Static menu bug (glitch)

There is a bug/glitch when static menu is selected.
Here is the website:
http://studenabrac.com/
You can see when you scroll to down, and when js add .fixed class on .site-header there is a jump.
How to fix this?

Thanks!

Hello,

I’ve just checked your website but didn’t notice the issue that you’ve described above. Can you please provide a link to affected page?

Kind Regards, Roman.