[HarfBuzz] Display issue with DejaVu Sans Mono font

[Khaled Hosny]

> On Aug 18, 2019, at 4:40 AM, Eli Zaretskii <eliz at gnu.org> wrote:
> 
>> From: Khaled Hosny <dr.khaled.hosny at gmail.com>
>> Date: Sun, 18 Aug 2019 00:48:14 +0200
>> Cc: Harfbuzz <harfbuzz at lists.freedesktop.org>
>> 
>>> https://lists.gnu.org/archive/html/bug-gnu-emacs/2019-08/msg01082.html
>>> 
>>> Is there something wrong with this font when displaying this sequence,
>>> or is there some kind of bug in Emacs and/or HarfBuzz?
>> 
>> The second accent is placed next to the glyph, but hb-view is incorrectly clipping the image, as you can see from hb-shape output:
>> 
>> $ hb-shape DejaVuSansMono.ttf -u '061,301,302'
>> [aacute=0+1233|uni0302=0+0]
>> 
>> Adding some margins gives:
>> 
>> $ hb-view DejaVuSansMono.ttf -u '061,301,302’ --margin=0,150,0,0
>> 
>> 
>> 
>> HarfBuzz will compose U+0061 + U+0301 to U+00E1 (since it prefers composed form when supported by the font), and that glyph does not have anchors to position any marks above it, so the circumflex ends up with its default position next to the glyph.
> 
> So this is indeed some problem with that particular font?

It is partly a font issue (missing anchors and combining marks default position to the left of base glyph), and partly HarfBuzz design decision of preferring composed forms. See https://github.com/harfbuzz/harfbuzz/issues/653.

Regards,
Khaled