diff --git a/gfx/layers/FrameMetrics.h b/gfx/layers/FrameMetrics.h
index 2cf689faef5..37246c94013 100644
--- a/gfx/layers/FrameMetrics.h
+++ b/gfx/layers/FrameMetrics.h
@@ -681,6 +681,16 @@ private:
 
   // Whether or not the frame can be vertically scrolled with a mouse wheel.
   bool mAllowVerticalScrollWithWheel;
+
+  // WARNING!!!!
+  //
+  // When adding new fields to FrameMetrics, the following places should be
+  // updated to include them (as needed):
+  //    FrameMetrics::operator ==
+  //    AsyncPanZoomController::NotifyLayersUpdated
+  //    The ParamTraits specialization in GfxMessageUtils.h
+  //
+  // Please add new fields above this comment.
 };
 
 /**
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
index c1f75869f57..35e4eb69554 100644
--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -2855,6 +2855,8 @@ void AsyncPanZoomController::NotifyLayersUpdated(const FrameMetrics& aLayerMetri
     mFrameMetrics.SetPresShellResolution(aLayerMetrics.GetPresShellResolution());
     mFrameMetrics.SetCumulativeResolution(aLayerMetrics.GetCumulativeResolution());
     mFrameMetrics.SetHasScrollgrab(aLayerMetrics.GetHasScrollgrab());
+    mFrameMetrics.SetLineScrollAmount(aLayerMetrics.GetLineScrollAmount());
+    mFrameMetrics.SetPageScrollAmount(aLayerMetrics.GetPageScrollAmount());
 
     if (scrollOffsetUpdated) {
       APZC_LOG("%p updating scroll offset from %s to %s\n", this,