The overflow:hidden is what stops the page being scrolled.
Why do you need it? What is its purpose.
Also if the page is only height:100% then theoretically there is nothing to scroll as its height will be limited to the viewport height. You can’t have it both ways.
If something has been made viewport height (effectively 100% height in your use case) then what happens when the content is taller than the viewport (such as when you reveal your item). ?
The portion outside of the viewport is basically ignored and background will not follow around the element. If you say something is xx tall then you can’t put something bigger inside it.