--- a/gfx/ipc/GfxMessageUtils.h
+++ b/gfx/ipc/GfxMessageUtils.h
@@ -669,16 +669,24 @@ struct ParamTraits<nsRect>
   }
 };
 
 template<>
 struct ParamTraits<nsRegion>
   : RegionParamTraits<nsRegion, nsRect, nsRegion::RectIterator>
 {};
 
+template<>
+struct ParamTraits<mozilla::layers::FrameMetrics::ScrollOffsetUpdateType>
+  : public ContiguousEnumSerializer<
+             mozilla::layers::FrameMetrics::ScrollOffsetUpdateType,
+             mozilla::layers::FrameMetrics::ScrollOffsetUpdateType::eNone,
+             mozilla::layers::FrameMetrics::ScrollOffsetUpdateType::eSentinel>
+{};
+
 template <>
 struct ParamTraits<mozilla::layers::FrameMetrics>
 {
   typedef mozilla::layers::FrameMetrics paramType;
 
   static void Write(Message* aMsg, const paramType& aParam)
   {
     WriteParam(aMsg, aParam.mScrollId);
@@ -699,19 +707,19 @@ struct ParamTraits<mozilla::layers::Fram
     WriteParam(aMsg, aParam.mPresShellId);
     WriteParam(aMsg, aParam.mViewport);
     WriteParam(aMsg, aParam.mExtraResolution);
     WriteParam(aMsg, aParam.mBackgroundColor);
     WriteParam(aMsg, aParam.GetContentDescription());
     WriteParam(aMsg, aParam.mLineScrollAmount);
     WriteParam(aMsg, aParam.mPageScrollAmount);
     WriteParam(aMsg, aParam.mPaintRequestTime);
+    WriteParam(aMsg, aParam.mScrollUpdateType);
     WriteParam(aMsg, aParam.mIsRootContent);
     WriteParam(aMsg, aParam.mHasScrollgrab);
-    WriteParam(aMsg, aParam.mUpdateScrollOffset);
     WriteParam(aMsg, aParam.mDoSmoothScroll);
     WriteParam(aMsg, aParam.mUseDisplayPortMargins);
     WriteParam(aMsg, aParam.mAllowVerticalScrollWithWheel);
     WriteParam(aMsg, aParam.mIsLayersIdRoot);
     WriteParam(aMsg, aParam.mUsesContainerScrolling);
     WriteParam(aMsg, aParam.mIsScrollInfoLayer);
     WriteParam(aMsg, aParam.mForceDisableApz);
   }
@@ -760,19 +768,19 @@ struct ParamTraits<mozilla::layers::Fram
             ReadParam(aMsg, aIter, &aResult->mPresShellId) &&
             ReadParam(aMsg, aIter, &aResult->mViewport) &&
             ReadParam(aMsg, aIter, &aResult->mExtraResolution) &&
             ReadParam(aMsg, aIter, &aResult->mBackgroundColor) &&
             ReadContentDescription(aMsg, aIter, aResult) &&
             ReadParam(aMsg, aIter, &aResult->mLineScrollAmount) &&
             ReadParam(aMsg, aIter, &aResult->mPageScrollAmount) &&
             ReadParam(aMsg, aIter, &aResult->mPaintRequestTime) &&
+            ReadParam(aMsg, aIter, &aResult->mScrollUpdateType) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetIsRootContent) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetHasScrollgrab) &&
-            ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetUpdateScrollOffset) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetDoSmoothScroll) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetUseDisplayPortMargins) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetAllowVerticalScrollWithWheel) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetIsLayersIdRoot) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetUsesContainerScrolling) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetIsScrollInfoLayer) &&
             ReadBoolForBitfield(aMsg, aIter, aResult, &paramType::SetForceDisableApz));
   }

--- a/gfx/layers/FrameMetrics.h
+++ b/gfx/layers/FrameMetrics.h
@@ -36,16 +36,23 @@ struct FrameMetrics {
   friend struct IPC::ParamTraits<mozilla::layers::FrameMetrics>;
 public:
   // We use IDs to identify frames across processes.
   typedef uint64_t ViewID;
   static const ViewID NULL_SCROLL_ID;   // This container layer does not scroll.
   static const ViewID START_SCROLL_ID = 2;  // This is the ID that scrolling subframes
                                         // will begin at.
 
