--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -183,59 +183,61 @@ private:
 public:
   typedef bool ReasonType;
   enum Reason : ReasonType
   {
     eReal,
     eSynthesized
   };
 
-  enum contextType
+  typedef bool ContextMenuTriggerType;
+  enum ContextMenuTrigger : ContextMenuTriggerType
   {
     eNormal,
     eContextMenuKey
   };
 
   enum exitType
   {
     eChild,
     eTopLevel
   };
 
 protected:
   WidgetMouseEvent()
     : reason(eReal)
+    , context(eNormal)
     , acceptActivation(false)
     , ignoreRootScrollFrame(false)
     , clickCount(0)
   {
   }
 
   WidgetMouseEvent(bool aIsTrusted, EventMessage aMessage, nsIWidget* aWidget,
                    EventClassID aEventClassID, Reason aReason)
     : WidgetMouseEventBase(aIsTrusted, aMessage, aWidget, aEventClassID)
     , reason(aReason)
+    , context(eNormal)
     , acceptActivation(false)
     , ignoreRootScrollFrame(false)
-    , context(eNormal)
     , exit(eChild)
     , clickCount(0)
   {
   }
 
 public:
   virtual WidgetMouseEvent* AsMouseEvent() override { return this; }
 
   WidgetMouseEvent(bool aIsTrusted, EventMessage aMessage, nsIWidget* aWidget,
-                   Reason aReason, contextType aContext = eNormal)
+                   Reason aReason, ContextMenuTrigger aContext = eNormal)
     : WidgetMouseEventBase(aIsTrusted, aMessage, aWidget, eMouseEventClass)
     , reason(aReason)
+    , context(aContext)
     , acceptActivation(false)
     , ignoreRootScrollFrame(false)
-    , context(aContext)
     , exit(eChild)
     , clickCount(0)
   {
     if (aMessage == eContextMenu) {
       button = (context == eNormal) ? eRightButton : eLeftButton;
     }
   }
 
@@ -258,23 +260,24 @@ public:
       new WidgetMouseEvent(false, mMessage, nullptr, reason, context);
     result->AssignMouseEventData(*this, true);
     result->mFlags = mFlags;
     return result;
   }
 
   Reason reason;
 
+  ContextMenuTrigger context;
+
   // Special return code for MOUSE_ACTIVATE to signal.
   // If the target accepts activation (1), or denies it (0).
   bool acceptActivation;
   // Whether the event should ignore scroll frame bounds during dispatch.
   bool ignoreRootScrollFrame;
 
-  contextType context : 4;
   exitType exit;
 
   /// The number of mouse clicks.
   uint32_t clickCount;
 
   void AssignMouseEventData(const WidgetMouseEvent& aEvent, bool aCopyTargets)
   {
     AssignMouseEventBaseData(aEvent, aCopyTargets);

--- a/widget/nsGUIEventIPC.h
+++ b/widget/nsGUIEventIPC.h
@@ -227,36 +227,38 @@ struct ParamTraits<mozilla::WidgetMouseE
 {
   typedef mozilla::WidgetMouseEvent paramType;
 
   static void Write(Message* aMsg, const paramType& aParam)
   {
     WriteParam(aMsg, static_cast<mozilla::WidgetMouseEventBase>(aParam));
     WriteParam(aMsg, aParam.ignoreRootScrollFrame);
     WriteParam(aMsg, static_cast<paramType::ReasonType>(aParam.reason));
-    WriteParam(aMsg, (uint8_t) aParam.context);
+    WriteParam(aMsg,
+               static_cast<paramType::ContextMenuTriggerType>(aParam.context));
     WriteParam(aMsg, (uint8_t) aParam.exit);
     WriteParam(aMsg, aParam.clickCount);
   }
 
   static bool Read(const Message* aMsg, void** aIter, paramType* aResult)
   {
     bool rv;
     paramType::ReasonType reason = 0;
-    uint8_t context = 0, exit = 0;
+    paramType::ContextMenuTriggerType contextMenuTrigger = 0;
+    uint8_t exit = 0;
     rv = ReadParam(aMsg, aIter,
                    static_cast<mozilla::WidgetMouseEventBase*>(aResult)) &&
          ReadParam(aMsg, aIter, &aResult->ignoreRootScrollFrame) &&
          ReadParam(aMsg, aIter, &reason) &&
-         ReadParam(aMsg, aIter, &context) &&
+         ReadParam(aMsg, aIter, &contextMenuTrigger) &&
          ReadParam(aMsg, aIter, &exit) &&
          ReadParam(aMsg, aIter, &aResult->clickCount);
     aResult->reason = static_cast<paramType::Reason>(reason);
     aResult->context =
-      static_cast<mozilla::WidgetMouseEvent::contextType>(context);
+      static_cast<paramType::ContextMenuTrigger>(contextMenuTrigger);
     aResult->exit = static_cast<mozilla::WidgetMouseEvent::exitType>(exit);
     return rv;
   }
 };
 
 
 template<>
 struct ParamTraits<mozilla::WidgetDragEvent>