--- a/widget/InputData.cpp
+++ b/widget/InputData.cpp
@@ -302,16 +302,17 @@ PanGestureInput::ToWidgetWheelEvent(nsIW
   wheelEvent.modifiers = this->modifiers;
   wheelEvent.time = mTime;
   wheelEvent.timeStamp = mTimeStamp;
   wheelEvent.refPoint =
     RoundedToInt(ViewAs<LayoutDevicePixel>(mPanStartPoint,
       PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent));
   wheelEvent.buttons = 0;
   wheelEvent.deltaMode = nsIDOMWheelEvent::DOM_DELTA_PIXEL;
+  wheelEvent.mayHaveMomentum = true; // pan inputs may have momentum
   wheelEvent.isMomentum = IsMomentum();
   wheelEvent.lineOrPageDeltaX = mLineOrPageDeltaX;
   wheelEvent.lineOrPageDeltaY = mLineOrPageDeltaY;
   wheelEvent.deltaX = mPanDisplacement.x;
   wheelEvent.deltaY = mPanDisplacement.y;
   wheelEvent.mFlags.mHandledByAPZ = mHandledByAPZ;
   return wheelEvent;
 }
@@ -375,16 +376,17 @@ ScrollWheelInput::ScrollWheelInput(const
   mScrollMode(SCROLLMODE_INSTANT),
   mHandledByAPZ(aWheelEvent.mFlags.mHandledByAPZ),
   mDeltaX(aWheelEvent.deltaX),
   mDeltaY(aWheelEvent.deltaY),
   mLineOrPageDeltaX(aWheelEvent.lineOrPageDeltaX),
   mLineOrPageDeltaY(aWheelEvent.lineOrPageDeltaY),
   mUserDeltaMultiplierX(1.0),
   mUserDeltaMultiplierY(1.0),
+  mMayHaveMomentum(aWheelEvent.mayHaveMomentum),
   mIsMomentum(aWheelEvent.isMomentum)
 {
   mOrigin =
     ScreenPoint(ViewAs<ScreenPixel>(aWheelEvent.refPoint,
       PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent));
 }
 
 WidgetWheelEvent
@@ -394,16 +396,17 @@ ScrollWheelInput::ToWidgetWheelEvent(nsI
   wheelEvent.modifiers = this->modifiers;
   wheelEvent.time = mTime;
   wheelEvent.timeStamp = mTimeStamp;
   wheelEvent.refPoint =
     RoundedToInt(ViewAs<LayoutDevicePixel>(mOrigin,
       PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent));
   wheelEvent.buttons = 0;
   wheelEvent.deltaMode = DeltaModeForDeltaType(mDeltaType);
+  wheelEvent.mayHaveMomentum = mMayHaveMomentum;
   wheelEvent.isMomentum = mIsMomentum;
   wheelEvent.deltaX = mDeltaX;
   wheelEvent.deltaY = mDeltaY;
   wheelEvent.lineOrPageDeltaX = mLineOrPageDeltaX;
   wheelEvent.lineOrPageDeltaY = mLineOrPageDeltaY;
   wheelEvent.mFlags.mHandledByAPZ = mHandledByAPZ;
   return wheelEvent;
 }

--- a/widget/InputData.h
+++ b/widget/InputData.h
@@ -580,16 +580,17 @@ public:
      mHandledByAPZ(false),
      mDeltaX(aDeltaX),
      mDeltaY(aDeltaY),
      mLineOrPageDeltaX(0),
      mLineOrPageDeltaY(0),
      mScrollSeriesNumber(0),
      mUserDeltaMultiplierX(1.0),
      mUserDeltaMultiplierY(1.0),
+     mMayHaveMomentum(false),
      mIsMomentum(false)
   {}
 
   explicit ScrollWheelInput(const WidgetWheelEvent& aEvent);
 
   WidgetWheelEvent ToWidgetWheelEvent(nsIWidget* aWidget) const;
   bool TransformToLocal(const ScreenToParentLayerMatrix4x4& aTransform);
 
@@ -623,14 +624,15 @@ public:
   // Indicates the order in which this event was added to a transaction. The
   // first event is 1; if not a member of a transaction, this is 0.
   uint32_t mScrollSeriesNumber;
 
   // User-set delta multipliers.
   double mUserDeltaMultiplierX;
   double mUserDeltaMultiplierY;
 
+  bool mMayHaveMomentum;
   bool mIsMomentum;
 };
 
 } // namespace mozilla
 
 #endif // InputData_h__

--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -444,16 +444,17 @@ public:
 
   WidgetWheelEvent(bool aIsTrusted, EventMessage aMessage, nsIWidget* aWidget)
     : WidgetMouseEventBase(aIsTrusted, aMessage, aWidget, eWheelEventClass)
     , deltaX(0.0)
     , deltaY(0.0)
     , deltaZ(0.0)
     , deltaMode(nsIDOMWheelEvent::DOM_DELTA_PIXEL)
     , customizedByUserPrefs(false)
+    , mayHaveMomentum(false)
     , isMomentum(false)
     , mIsNoLineOrPageDelta(false)
     , lineOrPageDeltaX(0)
     , lineOrPageDeltaY(0)
     , scrollType(SCROLL_DEFAULT)
     , overflowDeltaX(0.0)
     , overflowDeltaY(0.0)
     , mViewPortIsOverscrolled(false)
@@ -494,16 +495,18 @@ public:
   uint32_t deltaMode;
 
   // Following members are for internal use only, not for DOM event.
 
