How to make the orginial website background of a navigation menu fixed and 100% wide?

I have a responsive navigation menu and when you open it on a smartphone or tablet, the original background of the website gets fixed to prevent the user from scrolling:

.cover-bg {
    position: fixed;
}

But this also results to the scrolling bar to be hidden and the whole background moves to the left. This is not wanted and really weird, how can I fix this?

Heres my website, open it on a smartphone and click on the menu. (antonrave.de)