Hi,
Normally, when you have a dropdown using the "select" element, the browser will replace it with its own dropdown system which works properly and allows you to scroll it.
So I'm not sure why it wouldn't work for you. Maybe you have the chosen library activated (either in the settings of HikaShop, or via your template) which replaces the dropdown select element by a HTML/javascript element and it doesn't work properly on safari ?
I would need to check the situation on the page to confirm that theory but you didn't give a link to the page so I'm not sure.