Can we have a root cause analysis for this?

This uses $.client.test( blacklist, null, true ), and has that changed in 1.21? Should that be backported to 1.20, or even 1.19?

I think this is the cause:

Along the way, a 3rd parameter was added to "test":

@param {boolean} [exactMatchOnly=false] Only return true if the browser is matched, otherwise

As default is false, and this parameter was not present, we can have different behaviour. The question is what exactly goes wrong on MediaWiki <1.21 for which browser(s)? Our blacklist is IE7 and below ('msie': [['<=', 7]]).

Change 80339 had a related patch set uploaded by Amire80:
Use the old way of checking browser support for MW < 1.22

https://gerrit.wikimedia.org/r/80339

Change 80339 merged by jenkins-bot:
Use the old way of checking browser support for MW < 1.22

https://gerrit.wikimedia.org/r/80339