Sticky Header Problem

Hi,

on my site i want to use a sticky header - with a logo left and the menu right.

For the sticky header function i found a code from you on this board:

#masthead {
    position: -webkit-sticky;
    position: sticky;
    top: 0;
}

So far so good. The only problem i see is, that sometimes when you scroll down, that content from the site or the sidebar is laying over the sticky header - my url: shorturl.at/isCIT

When you make the site a little bit smaller, then you see that the links from the sidebar for example are laying over the sticky header.

How can i fix this?

Greets