Consider opening private messages with the direct JID instead of mediated by the MUC when it is available
Users get really confused when clicking on someone in their roster and clicking on the same person in a MUC don’t end up in the same tab, and have wildly different features (such as no offline messages for the latter).
It would be very useful to always open a chat with the real JID in case the recipient’s JID is known, and maybe also in the roster, to avoid woes when communication is possible through a MUC but blocked for whichever reason directly.