As part of the effort to move WikibaseClient to extension registration, we stopped loading $wgWBClientSettings in the PHP entry point (WikibaseClient.php) – instead, the default settings are loaded (and merged with the custom settings) at a later time, in the WikibaseSettings class.

However, we forgot about the Settings class (\Wikibase\Lib\Settings), which is a kind of duplicate settings system, separate from WikibaseSettings (deprecated a while ago, two remaining users). This class still directly accesses the $wgWBClientSettings global, so now it’s missing all the default settings.

The proper solution for that, we believe, is to move the remaining two users of this class to WikibaseSettings, and then to get rid of the class. (Note that for the parent task, to unblock the train, we’re going with the simpler solution of reverting the change that stopped loading $wgWBClientSettings in WikibaseClient.php.)