Use our own grapheme boundary handling.

third_party/WebKit/Source/core/editing/EditingUtilities.cpp

Index: third_party/WebKit/Source/core/editing/EditingUtilities.cpp
diff --git a/third_party/WebKit/Source/core/editing/EditingUtilities.cpp b/third_party/WebKit/Source/core/editing/EditingUtilities.cpp
index 1a7dc9d74831dcea9f16d3e7d42fa0ccdb41cffe..7b5fcebb0db61ab4c6b644714373bff78ae9248b 100644
--- a/third_party/WebKit/Source/core/editing/EditingUtilities.cpp
+++ b/third_party/WebKit/Source/core/editing/EditingUtilities.cpp
@@ -42,6 +42,8 @@
 #include "core/editing/iterators/TextIterator.h"
 #include "core/editing/serializers/HTMLInterchange.h"
 #include "core/editing/state_machines/BackspaceStateMachine.h"
+#include "core/editing/state_machines/BackwardGraphemeBoundaryStateMachine.h"
+#include "core/editing/state_machines/ForwardGraphemeBoundaryStateMachine.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/UseCounter.h"
 #include "core/html/HTMLBRElement.h"
@@ -555,7 +557,7 @@ int findNextBoundaryOffset(const String& str, int current)
         if (state != TextSegmentationMachineState::NeedMoreCodeUnit)
             break;
     }
-    if (state == TextSegmentationMachineState::NeedMoreCodeUnit)
+    if (current == 0 || state == TextSegmentationMachineState::NeedMoreCodeUnit)
         state = machine.tellEndOfPrecedingText();
     if (state == TextSegmentationMachineState::Finished)
         return current + machine.finalizeAndGetBoundaryOffset();
@@ -571,16 +573,12 @@ int findNextBoundaryOffset(const String& str, int current)
 
 int uncheckedPreviousOffset(const Node* node, int current)

[yosin_UTC9, 2016/04/06 07:33:43]

It seems make |uncheckedPreviousOffset()| and |uncheckNextOffset()| to work on
grapheme is right approach.

We need to introduce |PositionMoveType::GraphemeCluster| and make
|{previous,next}PositionOf()| to call |unchecked{Previous,Next}Of()| to use them
for |PositionMoveType::GraphemeCluster|, and, change
|PositionMoveType::CodePoint| of |{previous,next}PositionOf()| to work on code
point instead of calling 
|PositionMoveType::GraphemeCluster|.

 {
-    if (!node->isTextNode())
+    DCHECK_GE(current, 0);
+    if (current <= 1 || !node->isTextNode())
         return current - 1;
     const String& text = toText(node)->data();
-    if (text.is8Bit())
-        return current - 1; // TODO(nona): Good to support CR x LF.
-    TextBreakIterator* iterator = cursorMovementIterator(text.characters16(), text.length());
-    if (!iterator)
-        return current - 1;
-    const int result = iterator->preceding(current);
-    return result == TextBreakDone ? current - 1 : result;
+    return findNextBoundaryOffset<BackwardGraphemeBoundaryStateMachine>(
+        text, current);
 }
 
 static int uncheckedPreviousOffsetForBackwardDeletion(const Node* node, int current)
@@ -601,13 +599,12 @@ int uncheckedNextOffset(const Node* node, int current)
     if (!node->isTextNode())
         return current + 1;
     const String& text = toText(node)->data();
-    if (text.is8Bit())
-        return current + 1; // TODO(nona): Good to support CR x LF.
-    TextBreakIterator* iterator = cursorMovementIterator(text.characters16(), text.length());
-    if (!iterator)
+    const int length = text.length();
+    DCHECK_LE(current, length);
+    if (current >= (length - 1))
         return current + 1;
-    const int result = iterator->following(current);
-    return result == TextBreakDone ? current + 1 : result;
+    return findNextBoundaryOffset<ForwardGraphemeBoundaryStateMachine>(
+        text, current);
 }
 
 template <typename Strategy>