HTML as fast as possible.

Why not React? Our main pain point was rendering, and we didn't need all the extra stuff React provided as those bases were already covered by Ember. Why include 26k min+gzip when we would only use a small part of it that was already provided by another excellent virtual-dom library?