Room sorting by activity shouldn't consider join/leave events (and maybe others)?
Currently, rooms are sorted by timestamp of the last event of any type. I don't think join/leave events should be included while determining that timestamp. Messages should certainly be included. I don't really know about other types of events, like power level changes or room name/topic changes.
I propose we limit it to messages (m.room.message) only, because if another change is "worth reading about" then there will probably be relevant room messages.