[CarCdr]

You are not worried that while you are entering a post, someone else is as well? If your post hits the database after any others since you last visited the thread, you will not know there are unseen/new posts in the thread.

Of course, you could make the hack you require more complicated by including a check for this case.

If you just want any reply to mark the thread as viewed (not new), then try this:

In version 3.0.3, showthread.php, CHANGE line 1373 from:

if (!$posted AND $displayed_dateline AND $displayed_dateline > fetch_bbarray_cookie('thread_lastview', $threadid))

to:

if (/* !$posted AND */ $displayed_dateline AND $displayed_dateline > fetch_bbarray_cookie('thread_lastview', $threadid))