--- a/dom/ipc/TabChild.cpp
+++ b/dom/ipc/TabChild.cpp
@@ -1809,17 +1809,17 @@ TabChild::HandleLongTap(const CSSPoint& 
 }
 
 bool
 TabChild::NotifyAPZStateChange(const ViewID& aViewId,
                                const layers::GeckoContentController::APZStateChange& aChange,
                                const int& aArg)
 {
   mAPZEventState->ProcessAPZStateChange(GetDocument(), aViewId, aChange, aArg);
-  if (aChange == layers::GeckoContentController::APZStateChange::TransformEnd) {
+  if (aChange == layers::GeckoContentController::APZStateChange::eTransformEnd) {
     // This is used by tests to determine when the APZ is done doing whatever
     // it's doing. XXX generify this as needed when writing additional tests.
     nsCOMPtr<nsIObserverService> observerService = mozilla::services::GetObserverService();
     observerService->NotifyObservers(nullptr, "APZ:TransformEnd", nullptr);
   }
   return true;
 }
 

--- a/gfx/ipc/GfxMessageUtils.h
+++ b/gfx/ipc/GfxMessageUtils.h
@@ -1171,18 +1171,18 @@ struct ParamTraits<mozilla::gfx::FilterD
 };
 
 typedef mozilla::layers::GeckoContentController::APZStateChange APZStateChange;
 
 template <>
 struct ParamTraits<APZStateChange>
   : public ContiguousEnumSerializer<
              APZStateChange,
-             APZStateChange::TransformBegin,
-             APZStateChange::APZStateChangeSentinel>
+             APZStateChange::eTransformBegin,
+             APZStateChange::eSentinel>
 {};
 
 template<>
 struct ParamTraits<mozilla::layers::EventRegionsOverride>
   : public BitFlagsEnumSerializer<
             mozilla::layers::EventRegionsOverride,
             mozilla::layers::EventRegionsOverride::ALL_BITS>
 {};

--- a/gfx/layers/apz/public/GeckoContentController.h
+++ b/gfx/layers/apz/public/GeckoContentController.h
@@ -84,40 +84,43 @@ public:
    * APZ can then get the correct touch-sensitive region for each frame
    * directly from the layer.
    */
   virtual bool GetTouchSensitiveRegion(CSSRect* aOutRegion)
   {
     return false;
   }
 
-  enum APZStateChange {
+  enum class APZStateChange {
     /**
      * APZ started modifying the view (including panning, zooming, and fling).
      */
-    TransformBegin,
+    eTransformBegin,
     /**
      * APZ finished modifying the view.
      */
-    TransformEnd,
+    eTransformEnd,
     /**
      * APZ started a touch.
      * |aArg| is 1 if touch can be a pan, 0 otherwise.
      */
-    StartTouch,
+    eStartTouch,
     /**
      * APZ started a pan.
      */
-    StartPanning,
+    eStartPanning,
     /**
      * APZ finished processing a touch.
      * |aArg| is 1 if touch was a click, 0 otherwise.
      */
-    EndTouch,
-    APZStateChangeSentinel
+    eEndTouch,
+
+    // Sentinel value for IPC, this must be the last item in the enum and
+    // should not be used as an actual message value.
+    eSentinel
   };
   /**
    * General notices of APZ state changes for consumers.
    * |aGuid| identifies the APZC originating the state change.
    * |aChange| identifies the type of state change
    * |aArg| is used by some state changes to pass extra information (see
    *        the documentation for each state change above)
    */

--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -1088,17 +1088,17 @@ nsEventStatus AsyncPanZoomController::On
     case PAN_MOMENTUM:
       CurrentTouchBlock()->GetOverscrollHandoffChain()->CancelAnimations(ExcludeOverscroll);
       MOZ_FALLTHROUGH;
     case NOTHING: {
       mX.StartTouch(point.x, aEvent.mTime);
       mY.StartTouch(point.y, aEvent.mTime);
       if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
         controller->NotifyAPZStateChange(
-            GetGuid(), APZStateChange::StartTouch,
+            GetGuid(), APZStateChange::eStartTouch,
             CurrentTouchBlock()->GetOverscrollHandoffChain()->CanBePanned(this));
       }
       SetState(TOUCHING);
       break;
     }
     case TOUCHING:
     case PANNING:
     case PANNING_LOCKED_X:
@@ -2045,17 +2045,17 @@ nsEventStatus AsyncPanZoomController::Ge
     }
   }
   return nsEventStatus_eIgnore;
 }
 
