For example, I have `gnus-thread-hide-subtree' set to `(not
gnus-article-unread-p)', so that when I enter a summary, only threads
with unread messages are expanded.  When I do `A T' so I can see a whole
thread, if the other articles in the thread are already read, I don't
see any change in my summary buffer, or my thread might collapse.  I
actually need to `T s' (`gnus-summary-show-thread') to see them.

The enclosed patch fixes the problem.