[HarfBuzz] HarfBuzz shaping of R2L text

[Eli Zaretskii]

Hi,

While testing the results of hb_shape_full called to shape R2L text, I
observed behavior that surprised me: shaping an R2L base letter with a
diacritical produces a sequence of glyphs in reverse order, i.e. the
glyph for the diacritical comes first, before the base letter.

For example, if I shape the sequence (in the logical order)

  U+05EA HEBREW LETTER TAV
  U+05BB HEBREW POINT QUBUTS

the glyphs left in the buffer by the shaper are in reverse order,
first QUBUTS, then TAV.  I thought that this was because of bidi
reordering, but the result doesn't change if I set the buffer
direction to LTR before calling the shaper.  The order of the clusters
does change with the direction, i.e. with LTR the first cluster is
zero, followed by 1, etc., whereas with RTL the clusters are in the
decreasing order.  But the glyphs are always in the same order: the
point first, then the letter.

I see the same with the Arabic script if I shape U+0633 followed by
U+0651 (in logical order).

This doesn't happen with LTR text in unidirectional scripts, including
with Latin text when shaping a base letter followed by a diacritical.

Is this expected behavior?  If so, what are the reasons?  Also, can it
be controlled by the client application?  E.g., Uniscribe can be told
to produce glyphs in the logical order, after shaping them for RTL
display.

TIA