One-page-site / problem with left side main menu Firefox & Chrome

Hello

building up Enfold one-page-site. It’s working fine otherwice but on laptop there’s a problem with left side main menu on Firefox and Chrome. Safari is fine.

Main menu is on left side and it’s supposed be still when scrolling the content on right. That’s ok on desktop with all browsers (Safari, Firefox, Chrome). The problem comes out with laptop (only with Firefox & Chrome). Left side main menu disappears when scrolling the right side content (or if you click to some of the main menu links and the right side content runs lower).

Any help for this?

Thank’s,
Saija