--- a/widget/windows/KeyboardLayout.cpp
+++ b/widget/windows/KeyboardLayout.cpp
@@ -3572,16 +3572,19 @@ KeyboardLayout::ComputeScanCodeForVirtua
            ::MapVirtualKeyEx(aVirtualKeyCode, MAPVK_VK_TO_VSC, GetLayout()));
 }
 
 bool
 KeyboardLayout::IsDeadKey(uint8_t aVirtualKey,
                           const ModifierKeyState& aModKeyState) const
 {
   int32_t virtualKeyIndex = GetKeyIndex(aVirtualKey);
+
+  // XXX KeyboardLayout class doesn't support unusual keyboard layout which
+  //     maps some function keys as dead keys.
   if (virtualKeyIndex < 0) {
     return false;
   }
 
   return mVirtualKeys[virtualKeyIndex].IsDeadKey(
            VirtualKey::ModifiersToShiftState(aModKeyState.GetModifiers()));
 }
 
@@ -3648,17 +3651,17 @@ KeyboardLayout::InitNativeKey(NativeKey&
     "At handling VK_PACKET, we shouldn't refer keyboard layout");
   MOZ_ASSERT(aNativeKey.mKeyNameIndex == KEY_NAME_INDEX_USE_STRING,
     "Printable key's key name index must be KEY_NAME_INDEX_USE_STRING");
 
   bool isKeyDown = aNativeKey.IsKeyDownMessage();
   uint8_t shiftState =
     VirtualKey::ModifiersToShiftState(aModKeyState.GetModifiers());
 
-  if (mVirtualKeys[virtualKeyIndex].IsDeadKey(shiftState)) {
+  if (IsDeadKey(virtualKey, aModKeyState)) {
     if ((isKeyDown && mActiveDeadKey < 0) ||
         (!isKeyDown && mActiveDeadKey == virtualKey)) {
       //  First dead key event doesn't generate characters.
       if (isKeyDown) {
         // Dead-key state activated at keydown.
         mActiveDeadKey = virtualKey;
         mDeadKeyShiftState = shiftState;
       }