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

[Eli Zaretskii]

> Date: Wed, 07 Oct 2020 10:25:27 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 43835@debbugs.gnu.org
> 
> > I haven't yet rebuilt with --enable-checking='yes,glyphs', but I just
> > made a new, perhaps relevant, observation: after step 8 of the recipe,
> > i.e. with point at the end of the line but hscrolling undone, if I move
> > the mouse pointer to a position that pops up a tooltip (i.e., over a
> > tool-bar icon or a mode-line element), then with
> > x-gtk-use-system-tooltips set to t nothing changes but with
> > x-gtk-use-system-tooltips set to nil, the hscroll is restored, like with
> > `M-x' before `C-g', and the hscroll stays when I move the mouse so that
> > the tooltip vanishes, but if I then type `C-g', the hscroll is undone
> > again (and point remains at the end of the line, out of view).
> 
> I think popping up the native tooltip has the same effect as typing
> M-x: both trigger a thorough redisplay cycle.  The more important fact
> is that C-g "breaks" the display again, which is... unexpected.

Could the reason for this problem somehow be related to
gnus-horizontal-recenter, which is called by gnus-recenter basically
whenever you do something in the summary buffer?  If you disable this
horizontal recentering (e.g., by setting gnus-auto-center-summary to
the value 'vertical'), does the problem go away?