I have this web-app that has mostly web pages with search results (such as google is), so there's scrolling normally. But I have this scenario that you can open an overlay that shows additional content. The idea is that the content is complementary to the one shown by default, and showing the new content by navigating to a new page could break the flow of exploring the other search results.
Initially I made the overlay so it's always in a fixed position, occupying about 90% of the screen's height. If the content overflows, scrollbars are added inside the overlay (behaviour similar for a popup window). The content sitting beneath the overlay cannot be scrolled.
I want to add that the rest of the page is behind a transparent black curtain, so one cannot really use the content beneath directly without closing the overlay.
Now I received feedback from a user that this is counter intuitive and the user would expect the whole page to move when scrolling.
So I'm asking myself, and all of you UX experts, can you guide me in what makes most sense:
1) When the overlay opens, scrolling should be done only inside the overlay and the rest of the page should not move until you close the overlay.
2) When the overlay opens, the content in the overlay expands as much as it needs to, making the scroll action perform on the whole page, including the content beneath (that is shadowed by the curtain)
I hope I made my question clear enough. If not, let me know and I'll post some sketches.
Update: As I promised and you guys requested it, here are some sketches that hopefully illustrate my question: