Yep, that is correct. This is appear when you open user menu on narrow desktop view. This line isn’t visible when the user menu cover it.
It’s add a line below user avatar.
It can be removed with using media queries. The slide in menu activates under 768px.
This is the desktop view.
You can access the mobile view on desktop with append ?mobile_view=1 to the url.
To switch back to desktop view ?mobile_view=0 or in the sidebar footer desktop icon.