This is due to a change in YUI's event system where it's no longer automatically stopping the event from going into an infinite loop.
This is actually due to the change in Bootstrap 3 in the box-sizing to border-box. However, the problem is a from a couple of things:
1. Our height/width methods are subtracting the border width from the box, even though box-sizing is set to border-box.
2. The autosize module is doing a get/set from inside of an after listener. This wasn't breaking before because it was always setting maxHeight to the same value, so it never called the after listener
The fix, of course, is the one I sent up (which is to not fire the _syncHeight method if event.src == 'ui'), but we should also look into fixing our width/height methods as well to account for border-box being set.