When you press the button, does it go up somewhere?

Question

Layout

When you press the button, it goes somewhere up (disappears from sight). I thought that because of the hidden-sm-down class, I removed it, but it did not help. I use Bootstrap 4.
hobotrobots.devshift.co.uk

Answer 1

Hardcoded .navbar height to 75px. Therefore, the block that appears from below pushes the rest of the content to the top, where it is not visible.