This is a visual guidance to connect pathbar with search button when the window is “large enough”.
This actually comes from Adwaita, but it’s a fair solution (I think) to the fact that pathbar and search button move together and, without a visual connection, they’d look floating in the headerbar without reason
Ofc I understand also that it looks like an entry and might misguide the user, it’s hard to find a solution for both problems, but we’re open to suggestions
May the 4th be with you 
I don’t think we’ll propose a fourth version, in case we can tweak Yaru-light one (I actually love Adwaita headerbar color)
Yaru dark shell it’s actually there already (even if it needs some work to make it polished as it was at the beginning), but gnome-shell does not allow for light/dark selection