Severity: minor

This concerns a peculiarity affecting help buffers not named "*Help*".
It was noticed while investigating bug#73637 and bug#73686 but is
ultimately unrelated and dates back to at least Emacs 27.

Normally, returning to a previous help buffer via a back button replaces
the window buffer with the previous one. And though typing "l" does that
too, the latter also re-creates the previous buffer in a new window,
resulting in different buffers with the same content (the new one being
the default "*Help*").

From emacs -Q (on Emacs 30):

1. M-x list-faces-display RET
2. C-x o TAB TAB RET
3. In the help buffer for `abbrev-table-name':
   <backtab> <backtab> RET
4. In the help buffer for `font-lock-function-name-face':
   <backtab> RET
5. Back in `abbrev-table-name':
   TAB RET (should take you back to "*Faces*")

Now start a new session, repeating 1-3. For 4, hit "l" instead, and
witness an unwanted duplicate "*Help*" buffer pop up in another window.
A similar phenomenon exists between forward buttons and "r".

Superficially, I think the disparity results from `help-xref-following'
being t in the base recipe (at the hands of `help-do-xref'):

  * help-xref-go-back(#<buffer *Faces*>)
    help-do-xref(nil help-xref-go-back (#<buffer *Faces*>))
    help-button-action(#<marker (moves after insertion) at 751 in *Faces*>)
    button-activate(#<marker (moves after insertion) at 751 in *Faces*> nil)
    push-button(751)

Which allows for buffer reuse in `help-buffer'. No such arrangement
occurs with the "l" variant:

  * help-xref-go-back(#<buffer *Faces*>)
    help-go-back()
    funcall-interactively(help-go-back)

As an experiment, I tried using `help-do-xref' instead of `apply' in
both `help-xref-go-back' and `help-xref-go-forward' (see attached diff).
While this appears to work, I'm guessing it's breaking something
somewhere. Hopefully, someone who understands the intricacies of the
Emacs help system will know what to do.

Thanks.


In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.43, cairo version 1.18.0) of 2024-10-07 built on localhost
Repository revision: ff4de9eff30ade164655354f71e7cbca48135858
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12401002
System Description: Fedora Linux 40 (Workstation Edition)

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