If a contact has more that one resource online, this contact should have a [x] or [-], and should be un-foldable (like groups too).
When the roster cursor is on "[-] nick@server", the enter key should
open a conversation with the bare JID. So all messages sent in the
corresponding tab will be sent to the bare JID, and routed by the
contact's server to the highest resource. Displayed information should
only be what all the resources have in common (so, not much).
When the roster cursor is on a specific resource, the enter key should open a conversation with this specific JID. And messages should then be sent to this specific resource. If this resource goes offline, no fallback should be done (so, the messages should not be sent to an other resource neither to the bare jid). The information displayed should be the one of this specific resource.