Safari position:sticky not working in an overflow:auto element

simply add position: -webkit-sticky;

I got this solution from someone else:

http://cssdeck.com/labs/bu0nx69w

Basically, instead of position:sticky, use position:fixed for the right panel. The key is to also you will-change:transform in a parent div (in the above example, in .modal-content) so position:fixed becomes fixed relative to that parent, and not the viewport. It's a neat little trick

Adding display: block to the .sticky-element worked for me without having to add position: -webkit-sticky. Found this solution at this Codepen.