use this css!
i think the issue with your overlay is it spans the whole width of your page, you cant see it though cause the text inside of the overlay is 50% witdth and the overlay doesn't have a background color
also the overlay has a pretty big 20% frame bottom which further takes up more space where we cant scroll the actual page
the setting that enables scrolling of the actual page only works when the mouse is not on top of the overlay. this is how the browser itself works i think!
and the overlay in your case takes up a lot of space.
even though the space of the text is smaller.
so instead of using the css you could also set the overlay width to 50%.
and inside of the overlay page in its gridder, resize the text box to 100% width. and set frame bottom to 0.
but using the css above fixes the whole issue
anyways sorry for the really late response, this post somehow got lost :O