User blog comment:Kirkburn/Technical Update: October 14, 2015/@comment-24531999-20151016011938/@comment-11733175-20151016150124

I suspect you're referring to a hack of some description, that's not officially supported. Could you clarify what you mean by "hidden class"? If it's a legitimate thing, I'd love to use it myself.

Edit: perhaps you mean https://github.com/Wikia/mercury/blob/dev/front/styles/main/state/_hidden.scss (which I was unaware of until now)?