This is because of a typo in rMWe08e9609ffa0: OutputPage: Only export wgUserNewMsgRevisionId if non-null, which tries to export $$userNewMsgRevId instead of $userNewMsgRevId. This causes an undefined variable notice, which we don't seem to capture in logstash in production.
|Resolved||Release||hashar||T233872 1.35.0-wmf.24 deployment blockers|
Should be fixed now, though I was unable to produce a test case where that JS variable would be non-null during SWAT (I tried leaving myself a talk page message from another account but it didn’t seem to have any effect). Please verify :)