   // If the delta values are computed from prefs, this value is true.
   // Otherwise, i.e., they are computed from native events, false.
   bool customizedByUserPrefs;
 
+  // true if the momentum events directly tied to this event may follow it.
+  bool mayHaveMomentum;
   // true if the event is caused by momentum.
   bool isMomentum;
 
   // If device event handlers don't know when they should set lineOrPageDeltaX
   // and lineOrPageDeltaY, this is true.  Otherwise, false.
   // If mIsNoLineOrPageDelta is true, ESM will generate
   // eLegacyMouseLineOrPageScroll events when accumulated delta values reach
   // a line height.
@@ -574,16 +577,17 @@ public:
   {
     AssignMouseEventBaseData(aEvent, aCopyTargets);
 
     deltaX = aEvent.deltaX;
     deltaY = aEvent.deltaY;
     deltaZ = aEvent.deltaZ;
     deltaMode = aEvent.deltaMode;
     customizedByUserPrefs = aEvent.customizedByUserPrefs;
+    mayHaveMomentum = aEvent.mayHaveMomentum;
     isMomentum = aEvent.isMomentum;
     mIsNoLineOrPageDelta = aEvent.mIsNoLineOrPageDelta;
     lineOrPageDeltaX = aEvent.lineOrPageDeltaX;
     lineOrPageDeltaY = aEvent.lineOrPageDeltaY;
     scrollType = aEvent.scrollType;
     overflowDeltaX = aEvent.overflowDeltaX;
     overflowDeltaY = aEvent.overflowDeltaY;
     mViewPortIsOverscrolled = aEvent.mViewPortIsOverscrolled;

--- a/widget/nsGUIEventIPC.h
+++ b/widget/nsGUIEventIPC.h
@@ -160,16 +160,17 @@ struct ParamTraits<mozilla::WidgetWheelE
   static void Write(Message* aMsg, const paramType& aParam)
   {
     WriteParam(aMsg, static_cast<mozilla::WidgetMouseEventBase>(aParam));
     WriteParam(aMsg, aParam.deltaX);
     WriteParam(aMsg, aParam.deltaY);
     WriteParam(aMsg, aParam.deltaZ);
     WriteParam(aMsg, aParam.deltaMode);
     WriteParam(aMsg, aParam.customizedByUserPrefs);
+    WriteParam(aMsg, aParam.mayHaveMomentum);
     WriteParam(aMsg, aParam.isMomentum);
     WriteParam(aMsg, aParam.mIsNoLineOrPageDelta);
     WriteParam(aMsg, aParam.lineOrPageDeltaX);
     WriteParam(aMsg, aParam.lineOrPageDeltaY);
     WriteParam(aMsg, static_cast<int32_t>(aParam.scrollType));
     WriteParam(aMsg, aParam.overflowDeltaX);
     WriteParam(aMsg, aParam.overflowDeltaY);
     WriteParam(aMsg, aParam.mViewPortIsOverscrolled);
@@ -182,16 +183,17 @@ struct ParamTraits<mozilla::WidgetWheelE
     bool rv =
       ReadParam(aMsg, aIter,
                 static_cast<mozilla::WidgetMouseEventBase*>(aResult)) &&
       ReadParam(aMsg, aIter, &aResult->deltaX) &&
       ReadParam(aMsg, aIter, &aResult->deltaY) &&
       ReadParam(aMsg, aIter, &aResult->deltaZ) &&
       ReadParam(aMsg, aIter, &aResult->deltaMode) &&
       ReadParam(aMsg, aIter, &aResult->customizedByUserPrefs) &&
+      ReadParam(aMsg, aIter, &aResult->mayHaveMomentum) &&
       ReadParam(aMsg, aIter, &aResult->isMomentum) &&
       ReadParam(aMsg, aIter, &aResult->mIsNoLineOrPageDelta) &&
       ReadParam(aMsg, aIter, &aResult->lineOrPageDeltaX) &&
       ReadParam(aMsg, aIter, &aResult->lineOrPageDeltaY) &&
       ReadParam(aMsg, aIter, &scrollType) &&
       ReadParam(aMsg, aIter, &aResult->overflowDeltaX) &&
       ReadParam(aMsg, aIter, &aResult->overflowDeltaY) &&
       ReadParam(aMsg, aIter, &aResult->mViewPortIsOverscrolled) &&