[morx] Fix cluster-merging when ligating

Only merge when actual ligature happened.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1305

src/hb-aat-layout-morx-table.hh

@@ -399,9 +399,6 @@ struct LigatureSubtable
 	if (unlikely (!match_length))
 	  return true;
 
-	/* TODO Only when ligation happens? */
-	buffer->merge_out_clusters (match_positions[0], buffer->out_len);
-
 	unsigned int cursor = match_length;
         do
 	{
@@ -437,6 +434,7 @@ struct LigatureSubtable
 		     bool (action & LigActionLast));
 	  if (action & (LigActionStore | LigActionLast))
 	  {
+
 	    const GlyphID &ligatureData = ligature[ligature_idx];
 	    if (unlikely (!ligatureData.sanitize (&c->sanitizer))) return false;
 	    hb_codepoint_t lig = ligatureData;
@@ -452,6 +450,9 @@ struct LigatureSubtable
 	      buffer->skip_glyph ();
 	      end--;
 	    }
+
+	    buffer->move_to (end + 1);
+	    buffer->merge_out_clusters (match_positions[cursor], buffer->out_len);
 	  }
 
 	  action_idx++;

test/shaping/data/in-house/Makefile.sources

 TESTS = \
 	tests/aat-trak.tests \
+	tests/aat-morx.tests \
 	tests/arabic-fallback-shaping.tests \
 	tests/arabic-feature-order.tests \
 	tests/arabic-like-joining.tests \

test/shaping/data/in-house/tests/aat-morx.tests

+../fonts/TestMORXTwentyeight.ttf::U+0041,U+0078,U+0045,U+0079,U+0044,U+0079,U+0079:[A_E_D=0+1394|x=0+529|y=0+510|y=5+510|y=6+510]