Mobile Menu problem with zoom in of boxed layout page

Hi guys,
first of all: I’m loving your theme and I work with him now since years.

This is the website where i current work on:
http://isabeldoerfler.de/

The problem is here the mobile menu.
If I open the webseite for e.g. with my iphone and I zoom-in the menu above is no never visible?
Any chance to change that?

I need to have a boxed layout because of the background image.

Thanks for your quick help.

Cheers,
Karo