Why does navbar-fixed-top overflow content?

Question

css

Good afternoon!
The situation is this - there is navbar-fixed-top. When resizing or opening the site on mobile, for example at 320px, navbar-fixed-top shows a width of 420px. Overflow:hidden has already set everything possible.
I really need help.

Answer 1

Remove all blocks through DevTools, except for <nav>and you will see what's the matter.