This happens semi-frequently to me and others - we reply to a user who
has left the room. Sometimes this is useful (for example, bringing up a
previous topic) but in most cases this is accidental, and almost
guaranteed to happen if you turned off join/leave events.
So I added a reminder when replying - which manifests as a small label
in the chatbar - that the user has left and can't be notified of your
reply.
We can un-clutter our message context menu, which we had to share
with user actions. (Even though we only had one so far.) I added one new
user-specific action which allows you to quickly mention the user in
chat. Otherwise you would've had to copy their username or use the
completion menu.
It's convergent on mobile, it still has the hover indicator and it also
is available through the AuthorComponent.
BUG: 486252
