Here's one dumb possibility I just found:
I always used to click on the "new thread" row of the Thread Watcher. Of course if that was the ONLY new post, when I finish reading the new post and go to the forum, the tor row far-left column has black dashes!
I may have confused that obvious case with the idea that forums often have black dashes in the top-left corner "when I go into them".
I'll watch for that DUHH.
