Make first-letter style to work with editing

Source/core/rendering/RenderText.cpp

Index: Source/core/rendering/RenderText.cpp
diff --git a/Source/core/rendering/RenderText.cpp b/Source/core/rendering/RenderText.cpp
index 6e1703a0a230f11e817ba55611458abdc7956828..9db009a45b3958c4e1daa96eab91fec6305bc695 100644
--- a/Source/core/rendering/RenderText.cpp
+++ b/Source/core/rendering/RenderText.cpp
@@ -532,7 +532,8 @@ static PositionWithAffinity createPositionWithAffinityForBox(const InlineBox* bo
         affinity = offset > box->caretMinOffset() ? VP_UPSTREAM_IF_POSSIBLE : DOWNSTREAM;
         break;
     }
-    return box->renderer()->createPositionWithAffinity(offset, affinity);
+    int textStartOffset = box->renderer()->isText() ? toRenderText(box->renderer())->textStartOffset() : 0;
+    return box->renderer()->createPositionWithAffinity(offset + textStartOffset, affinity);
 }
 
 static PositionWithAffinity createPositionWithAffinityForBoxAfterAdjustingOffsetForBiDi(const InlineTextBox* box, int offset, ShouldAffinityBeDownstream shouldAffinityBeDownstream)