--- a/widget/windows/KeyboardLayout.cpp
+++ b/widget/windows/KeyboardLayout.cpp
@@ -3752,34 +3752,29 @@ bool
 KeyboardLayout::MaybeInitNativeKeyWithCompositeChar(
                   NativeKey& aNativeKey,
                   const ModifierKeyState& aModKeyState)
 {
   if (mActiveDeadKey < 0) {
     return false;
   }
 
-  int32_t activeDeadKeyIndex = GetKeyIndex(mActiveDeadKey);
-  if (NS_WARN_IF(activeDeadKeyIndex < 0)) {
-    return false;
-  }
-
-  if (NS_WARN_IF(!IsPrintableCharKey(aNativeKey.mOriginalVirtualKeyCode))) {
+  if (NS_WARN_IF(!IsPrintableCharKey(mActiveDeadKey)) ||
+      NS_WARN_IF(!IsPrintableCharKey(aNativeKey.mOriginalVirtualKeyCode))) {
     return false;
   }
 
   UniCharsAndModifiers baseChars =
     GetUniCharsAndModifiers(aNativeKey.mOriginalVirtualKeyCode, aModKeyState);
   if (baseChars.IsEmpty() || !baseChars.mChars[0]) {
     return false;
   }
 
   char16_t compositeChar =
-    mVirtualKeys[activeDeadKeyIndex].GetCompositeChar(mDeadKeyShiftState,
-                                                      baseChars.mChars[0]);
+    GetCompositeChar(mActiveDeadKey, mDeadKeyShiftState, baseChars.mChars[0]);
   if (!compositeChar) {
     return false;
   }
 
   // Active dead-key and base character does produce exactly one composite
   // character.
   aNativeKey.mCommittedCharsAndModifiers.Append(compositeChar,
                                                 baseChars.mModifiers[0]);
@@ -3811,16 +3806,28 @@ KeyboardLayout::GetNativeUniCharsAndModi
   if (key < 0) {
     return UniCharsAndModifiers();
   }
   VirtualKey::ShiftState shiftState =
     VirtualKey::ModifierKeyStateToShiftState(aModKeyState);
   return mVirtualKeys[key].GetNativeUniChars(shiftState);
 }
 
+char16_t
+KeyboardLayout::GetCompositeChar(uint8_t aVirtualKeyOfDeadKey,
+                                 VirtualKey::ShiftState aShiftStateOfDeadKey,
+                                 char16_t aBaseChar) const
+{
+  int32_t key = GetKeyIndex(aVirtualKeyOfDeadKey);
+  if (key < 0) {
+    return 0;
+  }
+  return mVirtualKeys[key].GetCompositeChar(aShiftStateOfDeadKey, aBaseChar);
+}
+
 void
 KeyboardLayout::LoadLayout(HKL aLayout)
 {
   mIsPendingToRestoreKeyboardLayout = false;
 
   if (mKeyboardLayout == aLayout) {
     return;
   }

--- a/widget/windows/KeyboardLayout.h
+++ b/widget/windows/KeyboardLayout.h
@@ -663,16 +663,27 @@ private:
 
   /**
    * See the comment of GetUniCharsAndModifiers() below.
    */
   UniCharsAndModifiers GetUniCharsAndModifiers(
                          uint8_t aVirtualKey,
                          VirtualKey::ShiftState aShiftState) const;
 
+  /**
+   * GetCompositeChar() returns a composite character with dead character
+   * caused by aVirtualKeyOfDeadKey and aShiftStateOfDeadKey and a base
+   * character (aBaseChar).
+   * If the combination of the dead character and the base character doesn't
+   * cause a composite character, this returns 0.
+   */
+  char16_t GetCompositeChar(uint8_t aVirtualKeyOfDeadKey,
+                            VirtualKey::ShiftState aShiftStateOfDeadKey,
+                            char16_t aBaseChar) const;
+
 public:
   static KeyboardLayout* GetInstance();
   static void Shutdown();
   static void NotifyIdleServiceOfUserActivity();
 
   static bool IsPrintableCharKey(uint8_t aVirtualKey);
 
   /**