 void AsyncPanZoomController::OnTouchEndOrCancel() {
   if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
     controller->NotifyAPZStateChange(
-        GetGuid(), APZStateChange::EndTouch, CurrentTouchBlock()->SingleTapOccurred());
+        GetGuid(), APZStateChange::eEndTouch, CurrentTouchBlock()->SingleTapOccurred());
   }
 }
 
 nsEventStatus AsyncPanZoomController::OnSingleTapUp(const TapGestureInput& aEvent) {
   APZC_LOG("%p got a single-tap-up in state %d\n", this, mState);
   // If mZoomConstraints.mAllowDoubleTapZoom is true we wait for a call to OnSingleTapConfirmed before
   // sending event to content
   if (!(mZoomConstraints.mAllowDoubleTapZoom && CurrentTouchBlock()->TouchActionAllowsDoubleTapZoom())) {
@@ -2268,17 +2268,17 @@ nsEventStatus AsyncPanZoomController::St
       SetState(PANNING);
     } else {
       HandlePanning(angle);
     }
   }
 
   if (IsInPanningState()) {
     if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
-      controller->NotifyAPZStateChange(GetGuid(), APZStateChange::StartPanning);
+      controller->NotifyAPZStateChange(GetGuid(), APZStateChange::eStartPanning);
     }
     return nsEventStatus_eConsumeNoDefault;
   }
   // Don't consume an event that didn't trigger a panning.
   return nsEventStatus_eIgnore;
 }
 
 void AsyncPanZoomController::UpdateWithTouchAtDevicePoint(const MultiTouchInput& aEvent) {
@@ -3644,27 +3644,27 @@ void AsyncPanZoomController::DispatchSta
     if (mNotificationBlockers > 0) {
       return;
     }
   }
 
   if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
     if (!IsTransformingState(aOldState) && IsTransformingState(aNewState)) {
       controller->NotifyAPZStateChange(
-          GetGuid(), APZStateChange::TransformBegin);
+          GetGuid(), APZStateChange::eTransformBegin);
 #if defined(XP_WIN) || defined(MOZ_WIDGET_GTK)
       // Let the compositor know about scroll state changes so it can manage
       // windowed plugins.
       if (mCompositorBridgeParent) {
         mCompositorBridgeParent->ScheduleHideAllPluginWindows();
       }
 #endif
     } else if (IsTransformingState(aOldState) && !IsTransformingState(aNewState)) {
       controller->NotifyAPZStateChange(
-          GetGuid(), APZStateChange::TransformEnd);
+          GetGuid(), APZStateChange::eTransformEnd);
 #if defined(XP_WIN) || defined(MOZ_WIDGET_GTK)
       if (mCompositorBridgeParent) {
         mCompositorBridgeParent->ScheduleShowAllPluginWindows();
       }
 #endif
     }
   }
 }

--- a/gfx/layers/apz/test/gtest/TestBasic.cpp
+++ b/gfx/layers/apz/test/gtest/TestBasic.cpp
@@ -201,27 +201,27 @@ TEST_F(APZCBasicTester, PanningTransform
   // go into overscroll, and finally snap-back to recover from overscroll.
   // Again, ensure we only get one set of state change notifications for
   // this entire procedure.
 
   MockFunction<void(std::string checkPointName)> check;
   {
     InSequence s;
     EXPECT_CALL(check, Call("Simple pan"));
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::StartTouch,_)).Times(1);
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::TransformBegin,_)).Times(1);
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::StartPanning,_)).Times(1);
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::EndTouch,_)).Times(1);
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::TransformEnd,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eStartTouch,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eTransformBegin,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eStartPanning,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eEndTouch,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eTransformEnd,_)).Times(1);
     EXPECT_CALL(check, Call("Complex pan"));
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::StartTouch,_)).Times(1);
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::TransformBegin,_)).Times(1);
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::StartPanning,_)).Times(1);
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::EndTouch,_)).Times(1);
-    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::TransformEnd,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eStartTouch,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eTransformBegin,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eStartPanning,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eEndTouch,_)).Times(1);
+    EXPECT_CALL(*mcc, NotifyAPZStateChange(_,GeckoContentController::APZStateChange::eTransformEnd,_)).Times(1);
     EXPECT_CALL(check, Call("Done"));
   }
 
   check.Call("Simple pan");
   ApzcPanNoFling(apzc, 50, 25);
   check.Call("Complex pan");
   Pan(apzc, 25, 45);
   apzc->AdvanceAnimationsUntilEnd();

