Popups has browser tests. They run in mwext-mw-selenium-jessie e.g. https://integration.wikimedia.org/ci/job/mwext-mw-selenium-jessie/6239/console
These were passing despite the fact the code we had written was not compatible with jQuery 2. It seems the environment they run in enables $wgUsejQueryThree (per https://phabricator.wikimedia.org/rMWbf69459d5011f7970c159ae752ac399df174ccb3)
When disabling this, interestingly, despite failing locally it can be seen that our browser tests still pass - so the enablement must be happening somewhere else.
Given the beta cluster also enables $wgUsejQueryThree and we do locally, we failed to notice we'd introduced code that would cause an UBN (T174724) until the train had finished rolling out ( a Friday).
This seems like a big lesson learned for us and a reminder that the beta cluster or some other environment is expected to reliably mimic the code we run on production for the benefit of running these integration tests....
How can we prevent this from happening again?
Why are the browser tests passing when they should not be? Is our job not working correctly?