Overflow-x:hidden doesn’t prevent content from overflowing in mobile browsers

I have a website here.

Viewed in a desktop browser, the black menu bar properly extends only to edge of the window, since the body has overflow-x:hidden.

In any mobile browser, whether Android or iOS, the black menu bar displays its full width, which brings whitespace on the right of the page. As far as I can tell, this whitespace isn’t even a part of the html or body tags.

Even if I set the viewport to a specific width in the <head>:

<meta name="viewport" content="width=1100, initial-scale=1">

The site expands to the 1100px but still has the whitespace beyond the 1100.

What am I missing? How do I keep the viewport to 1100 and cut off the overflow?