I have discovered that Magento's modal-popup's do not scroll on safari.
You can see this is you log into a Magento 2 demo store with a user which has a saved shipping address. Go to the checkout and click on the New Address button.
You will notice that eventhough the .modal-popup has a CSS rule for
overflow-y: auto it does not scroll.
I've search online and this seems to be a common issue on Safari, but none of the solutions I have read of work for Magetno.