I have long been interested in xmpp as an IM solution but have always wanted the Invisibility feature of old school Instant Messengers (such as ICQ and MSN) but no implementation ever seemed to exist very well for it. I have tested a few things over the years and in recent times with the privacy lists/presences and the fact that a default openfire/spark combo does include a form of invisibility I've found there to be a slight oddity with which a person can easily determine whether or not someone is in fact online regardless of them being invisible or not. If you install a base openfire/spark setup you can add a friend, have them appear as invisible and use the "last activity" right click function to basically tell that they are in fact there despite them being invisible. Even if that person logs on as invisible, thus completely defeats the purpose. I'm not sure if this is a buggy implementation of the privacy lists or not, considering I tried the official ejabberd and another client... can't recall which but it had a similar function to view idle time and it had the exact same problem.
So the question is, is this an implementation issue, or a bug or an unintended issue with regards to how the privacy lists/invisibility is handled? Even when it is all setup properly invisibility vs a party doesn't work at all if that person uses the last activity/idle check because it will always say <1 minute when you are logged on as invisible. But if you aren't logged in at all... it starts adding up as per normal.