I often find myself replying a little later to a users original message. However, if they're no longer in the room, it's often a waste of time. When the user clicks reply either from the hover reply button or the right-click context menu it should do a quick check to see if the user is still a member of the room. If they are no longer a member, a small message could pop up to notify the user: "$username is no longer in this room to see your reply."
this has been implemented