bug#36858: 27.0.50; display bugs with display-fill-column-indicator-mode

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 7 Aug 2019 19:06:54 +0200
> From: Ergus <spacibba@aol.com>
> Cc: 36858@debbugs.gnu.org, rotim.davor@gmail.com
> 
> >(There's no need to make any changes for that, you can simply invoke
> >dump-glyph-row or dump-glyph-matrix.)
> >
> How is it?

Just invoke these functions, they dump the glyph row's contents,
including the ends_at_zv_p, to stderr.

> This issue is already fixed with the other condition I mentioned:
> 
> IT_CHARPOS (*it) < ZV
> 
> But ends_at_zv_p this also need to be fixed because there are some tests
> inside extend_face_to_end_of_line that compare with ends_at_zv_p. In the
> worst case we need to remove these comparisons.

Not sure I understand why the comparisons need to be removed.  Can you
elaborate?

> But ideally the flag must be set before right?

In a buffer showing only buffer text (no after-strings at EOB), all
the glyph rows starting from the one showing EOB have their
ends_at_zv_p flag set.