- Go to wiki A where you have no messages (or create a new account)
- The messages badge is not shown; this is expected
- Cause yourself to have a message (e.g. user talk edit) on wiki B
- The messages badge should be shown on wiki A, but isn't
- Cause yourself to have a message on wiki A
- The messages badge is now shown on A, and shows both the local message (from A) and the foreign message (from B)
In step 4, the messages badge should have been shown already. It appears to be hidden because there are no messages, without taking foreign messages into account.