[Emacs] Bug?: notmuch-search-show-thread shows several threads; only one containing matching messages

[Austin Clements]

Quoth Gregor Zattler on Jan 26 at  1:40 am:
> [sorry for the long and meandering explanation, I do not know how
> to express the issue more concise]
> 
> Dear notmuch developers,
> 
> may someone please enlighten me regarding this behaviour: 
> 
> I experienced a situation where the Emacs interface shows three
> (3) different threads in one notmuch show buffer.  I thought it
> should show only one.  Perhaps this is the result of notmuch show
> showing messages which do not match the search pattern and do not
> belong to one thread which contains messages which do so.[1]

It's always possible that this is a bug, but one potential legitimate
cause comes to mind.  If *any* of the messages in each of the
apparently unrelated threads contain a message ID of any of the
messages in the original thread (or one of the other unrelated
threads) in their References or In-reply-to headers, notmuch will
merge the threads and cause the exact behavior you're seeing.

One very common cause of this is someone using "reply" to get an
initial set of recipients, but then replacing the entire message and
subject (presumably without realizing that the mail is still tracking
what it was a reply to).  This can also happen if someone
intentionally replies to multiple messages (though few mail clients
support this), or if there was a message ID collision.