[Users] new-mail notifications on message open

[Pierre Fortin]

I've noticed some new (New vs Unread) messages (already filtered to
folders) trigger the new-mail sound when opened.

If I get multiple copies of a message (as I do on devel list -- leftover
from previous testing), it does not matter what order the copies arrive;
they are all filtered into my claws-mail-dev folder.  I don't know if they
trigger the new-mail sound when they arrive; but they do while still
marked as "new" and only if they are opened 1st or 2nd, never 3rd.

NS = Notification Sound
ie, messages arrive as A1 A2 A3 B2 B1 B3 C3 C2 C1
Opened in order:       NS NS    NS NS    NS NS
   in reverse order:      NS NS    NS NS    NS NS

Opened in this order:  A3 B1 C2 B2 C3 C1 A1 A2 B3
                       NS NS NS NS NS    NS      
  
The first 2 copies opened give:
[11:47:21] notification_core.c:440:Notification Plugin: Found msg 20181104115538.389dc8f8 at sleipner.datanom.net, checking if it is in hash...
[11:47:21] notification_core.c:449:no, added to table.
[11:47:21] notification_core.c:452:notification_new_unnotified_do_msg
[11:47:21] notification_core.c:472:notification_new_unnotified_do_msg
while the 3rd gives:
[11:47:29] notification_core.c:440:Notification Plugin: Found msg 20181104130414.47e84bd8 at sleipner.datanom.net, checking if it is in hash...
[11:47:29] notification_core.c:444:yes.

If I exit the folder and return before reading all the messages, they are
now "unread" and do not trigger a notification sound.

Is this a bug, or undocumented feature?

Pierre