Nevermind - found the issue. A block of JS setup to find out if Safari was miscalculating the Viewport Height stopped working - since Apple apparently fixed the calculation, but busted the view heights unit on the div height. I was setting
on a parent element. I know what some of you are thinking - why not
Well, I had a bit of js sniffing the height of the element. Safari's viewport height bug causes the element to calculate out higher than the body, which is impossible. Upon detecting this, I remove a class that enable viewport units entirely.
Now, Apple apparently fixed the height bug, but doesn't render backgrounds on elements with a vh unit height (or it's miscalculating the height as 0). Either way, I was forced to just turn off view units entirely with Safari using a specific browser sniff. Which is a technique I find distasteful.
It's one thing to not implement a CSS feature - quite another to implement it wrong, which Apple has been doing a lot of here recently.