I know that I have seen this request in the issue queue before, but
- I honestly can't find it anymore
- back then nobody had a clue how to achieve this.
I would love to see the feature (certainly optional) to maintain only a single thread between any two users or any given set of participants. Just like Facebook has it. This would be really nice for all of us who use Privatemsg more as a chat module than as a proper email module.
Anil Maharjan has posted a patch for Privatemsg on his blog that would take care of exactly this behavior: http://anilmaharjan.com.np/blog/2013/05/maintain-single-thread-between-t...
Could one of the maintainers have a look if this could be implemented in Privatemsg module?