Today, the server emits a system message containing one of `<username> left the channel.` or `<username> removed from the channel.` when such an event occurs.
While the client interprets this special kind of message to take into account the enduser's username display settings (e.g. to render `First Last left the channel.`), the server also processes this "message" to look for mentions.
With the user in question no longer in the channel, it incorrectly splits usernames containing a period into two tokens and tries to mentions users accordingly. So `christopher.poile` ends up trying to mention `christopher` and `poile`. If such a user exists, they receive a spurious mention for this event.
Change the message rendered by the server to explicitly prepend an `@`, which in turn is detected by the mention logic to avoid splitting. As above, this has no effect on the client-side rendered message.
4f7bd2200d