bug#71439: Acknowledgement (30.0.50; gnus-summary-refer-thread doesn't work)

[jao via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

It's actually not completely true that ^
(gnus-summary-refer-parent-article) works. There are circumstances in
which it also seem to fail. I think it's when a tree has multiple
subtrees and one reaches the top of one of them: then it fails.

    |
    |-
    | |-
    | |- 
    |-
      |-     <-- failure here
        -    <-- works here and belos

I think it fails also sometimes when reaching the top of the main tree,
which would be consistent to the failure reaching the top of subtrees,
and might be related to the failure collecting entire threads, but I am
just guessing.

These are the possibly relevant variables I set in my config:

(setq gnus-show-threads t
      gnus-thread-hide-subtree t
      gnus-build-sparse-threads nil
      gnus-refer-thread-use-search t
      gnus-summary-make-false-root 'adopt
      gnus-summary-gather-subject-limit nil
      gnus-summary-thread-gathering-function #'gnus-gather-threads-by-subject
      gnus-sort-gathered-threads-function 'gnus-thread-sort-by-date
      gnus-thread-sort-functions '(gnus-thread-sort-by-date))