Currently, when I drag (& drop) a collapsed thread of messages, only the top message gets dragged. This is unintuitive (and it's different from the behaviour of other applications that use some sort of hierarchical collapsing). All messages in the collapsed thread should be moved.
That makes sense. Note that there's been some changes in current master since the v0.7 release which at least make actions such as "mark as read" or "mark as deleted" work on an entire thread when operating on a collapsed root.