Hi Behdad,
If the harfbuzz buffer direction is vertical (TTB or BTT), I think we
should refrain from doing Arabic shaping.[1]
We could do this, for example, by adding an early return such as
if (unlikely (HB_DIRECTION_IS_VERTICAL (buffer->props.direction)))
return;
in setup_masks_arabic(), so that the shaping analysis and setting of
per-glyph features doesn't happen. WDYT?
(It's less clear to me what should be done in Indic and similar shapers,
so I'm ignoring that for now...)
JK
[1] See CSS Writing Modes, section 5.1.1 Vertical Typesetting and Font
Features: "...characters from horizontal cursive scripts (such as
Arabic) are shaped in their isolated forms when typeset upright".
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