Quick hack for Ublock Origin users to get everything lined up again (add this to your filters): github.com#$#body{ max-width: 1280px !important; margin-right: auto !important; margin-left: auto !important; }
Did GitHub only test their new layout on 4/3 screens? It looks so odd when the project header is fluid but the project itself is still centered and 1280px wide. Especially if you log out of GitHub, in which case the GitHub banner is no longer fluid and makes the project header look really out of place[1].