Fix text shaping of precomposed characters with HarfBuzz

* src/hbfont.c (hbfont_shape): Fix setting TO of lglyph for
the case when Lisp shape-gstring function performed some
compositions by itself.  This happens with hebrew.el.  See
https://lists.freedesktop.org/archives/harfbuzz/2019-June/007396.html
for the details of the problem this solves.

1 files changed, 11 insertions, 1 deletions

diff --git a/src/hbfont.c b/src/hbfont.c
index 5923efd488d..623bee61cc8 100644
--- a/src/hbfont.c
+++ b/src/hbfont.c
@@ -435,7 +435,17 @@ hbfont_shape (Lisp_Object lgstring, Lisp_Object direction)
 
       /* All the glyphs in a cluster have the same values of FROM and TO.  */
       LGLYPH_SET_FROM (lglyph, from);
-      LGLYPH_SET_TO (lglyph, to);
+      /* This heuristic is for when the Lisp shape-gstring function
+         substitutes known precomposed characters for decomposed
+         sequences.  E.g., hebrew.el does that.  This makes TEXT_LEN
+         be smaller than the original length of the composed character
+         sequence.  In that case, we must not alter the largest TO,
+         because the display engine must know that all the characters
+         in the original sequence were processed by the composition.
+         If we don't do this, some of the composed characters will be
+         displayed again as separate glyphs.  */
+      if (!(to == text_len - 1 && LGLYPH_TO (lglyph) > to))
+        LGLYPH_SET_TO (lglyph, to);
 
       /* Not every glyph in a cluster maps directly to a single
          character; in general, N characters can yield M glyphs, where