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)

diff --git a/src/xdisp.c b/src/xdisp.c
index 4031571dd9917126e56e0f45b99e1d89d2ff7fe7..dc5101d5aca522e9e153d91c39c2d149469eac36 100644 (file)
--- 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