+  enum ScrollOffsetUpdateType : uint8_t {
+    eNone,          // The default; the scroll offset was not updated
+    eMainThread,    // The scroll offset was updated by the main thread.
+
+    eSentinel       // For IPC use only
+  };
+
   FrameMetrics()
     : mScrollId(NULL_SCROLL_ID)
     , mScrollParentId(NULL_SCROLL_ID)
     , mPresShellResolution(1)
     , mCompositionBounds(0, 0, 0, 0)
     , mDisplayPort(0, 0, 0, 0)
     , mCriticalDisplayPort(0, 0, 0, 0)
     , mScrollableRect(0, 0, 0, 0)
@@ -60,19 +67,19 @@ public:
     , mPresShellId(-1)
     , mViewport(0, 0, 0, 0)
     , mExtraResolution()
     , mBackgroundColor()
     , mContentDescription()
     , mLineScrollAmount(0, 0)
     , mPageScrollAmount(0, 0)
     , mPaintRequestTime()
+    , mScrollUpdateType(eNone)
     , mIsRootContent(false)
     , mHasScrollgrab(false)
-    , mUpdateScrollOffset(false)
     , mDoSmoothScroll(false)
     , mUseDisplayPortMargins(false)
     , mAllowVerticalScrollWithWheel(false)
     , mIsLayersIdRoot(false)
     , mUsesContainerScrolling(false)
     , mIsScrollInfoLayer(false)
     , mForceDisableApz(false)
   {
@@ -101,19 +108,19 @@ public:
            mPresShellId == aOther.mPresShellId &&
            mViewport.IsEqualEdges(aOther.mViewport) &&
            mExtraResolution == aOther.mExtraResolution &&
            mBackgroundColor == aOther.mBackgroundColor &&
            // don't compare mContentDescription
            mLineScrollAmount == aOther.mLineScrollAmount &&
            mPageScrollAmount == aOther.mPageScrollAmount &&
            mPaintRequestTime == aOther.mPaintRequestTime &&
+           mScrollUpdateType == aOther.mScrollUpdateType &&
            mIsRootContent == aOther.mIsRootContent &&
            mHasScrollgrab == aOther.mHasScrollgrab &&
-           mUpdateScrollOffset == aOther.mUpdateScrollOffset &&
            mDoSmoothScroll == aOther.mDoSmoothScroll &&
            mUseDisplayPortMargins == aOther.mUseDisplayPortMargins &&
            mAllowVerticalScrollWithWheel == aOther.mAllowVerticalScrollWithWheel &&
            mIsLayersIdRoot == aOther.mIsLayersIdRoot &&
            mUsesContainerScrolling == aOther.mUsesContainerScrolling &&
            mIsScrollInfoLayer == aOther.mIsScrollInfoLayer &&
            mForceDisableApz == aOther.mForceDisableApz;
   }
@@ -358,29 +365,29 @@ public:
 
   const CSSToParentLayerScale2D& GetZoom() const
   {
     return mZoom;
   }
 
   void SetScrollOffsetUpdated(uint32_t aScrollGeneration)
   {
-    mUpdateScrollOffset = true;
+    mScrollUpdateType = eMainThread;
     mScrollGeneration = aScrollGeneration;
   }
 
   void SetSmoothScrollOffsetUpdated(int32_t aScrollGeneration)
   {
     mDoSmoothScroll = true;
     mScrollGeneration = aScrollGeneration;
   }
 
   bool GetScrollOffsetUpdated() const
   {
-    return mUpdateScrollOffset;
+    return mScrollUpdateType != eNone;
   }
 
   bool GetDoSmoothScroll() const
   {
     return mDoSmoothScroll;
   }
 
   uint32_t GetScrollGeneration() const
@@ -714,26 +721,26 @@ private:
   LayoutDeviceIntSize mLineScrollAmount;
 
   // The value of GetPageScrollAmount(), for scroll frames.
   LayoutDeviceIntSize mPageScrollAmount;
 
   // The time at which the APZC last requested a repaint for this scrollframe.
   TimeStamp mPaintRequestTime;
 
+  // Whether mScrollOffset was updated by something other than the APZ code, and
+  // if the APZC receiving this metrics should update its local copy.
+  ScrollOffsetUpdateType mScrollUpdateType;
+
   // Whether or not this is the root scroll frame for the root content document.
   bool mIsRootContent:1;
 
   // Whether or not this frame is for an element marked 'scrollgrab'.
   bool mHasScrollgrab:1;
 
-  // Whether mScrollOffset was updated by something other than the APZ code, and
-  // if the APZC receiving this metrics should update its local copy.
-  bool mUpdateScrollOffset:1;
-
   // When mDoSmoothScroll, the scroll offset should be animated to
   // smoothly transition to mScrollOffset rather than be updated instantly.
   bool mDoSmoothScroll:1;
 
   // If this is true then we use the display port margins on this metrics,
   // otherwise use the display port rect.
   bool mUseDisplayPortMargins:1;
 
@@ -762,19 +769,16 @@ private:
   //    FrameMetrics::operator ==
   //    AsyncPanZoomController::NotifyLayersUpdated
   //    The ParamTraits specialization in GfxMessageUtils.h
   //
   // Please add new fields above this comment.
 
 
   // Private helpers for IPC purposes
-  void SetUpdateScrollOffset(bool aValue) {
-    mUpdateScrollOffset = aValue;
-  }
   void SetDoSmoothScroll(bool aValue) {
     mDoSmoothScroll = aValue;
   }
 };
 
 struct ScrollSnapInfo {
   ScrollSnapInfo()
     : mScrollSnapTypeX(NS_STYLE_SCROLL_SNAP_TYPE_NONE)