Roll harfbuzz-ng to 1.0.5

third_party/harfbuzz-ng/src/hb-ot-shape-complex-thai.cc

 /*
  * Copyright © 2010,2012  Google, Inc.
  *
  *  This is part of HarfBuzz, a text shaping library.
  *
  * Permission is hereby granted, without written agreement and without
  * license or royalty fees, to use, copy, modify, and distribute this
  * software and its documentation for any purpose, provided that the
  * above copyright notice and the following two paragraphs appear in
  * all copies of this software.
@@ skipping 335 matching lines @@
       hb_glyph_info_t t = buffer->out_info[end - 2];
       memmove (buffer->out_info + start + 1,
                buffer->out_info + start,
                sizeof (buffer->out_info[0]) * (end - start - 2));
       buffer->out_info[start] = t;
     }
     else
     {
       /* Since we decomposed, and NIKHAHIT is combining, merge clusters with the
        * previous cluster. */
-      if (start)
+      if (start && buffer->cluster_level == HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES)
         buffer->merge_out_clusters (start - 1, end);
     }
   }
   buffer->swap_buffers ();
 
   /* If font has Thai GSUB, we are done. */
   if (plan->props.script == HB_SCRIPT_THAI && !plan->map.found_script[0])
     do_thai_pua_shaping (plan, buffer, font);
 }
 
 const hb_ot_complex_shaper_t _hb_ot_complex_shaper_thai =
 {
   "thai",
   NULL, /* collect_features */
   NULL, /* override_features */
   NULL, /* data_create */
   NULL, /* data_destroy */
   preprocess_text_thai,
   HB_OT_SHAPE_NORMALIZATION_MODE_DEFAULT,
   NULL, /* decompose */
   NULL, /* compose */
   NULL, /* setup_masks */
   HB_OT_SHAPE_ZERO_WIDTH_MARKS_DEFAULT,
   false,/* fallback_position */
 };