As I said before, I viewed your website home page on my iMac in Safari, Chrome and Firefox and I had no problems scrolling your home page back up after I had scrolled to the foot of the page. Also, we have more than 5,000 customers for this theme, and no one else is reporting this problem. This is why I think the problem appears to be specific to your setup or usage.
Setting the Smooth Scrolling option to the blank option disables that functionality, and if that fixes your problem then I would hazard a guess that the JavaScript that adds this functionality is conflicting with some third-party JavaScript resulting in a JavaScript error that causes the problem you describe. If disabling this functionality fixes the problem then you’ve found a solution.
We’ll keep an eye on this and if we get more reports then we’ll look into it further.
Thanks.