--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -111,17 +111,19 @@ public:
     eLeftButton   = 0,
     eMiddleButton = 1,
     eRightButton  = 2
   };
   // Pressed button ID of mousedown or mouseup event.
   // This is set only when pressing a button causes the event.
   ButtonType button;
 
-  enum buttonsFlag {
+  typedef int16_t ButtonsFlagType;
+  enum ButtonsFlag : ButtonsFlagType
+  {
     eNoButtonFlag     = 0x00,
     eLeftButtonFlag   = 0x01,
     eRightButtonFlag  = 0x02,
     eMiddleButtonFlag = 0x04,
     // typicall, "back" button being left side of 5-button
     // mice, see "buttons" attribute document of DOM3 Events.
     e4thButtonFlag    = 0x08,
     // typicall, "forward" button being right side of 5-button