@jbpasquier does this ring a bell? Alex gave me the workaround to fix the problem so I guess it already occured? Searching for it didn't gave me any issue though.
@sylvain even if you type @all it is suggesting you @_all. So it's a bit ugly but it works. When we saw the bug in that particular channel typing both @all or @_all didn't didn't show you anything... at all (if I may say).
From what I remember, the _ is to prevent the case where someone have all as username. Anyway, it's a little change on prosody's module to call it whatever we want.
In many chats like slack, you can mention specifically a group of users like:
"@here": Only online user of the channel
"@everyone": Online and offline users
"@channel": Every users of the channel, same as everyone
Some chats also offers "@groupName", with which you can mention users in a particular group
In our case, we stated to provide only the "@all" for now who works as the "@everyone". The "_" also ensure that it's always on top of the list even if there are an "alex" on the list. :)
In all platform I've tested, you've no username restriction, but the "all" or the "everyone" is not displayed the same way as an user.
On slack, you can even mention external users too.
For technical side, Prosody provide the list of each users to converse, we added a fake one named "_all", we also intercept on prosody mentions with this "_all" to send a notification to every users instead of only one. From converse side, it works like every other users.
You can on slack, it works fine, as the "all" is not treated as a user. If you want this kind of behavior on our system, that mean that we need to handle it on converse side instead of prosody. But as converse has no knowledge of who is linked with which jabberID, it'll have to ask prodosy for all of them - we will need a custom process all long as XEP-0372: References is made to mention only one user and not a set of users.
Notice that slack order is more a personal one, you'll always have "Person on this channel, ordered by last mention you did" then other persons and channel/here mention, with the same order - the last mention/talk you did/had.
I guess it's up to @rachel to decide what the right ux is, and how to deal with the priorities. I was just trying to point out that other systems seem more user friendly to me :)