Elements whose contents start with an astral Unicode symbol disappear when CSS `::first-letter` is …

Source/core/dom/FirstLetterPseudoElement.cpp

Index: Source/core/dom/FirstLetterPseudoElement.cpp
diff --git a/Source/core/dom/FirstLetterPseudoElement.cpp b/Source/core/dom/FirstLetterPseudoElement.cpp
index 08d3a1c00b881de7a045f75bbd5140ccb52c246d..b79a20da6c8819f116af0a487944363946c52c7a 100644
--- a/Source/core/dom/FirstLetterPseudoElement.cpp
+++ b/Source/core/dom/FirstLetterPseudoElement.cpp
@@ -30,6 +30,7 @@
 #include "core/layout/LayoutObjectInlines.h"
 #include "core/layout/LayoutText.h"
 #include "core/layout/LayoutTextFragment.h"
+#include "platform/text/TextBreakIterator.h"
 #include "wtf/TemporaryChange.h"
 #include "wtf/text/WTFString.h"
 #include "wtf/text/icu/UnicodeIcu.h"
@@ -77,7 +78,8 @@ unsigned FirstLetterPseudoElement::firstLetterLength(const String& text)
         return 0;
 
     // Account the next character for first letter.
-    length++;
+    const String substring = text.is8Bit() ? String::make16BitFrom8BitSource(text.substring(length).characters8(), textLength - length) : text.substring(length);
+    length += countCharactersAndGraphemesInCluster(substring.characters16(), substring.length(), 0, 1).characters;
 
     // Keep looking for allowed punctuation for the :first-letter.
     for (; length < textLength; ++length) {