Thank you. It's just CSS and the :hover pseudo-selector, the popover effect is done using CSS keyframe animations.
Fun geek fact: The Mac OS style close/minimize/expand buttons on the code editor are written in CSS, they're not images. A bit of a pointless exercise but a fun little easter egg.
My initial run-through of it leaves me thinking it's absolutely brilliant, which isn't generally a word I use lightly.
So long as non-Classy controllers are still operable (which I assume they are), then I should be able to port individual controllers one at a time without disturbing un-ported controllers, then this is exactly what Angular was needing, IMO.