On Wed 2019-01-23 08:29:05 -0700, Nicolas Bock wrote:

> Sometimes during a lengthy discussion someone inadvertently breaks 
> the email thread because their mail client doesn't get it. Is 
> there a way to merge the two fragmented threads again with 
> notmuch?

there's a manual way to do it as a janky workaround:

 * create a bogus e-mail message that References: a message-id from each
   split faction.

 * inject that message into your notmuch message store

 * delete that message

See also the thread on this maiing list with Subject "thread merge/split
proposal" from back in 2016, starting at
id:87mvp9uwi4.fsf@alice.fifthhorseman.net though i don't think any of
that has been implemented.

        --dkg