Use following font data and script for heading zero-width spaces while collecting candidate runs.

Source/platform/fonts/shaping/HarfBuzzShaper.cpp

 /*
  * Copyright (c) 2012 Google Inc. All rights reserved.
  * Copyright (C) 2013 BlackBerry Limited. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 605 matching lines @@
 {
     const UChar* normalizedBufferEnd = normalizedBuffer + bufferLength;
     SurrogatePairAwareTextIterator iterator(normalizedBuffer, 0, bufferLength, bufferLength);
     UChar32 character;
     unsigned clusterLength = 0;
     unsigned startIndexOfCurrentRun = 0;
 
     if (!iterator.consume(character, clusterLength))
         return false;
 
+    bool collectionForHeadingZWS = false;
+    if (Character::treatAsZeroWidthSpace(character))
+        collectionForHeadingZWS = true;
+
     const SimpleFontData* nextFontData = font->glyphDataForCharacter(character, false, isSpaceNormalize).fontData;
     UErrorCode errorCode = U_ZERO_ERROR;
     UScriptCode nextScript = uscript_getScript(character, &errorCode);
     if (U_FAILURE(errorCode))
         return false;
 
     do {
         const UChar* currentCharacterPosition = iterator.characters();
         const SimpleFontData* currentFontData = nextFontData;
         UScriptCode currentScript = nextScript;
 
         UChar32 lastCharacter = character;
         for (iterator.advance(clusterLength); iterator.consume(character, clusterLength); iterator.advance(clusterLength)) {
             if (Character::treatAsZeroWidthSpace(character))
                 continue;
 
             int length = handleMultipleUChar(character, clusterLength, currentFontData, currentCharacterPosition, iterator.characters() + clusterLength, normalizedBufferEnd);
             if (length) {
                 clusterLength = length;
                 continue;
             }
 
             nextFontData = font->glyphDataForCharacter(character, false, isSpaceNormalize).fontData;
             nextScript = uscript_getScript(character, &errorCode);
             if (U_FAILURE(errorCode))
                 return false;
             if (lastCharacter == zeroWidthJoiner)
                 currentFontData = nextFontData;
-            if ((nextFontData != currentFontData) || ((currentScript != nextScript) && (nextScript != USCRIPT_INHERITED) && (!uscript_hasScript(character, currentScript))))
-                break;
+            if ((nextFontData != currentFontData) || ((currentScript != nextScript) && (nextScript != USCRIPT_INHERITED) && (!uscript_hasScript(character, currentScript)))) {
+                if (!collectionForHeadingZWS)
+                    break;
+                currentFontData = nextFontData;
+                currentScript = nextScript;
+                collectionForHeadingZWS = false;
+            }
             currentCharacterPosition = iterator.characters();
             lastCharacter = character;
         }
 
         CandidateRun run = { character, startIndexOfCurrentRun, iterator.currentCharacter(), currentFontData, currentScript };
         runs->append(run);
 
         startIndexOfCurrentRun = iterator.currentCharacter();
     } while (iterator.consume(character, clusterLength));
 
@@ skipping 478 matching lines @@
         fromX = 0;
     if (!foundToX)
         toX = m_run.rtl() ? 0 : m_totalWidth;
 
     if (fromX < toX)
         return FloatRect(point.x() + fromX, point.y(), toX - fromX, height);
     return FloatRect(point.x() + toX, point.y(), fromX - toX, height);
 }
 
 } // namespace blink