full page overlay menu tablet portrait view – scrolling page in background

I’m using a full page overlay menu on https://dtpc.wpenginepowered.com/ (login in private) and it seems like there’s an issue in tablet/portrait view where, if I click/view the overlay menu in that view and then scroll down, the close “X” and search icon scroll up and out of view and the scrolling continues for the full height of the page in the background (you can see the page scrolling in the background, and the scrolling stops when the bottom of the page is reached). Here’s a little screen recording, to illustrate: https://capture.dropbox.com/i5YNiMmlA8GqX14B — I hunted around in the css to see if i could find what’s causing that, but it’s not coming easy — can anyone offer a clue for me? Thanks!