Steps to reproduce:

1. open https://meta.wikimedia.org/ in a fresh browser (incoginto mode, cookie purge etc)
2. log in
3. visit https://mediawiki.org/

After a second or so, CentralAuth autologin kicks in and logs you in without reloading the page; the user toolbar is replaced via AJAX. (mediawiki/extensions/CentralAuth/modules/ext.centralauth.centralautologin.js fires the request, after some back and forth it returns CentralAuth's modules/inline/autologin.js which, when executed, calls to Special:CentralAutoLogin/toolslist and replaces the tool list with that.) The accompanying CSS is not loaded though; there is just 0 0 whithout the icons or the background color.