[HarfBuzz] Display issue with DejaVu Sans Mono font

[Khaled Hosny]

> On Aug 17, 2019, at 8:38 PM, Eli Zaretskii <eliz at gnu.org> wrote:
> 
> We have some strange display problem in Emacs with this sequence:
> 
>  u+0097 u+0301 u+0302
> 
> The problem seems to happen only with DejaVu Sans Mono font.  It looks
> like hb-view also displays the sequence with only one of the two
> combining accents, see the images in
> 
>  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.
Regards,
Khaled

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