Allegedly, on or about 29 November 2013, David sent:
So, for example, how would one, legitimately , add 'Solved' to a Subject: line and still expect it to follow the thread?
By not using gmail... I think you're screwed, using their webmail interface. They've taken the stance that a subject change means new thread. And it probably does, most of the time.
Most email clients will not do that, and we continually see people hijack threads by replying to something at random, then changing the subject.
Either way of the client working has its problem.