Unable to scroll page with an overlay

Hi,

I have a prototype that has an artboard page 2000px high.  An overlay is used the same size as the page. 

Whilst viewing a prototype if I add an overlay then and then scroll the page, the overlay scrolls over the page itself... the page below stays still.. it looks rubbish.

... how do you get the overlay and page scroll together?

I've seen similar posts and quations but a while ago so thinking this is fixed and i'm setting it up incorrectly?

Thanks

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