first off i thought you were talking about the chrome icon lol . I’m on ubuntu 17.10 and gnome 3.26.2 and its on settings, files, terminal, software, cheese and probably more apps. I dont know what its called. I reinstalled from 18.04 but the issue was still there when i left 18.04.
Its in tweak tool "attach Modal Dialogs . I turn it of and logged out . No change.
I just checked adwaita theme and its fine no issue but adwaita dark there is a line from top to bottom.
Would the header bar on adwaita be slightly wider ?
I have figured out that the step is part of the dock . On auto hide i can see it slide in and out with the dock.