--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -99,16 +99,17 @@ public:
   virtual WidgetEvent* Duplicate() const override
   {
     MOZ_CRASH("WidgetMouseEventBase must not be most-subclass");
   }
 
   /// The possible related target
   nsCOMPtr<nsISupports> relatedTarget;
 
+  // Defined ButtonTypeType as an int16_t.
   typedef int16_t ButtonTypeType;
   enum ButtonType : ButtonTypeType
   {
     eLeftButton   = 0,
     eMiddleButton = 1,
     eRightButton  = 2
   };
   // Pressed button ID of mousedown or mouseup event.