--- a/gfx/layers/apz/util/APZEventState.cpp
+++ b/gfx/layers/apz/util/APZEventState.cpp
@@ -371,17 +371,17 @@ APZEventState::ProcessMouseEvent(const W
 void
 APZEventState::ProcessAPZStateChange(const nsCOMPtr<nsIDocument>& aDocument,
                                      ViewID aViewId,
                                      APZStateChange aChange,
                                      int aArg)
 {
   switch (aChange)
   {
-  case APZStateChange::TransformBegin:
+  case APZStateChange::eTransformBegin:
   {
     nsIScrollableFrame* sf = nsLayoutUtils::FindScrollableFrameFor(aViewId);
     if (sf) {
       sf->SetTransformingByAPZ(true);
     }
     nsIScrollbarMediator* scrollbarMediator = do_QueryFrame(sf);
     if (scrollbarMediator) {
       scrollbarMediator->ScrollbarActivityStarted();
@@ -392,17 +392,17 @@ APZEventState::ProcessAPZStateChange(con
       if (docshell && sf) {
         nsDocShell* nsdocshell = static_cast<nsDocShell*>(docshell.get());
         nsdocshell->NotifyAsyncPanZoomStarted();
       }
     }
     mActiveAPZTransforms++;
     break;
   }
-  case APZStateChange::TransformEnd:
+  case APZStateChange::eTransformEnd:
   {
     mActiveAPZTransforms--;
     nsIScrollableFrame* sf = nsLayoutUtils::FindScrollableFrameFor(aViewId);
     if (sf) {
       sf->SetTransformingByAPZ(false);
     }
     nsIScrollbarMediator* scrollbarMediator = do_QueryFrame(sf);
     if (scrollbarMediator) {
@@ -413,36 +413,37 @@ APZEventState::ProcessAPZStateChange(con
       nsCOMPtr<nsIDocShell> docshell(aDocument->GetDocShell());
       if (docshell && sf) {
         nsDocShell* nsdocshell = static_cast<nsDocShell*>(docshell.get());
         nsdocshell->NotifyAsyncPanZoomStopped();
       }
     }
     break;
   }
-  case APZStateChange::StartTouch:
+  case APZStateChange::eStartTouch:
   {
     mActiveElementManager->HandleTouchStart(aArg);
     break;
   }
-  case APZStateChange::StartPanning:
+  case APZStateChange::eStartPanning:
   {
     // The user started to pan, so we don't want anything to be :active.
     mActiveElementManager->ClearActivation();
     break;
   }
-  case APZStateChange::EndTouch:
+  case APZStateChange::eEndTouch:
   {
     mEndTouchIsClick = aArg;
     mActiveElementManager->HandleTouchEnd();
     break;
   }
-  default:
-    // APZStateChange has a 'sentinel' value, and the compiler complains
-    // if an enumerator is not handled and there is no 'default' case.
+  case APZStateChange::eSentinel:
+    // Should never happen, but we want this case branch to stop the compiler
+    // whining about unhandled values.
+    MOZ_ASSERT(false);
     break;
   }
 }
 
 void
 APZEventState::ProcessClusterHit()
 {
   // If we hit a cluster of links then we shouldn't activate any of them,

--- a/widget/android/AndroidContentController.cpp
+++ b/widget/android/AndroidContentController.cpp
@@ -122,21 +122,21 @@ AndroidContentController::NotifyAPZState
 {
   // This function may get invoked twice, if the first invocation is not on
   // the main thread then the ChromeProcessController version of this function
   // will redispatch to the main thread. We want to make sure that our handling
   // only happens on the main thread.
   ChromeProcessController::NotifyAPZStateChange(aGuid, aChange, aArg);
   if (NS_IsMainThread()) {
     nsCOMPtr<nsIObserverService> observerService = mozilla::services::GetObserverService();
-    if (aChange == layers::GeckoContentController::APZStateChange::TransformEnd) {
+    if (aChange == layers::GeckoContentController::APZStateChange::eTransformEnd) {
       // This is used by tests to determine when the APZ is done doing whatever
       // it's doing. XXX generify this as needed when writing additional tests.
       observerService->NotifyObservers(nullptr, "APZ:TransformEnd", nullptr);
       observerService->NotifyObservers(nullptr, "PanZoom:StateChange", MOZ_UTF16("NOTHING"));
-    } else if (aChange == layers::GeckoContentController::APZStateChange::TransformBegin) {
+    } else if (aChange == layers::GeckoContentController::APZStateChange::eTransformBegin) {
       observerService->NotifyObservers(nullptr, "PanZoom:StateChange", MOZ_UTF16("PANNING"));
     }
   }
 }
 
 } // namespace widget
 } // namespace mozilla