Never skip over CGJ

We might want to tweak this some more.  For now, never skipping over
it is better behavior than always skipping.

Part of https://github.com/behdad/harfbuzz/issues/554

src/hb-ot-layout-private.hh

@@ -226,7 +226,9 @@ _next_syllable (hb_buffer_t *buffer, unsigned int start)
  * - General_Category: 5 bits.
  * - A bit each for:
  *   * Is it Default_Ignorable(); we have a modified Default_Ignorable().
- *   * Whether it's one of the three Mongolian Free Variation Selectors.
+ *   * Whether it's one of the three Mongolian Free Variation Selectors,
+ *     CGJ, or other characters that are hidden but should not be ignored
+ *     like most other Default_Ignorable()s do during matching.
  *   * One free bit right now.
  *
  * The high-byte has different meanings, switched by the Gen-Cat:
@@ -264,20 +266,21 @@ _hb_glyph_info_set_unicode_props (hb_glyph_info_t *info, hb_buffer_t *buffer)
       buffer->scratch_flags |= HB_BUFFER_SCRATCH_FLAG_HAS_DEFAULT_IGNORABLES;
       props |=  UPROPS_MASK_IGNORABLE;
       if (u == 0x200Cu) props |= UPROPS_MASK_Cf_ZWNJ;
-      if (u == 0x200Du) props |= UPROPS_MASK_Cf_ZWJ;
+      else if (u == 0x200Du) props |= UPROPS_MASK_Cf_ZWJ;
       /* Mongolian Free Variation Selectors need to be remembered
        * because although we need to hide them like default-ignorables,
        * they need to non-ignorable during shaping.  This is similar to
        * what we do for joiners in Indic-like shapers, but since the
        * FVSes are GC=Mn, we have use a separate bit to remember them.
        * Fixes:
-       * https://github.com/behdad/harfbuzz/issues/234
-       */
-      if (unlikely (hb_in_range (u, 0x180Bu, 0x180Du))) props |= UPROPS_MASK_HIDDEN;
+       * https://github.com/behdad/harfbuzz/issues/234 */
+      else if (unlikely (hb_in_range (u, 0x180Bu, 0x180Du))) props |= UPROPS_MASK_HIDDEN;
       /* TAG characters need similar treatment. Fixes:
-       * https://github.com/behdad/harfbuzz/issues/463
-       */
-      if (unlikely (hb_in_range (u, 0xE0020u, 0xE007Fu))) props |= UPROPS_MASK_HIDDEN;
+       * https://github.com/behdad/harfbuzz/issues/463 */
+      else if (unlikely (hb_in_range (u, 0xE0020u, 0xE007Fu))) props |= UPROPS_MASK_HIDDEN;
+      /* COMBINING GRAPHEME JOINER should not be skipped; at least some times.
+       * https://github.com/behdad/harfbuzz/issues/554 */
+      else if (unlikely (u == 0x034Fu)) props |= UPROPS_MASK_HIDDEN;
     }
     else if (unlikely (HB_UNICODE_GENERAL_CATEGORY_IS_NON_ENCLOSING_MARK_OR_MODIFIER_SYMBOL (gen_cat)))
     {

test/shaping/tests/default-ignorables.tests

 fonts/sha1sum/051d92f8bc6ff724511b296c27623f824de256e9.ttf::U+0075,U+0361,U+034F,U+0301,U+0069:[gid2=0+1266|gid7=0@-617,442+0|gid5=0@-7,0+0|gid1=4+528]
+fonts/sha1sum/bf962d3202883a820aed019d9b5c1838c2ff69c6.ttf::U+0020,U+06CC,U+064E,U+034F,U+0651:[uni0651=1+0|space=1+0|uni064E=1@236,-432+0|uni06CC=1+1266|space=0+452]