Thanks for your reply.

I never tried gmail-conversation threading, but from your first reference I
understand it breaks threads on subject unconditionally.

Breaking on subject unconditionally would be even easier to implement, as
comparing the contents of previous messages takes performance and as long as
the crucial linking messages aren't read the outcome is ambiguous and would
lead to the annoying jumping of results.

I'll watch for 'client-end' solutions, but the mail that broke all those
mailers originated from my own mailprogram, I think Outlook 2010, so
automatic clearing references and in-reply-to when the user clears the
subject and body isn't common practice for MUA's.

Your point on patch-breaking related to gmail and my proposal isn't
completely clear to me, but I've probably addressed it well with my new
approach.

I'll study the code for adding the option of unconditional (stripped)
subject breaking on top of the existing thread-breaking.