The 'Welcome Back' banner isn't centered:
Here's another example on Physics.SE:
I'm currently on Chrome v64.0.3282.140, but I reproduced in Safari 11:
Fixed in the next build, thank you.
As part of our "make our CSS slightly less horrible" project, we finally switched to
box-sizing: border-box as the default (even the CSSWG these days thinks that should have been the default from the start). This notification bar is one of the places we missed where this requires a change.