msglist: On topic-narrow fetch after refreshing event queue, if topic is newly empty, detect/follow message move

[chrisbobbe]

If messages are bulk-moved while we have an active event queue, the topic-narrow message list understands this and follows the move, showing the messages in their new location.

But if you're looking at a topic-narrow message list while we don't have an active event queue, and the topic's messages get moved during that time, we won't follow the move. The result can be confusing: when we get the new event queue and refetch the message list, you'll be looking at the same topic as before and it will be empty.

Instead, if the refetch comes up empty, how about doing a quick check to see if the messages were moved, by fetching a single message that we know was there before, and re-narrowing to the new location if so.

[chrisbobbe]

One beta-feedback report that might (?) have been explained by this.