bug#43835: 28.0.50; auto-hscroll-mode/current-line + show-paren-mode + gnus-summary-mode = bug

[Eli Zaretskii <eliz@gnu.org>]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: 43835@debbugs.gnu.org
> Date: Thu, 08 Oct 2020 12:18:55 +0200
> 
> I can see the problem is a real .el file, but it seems to depend on the
> form of the line and other conditions I don't understand.  I visited
> bindings.el and added the following line (with no line break, in case
> one gets added to it in the email) at the end of the file:

Like I said: this happens when the problematic line is either the very
first or the very last line of the buffer.  I think you are describing
the latter situation?

> (setq bla "Subject: Re: bug#39280: 27.0.60; wdired-get-filename ignores first argument")
> 
> Then I added a newline, evaluated (setq auto-hscroll-mode 'current-line
> truncate-lines t) and enabled show-paren-mode.  With point at the
> beginning of the above line I typed `C-e' and the line scrolled left and
> the end of the line remained visible, i.e., no problem.  Then I deleted
> the final `)', typed `C-a C-e' and the line scrolled left and then
> immediately undid the hscroll so the end of the line was not visible but
> after a fraction of a second it scrolled left again and the end of the
> line remained visible.  Then I additionally deleted the final `"', typed
> `C-a C-e' and the line scrolled left and now the end of the line
> remained out of view indefinitely.  This even though the final line does
> now does not end with a parenthesis group, in contrast to all previous
> cases where I've seen the problem.  And indeed, if I restart the
> experiment in a fresh emacs -Q and insert just `(setq bla "Subject: Re:
> bug#39280: 27.0.60; wdired-get-filename ignores first argument' --
> i.e. without the closing `")' -- plus a newline and make the other
> necessary settings, then I see no hscrolling problem.  Can you replicate
> these observations?

I don't see them after fixing the problem.  Is it still necessary to
see if they existed before the fix?

Thanks.