If you are struggling to find the cause for Safari not allowing scrolling on a modal, check if you're using a CSS animation. For example I was using

Copycopy code to clipboard
1animation: drop .5s;

I was struggling for a little while over why a modal that popped in using CSS animations would not scroll in Safari (but would briefly be scrollable before the first .5 seconds I noticed).

Apparently its a Safari behavior (bug?) that causes content to be un-scrollable if the animation property applied to it has run. Removing this property fixed the issue with scrolling in the modal. This does not impact Chrome or Firefox.