Hmm, is it possible to share an URL? It's hard to figure this out just by a screenshot :(
What I tend to do in these cases is to comment out all the CSS (yes ALL of it) and then turn on a couple of classes to see what happens and repeat that until you see the white space again. Then you've narrowed it down a bit. It's bug hunting, that's for sure :(
Oh and check the HTML for open tags. Most browsers are pretty relaxed about it, but that varies per browser. Maybe Safari 9.1 is more strict?