Css issue when loading the site

Here is my site https://www.what-really-counts.com/.
When loading there is a video which loads properly but the problem is it shows extra white space at bottom when scrolling in mobile devices . The problem only shows up in mobile devices. Can someone help me identify the problem.

i don’t see a mark in the pic you sent, or in the site, unless it is on the orange loading screen that goes diagonal from the top right to bottom left?