Hello,

After tweaking gnus-sum-thread-tree-* options and messing with
gnus-summary-make-false-root before settling for 'dummy, I found a setup
I am mostly satisfied with for displaying loose threads:

(example from bug-gnu-emacs)
                     ┌◈                          bug#40337: 28.0.50; Enable case-fold-search in hi-lock
    02:08            ├► Juri Linkov
    05:33            ╰► Stefan Monnier

Yet sometimes I end up with threads where both the dummy line and the
first article show the title:

(example from emacs-devel)
                     ┌◈                          Re: Emacs's set-frame-size can not work well with gnome-shell?
    10:32            ├► martin rudalics          Re: Emacs's set-frame-size can not work well with gnome-shell?
    16:04            │╰► Eli Zaretskii
    10:33            ├► martin rudalics
    15:19            │╰► Dmitry Gutov
    10:33            ╰► martin rudalics

Ideally, I would like for loose threads to show the subject only once,
on the dummy root line.  IIUC, "%S" in gnus-summary-dummy-line-format
and "%s" in gnus-summary-line-format should get me that; quoting the
latter's docstring:

> %S          Subject (string)
> %s          Subject if it is at the root of a thread, and ""
>             otherwise (string)

I don't know if the duplicate subject I see is due to me misconfiguring
something[1], if this is a corner case that needs to be addressed in
Gnus, or if I am misreading the docstring, i.e. "at the root of a
thread" means "at the non-dummy root of a thread".

In case it's a corner case, here is my stab at fixing it: