Hi @sherlyntandinata, thanks for your reply. If you take a close look at the original example site you shared, the effect is on a div having relative position.
That Example is also a single page example, not a multi-section example, so I would give your overlay a position relative and set the overflow to scroll:
See if that helps, if not, you may have to contact to the script maker for what is the best structure to use for that script.