I just upgraded from 12.0.1 to 12.0.3RC2 and it broke the header bar.
Analysing the HTML I noticed that the header changed from an ID in <=12.0.2 :
<div id="header-left">
to a class in 12.0.3RC2 :
<div class="header-left">
Adding back the id="left-header" in core/templates/layout.user.php:46 fixes the issue.