Fix shaping of Arabic test when the region is extended

* src/xdisp.c (compute_stop_pos): Set the limit for searching
for changes in text properties such that the limit is never in
the middle of composable text.  (Bug#28312)

1 files changed, 39 insertions, 1 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 4031571dd99..dc5101d5aca 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3632,7 +3632,45 @@ compute_stop_pos (struct it *it)
       /* Set up variables for computing the stop position from text
          property changes.  */
       XSETBUFFER (object, current_buffer);
-      limit = make_fixnum (IT_CHARPOS (*it) + TEXT_PROP_DISTANCE_LIMIT);
+      pos = charpos + TEXT_PROP_DISTANCE_LIMIT;
+      /* Make sure the above arbitrary limit position is not in the
+         middle of composable text, so we don't break compositions by
+         submitting the composable text to the shaper in separate
+         chunks.  We play safe here by assuming that only SPC, TAB,
+         FF, and NL cannot be in some composition; in particular, most
+         ASCII punctuation characters could be composed into ligatures.  */
+      if (!NILP (BVAR (current_buffer, enable_multibyte_characters))
+          && !NILP (Vauto_composition_mode))
+        {
+          ptrdiff_t endpos = charpos + 10 * TEXT_PROP_DISTANCE_LIMIT;
+          bool found = false;
+
+          if (pos > ZV)
+            pos = ZV;
+          if (endpos > ZV)
+            endpos = ZV;
+          ptrdiff_t bpos = CHAR_TO_BYTE (pos);
+          while (pos < endpos)
+            {
+              int ch;
+              FETCH_CHAR_ADVANCE_NO_CHECK (ch, pos, bpos);
+              if (ch == ' ' || ch == '\t' || ch == '\n' || ch == '\f')
+                {
+                  found = true;
+                  break;
+                }
+            }
+          if (found)
+            pos--;
+          else if (it->stop_charpos < endpos)
+            pos = it->stop_charpos;
+          else
+            {
+              /* Give up and use the original arbitrary limit.  */
+              pos = charpos + TEXT_PROP_DISTANCE_LIMIT;
+            }
+        }
+      limit = make_fixnum (pos);
     }
 
   /* Get the interval containing IT's position.  Value is a null