In my app, I have a group you can scroll down through, and its inside another group that just has a background image set. The problem is, when the user scrolls down the main group… it doesn’t look like you’re scrolling the page, it stays there (like first image) and can stay that way as you keep scrolling in the group. You’d have to scroll from the background group to scroll the entire page/have your mouse over the white area.
So, is there a way to detect when an user scrolls to a certain point of a child group to then scroll the entire page downward? I can’t just use “when current scroll position is x” because it will/can always be 0 even if the user keeps scrolling down inside the other group like in the first image which is the initial stae. Ideas? 
Or tips?
Imagine you open an app where initially there’s a half-shown screen, and it becomes full page-height as you scroll down, going to the top of your page more the more you scroll down. Ideally vice versa, when you scroll up to the same spot it goes back to its half-shown way, kind of like it’s peaking out into view like this:
