Issues with messages related by subject

The code currently looks for messages with the same subject in order to construct threads.
That often results in spurious matches, so personally, I don't see the point of doing this, at least not by default.

However it seems especially futile to do so when the subject is missing or empty.

Start by locating the message-thread construction entry point and inspect how subjects are compared, especially when a subject is missing or empty. Reproduce the spurious-match case and determine the intended default behavior from the issue; done means empty subjects no longer create matches and same-subject threading follows the agreed behavior.

Written by the indexing model from the issue text.