--- a/widget/TextEventDispatcher.cpp
+++ b/widget/TextEventDispatcher.cpp
@@ -675,47 +675,59 @@ TextEventDispatcher::PendingComposition:
       if (NS_WARN_IF(NS_FAILED(rv))) {
         return rv;
       }
     }
     return NS_OK;
   }
 
   // Adjust offsets in the ranges for XP linefeed character (only \n).
-  // XXX Following code is the safest approach.  However, it wastes performance.
-  //     For ensuring the clauses do not overlap each other, we should redesign
-  //     TextRange later.
   for (uint32_t i = 0; i < aRanges->Length(); ++i) {
     TextRange range = aRanges->ElementAt(i);
-    TextRange nativeRange = range;
-    if (nativeRange.mStartOffset > 0) {
-      nsAutoString preText(Substring(aString, 0, nativeRange.mStartOffset));
-      preText.ReplaceSubstring(NS_LITERAL_STRING("\r\n"),
-                               NS_LITERAL_STRING("\n"));
-      range.mStartOffset = preText.Length();
-    }
-    if (nativeRange.Length() == 0) {
-      range.mEndOffset = range.mStartOffset;
-    } else {
-      nsAutoString clause(
-        Substring(aString, nativeRange.mStartOffset, nativeRange.Length()));
-      clause.ReplaceSubstring(NS_LITERAL_STRING("\r\n"),
-                              NS_LITERAL_STRING("\n"));
-      range.mEndOffset = range.mStartOffset + clause.Length();
-    }
+    AdjustRange(range, aString);
     if (range.mRangeType == TextRangeType::eCaret) {
       mCaret = range;
     } else {
       EnsureClauseArray();
       mClauses->AppendElement(range);
     }
   }
   return NS_OK;
 }
 
+// static
+void
+TextEventDispatcher::PendingComposition::AdjustRange(
+                                           TextRange& aRange,
+                                           const nsAString& aNativeString)
+{
+  TextRange nativeRange = aRange;
+  // XXX Following code wastes runtime cost because this causes computing
+  //     mStartOffset for each clause from the start of composition string.
+  //     If we'd make TextRange have only its length, we don't need to do
+  //     this.  However, this must not be so serious problem because
+  //     composition string is usually short and separated as a few clauses.
+  if (nativeRange.mStartOffset > 0) {
+    nsAutoString preText(
+      Substring(aNativeString, 0, nativeRange.mStartOffset));
+    preText.ReplaceSubstring(NS_LITERAL_STRING("\r\n"),
+                             NS_LITERAL_STRING("\n"));
+    aRange.mStartOffset = preText.Length();
+  }
+  if (nativeRange.Length() == 0) {
+    aRange.mEndOffset = aRange.mStartOffset;
+  } else {
+    nsAutoString clause(
+      Substring(aNativeString, nativeRange.mStartOffset, nativeRange.Length()));
+    clause.ReplaceSubstring(NS_LITERAL_STRING("\r\n"),
+                            NS_LITERAL_STRING("\n"));
+    aRange.mEndOffset = aRange.mStartOffset + clause.Length();
+  }
+}
+
 nsresult
 TextEventDispatcher::PendingComposition::Flush(
                                            TextEventDispatcher* aDispatcher,
                                            nsEventStatus& aStatus,
                                            const WidgetEventTime* aEventTime)
 {
   aStatus = nsEventStatus_eIgnore;
 

--- a/widget/TextEventDispatcher.h
+++ b/widget/TextEventDispatcher.h
@@ -327,16 +327,26 @@ private:
     void Clear();
 
   private:
     nsString mString;
     RefPtr<TextRangeArray> mClauses;
     TextRange mCaret;
 
     void EnsureClauseArray();
+
+    /**
+     * AdjustRange() adjusts aRange as in the string with XP line breakers.
+     *
+     * @param aRange            The reference to a range in aNativeString.
+     *                          This will be modified.
+     * @param aNativeString     The string with native line breakers.
+     *                          This may include "\r\n" and/or "\r".
+     */
+    static void AdjustRange(TextRange& aRange, const nsAString& aNativeString);
   };
   PendingComposition mPendingComposition;
 
   // While dispatching an event, this is incremented.
   uint16_t mDispatchingEvent;
 
   enum InputTransactionType : uint8_t
   {