bug#50951: 28.0.50; Urdu text is not displayed correctly

[Eli Zaretskii]

> Date: Tue, 20 Sep 2022 12:41:40 +0900
> From: YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
> Cc: Visuwesh <visuweshm@gmail.com>,
>       Eli Zaretskii <eliz@gnu.org>,
>       larsi@gnus.org,
>       50951@debbugs.gnu.org
> 
> The width of grapheme cluster corresponding to U+06AF (ARABIC LETTER
> GAF) is rounded to zero, and Emacs does not display such clusters:
> 
> xdisp.c:
>  32424              gstring = composition_gstring_from_id (it->cmp_it.id);
>  32425              it->pixel_width
>  32426                = composition_gstring_width (gstring, it->cmp_it.from, 
> it->cmp_it.to,
>  32427                                             &metrics);
>  32428              if (it->pixel_width == 0)
>  32429                {
>  32430                  it->glyph_not_available_p = true;
>  32431                  it->phys_ascent = it->ascent;
>  32432                  it->phys_descent = it->descent;
>  32433                  it->pixel_width = face->font->space_width;
>  32434                }
>  32435              else
> 
> The attached patch avoids zero-width grapheme clusters by adding 1 to
> the width of the last glyph in such clusters.

If the problem is rounding, I think we should do this adjustment only
when the last glyph has a non-zero width that was rounded to zero, no?
Otherwise, we are inventing adjustments out of thin air, which could
adversely affect the displayed result, I think?

Or maybe we should have a variable that controls this heuristic?

Bottom line: I'm uneasy with messing with the grapheme cluster data
without some sound basis.  We delegate this job to a text-shaping
engine for a reason.  But if there is a sound basis for this
adjustment, could you please elaborate on it?

Thanks.