Thank you for sharing the details with us, I checked your cart in multiple browsers and it scrolls just fine with multiple items. In order to see the scrollbar you'll need to hover in the cart box (see screenshot)
Just move the mouse cursor in the cart box and you'll see the scrollbar. I tried it in different browsers and couldn't replicate the issue you mentioned. Can you please let us know how to reproduce it or in which browser you're having this issue, if you can be more specific regarding the problem, it'd help us in finding the solution.