bug#24109: 24.5; Long lines in message mode make Emacs irresponsive

[Eli Zaretskii]

> From: Yuri D'Elia <wavexx@thregr.org>
> Cc: 24109@debbugs.gnu.org, djcb@djcbsoftware.nl, 
> Christophe.Troestler@umons.ac.be, rudalics@gmx.at
> Date: Wed, 31 Aug 2016 17:51:04 +0200
> 
> I've narrowed it down to redisplay_internal(), hitting a goto in
> xdisp.c:14144:
> 
>     if (hscroll_windows (selected_window))
> =>    goto retry;

OK, thanks.  Next question: why does hscroll_windows returns non-zero
when called repeatedly?  It is supposed to do that only once; the next
call should return zero.

For starters, in this line:

              if (w->hscroll != hscroll)

what are the values of those two, and do they change each time
hscroll_windows is called in the infloop, for the selected window?