--- a/gfx/layers/apz/src/HitTestingTreeNode.h
+++ b/gfx/layers/apz/src/HitTestingTreeNode.h
@@ -123,19 +123,25 @@ private:
   RefPtr<HitTestingTreeNode> mPrevSibling;
   RefPtr<HitTestingTreeNode> mParent;
 
   RefPtr<AsyncPanZoomController> mApzc;
   bool mIsPrimaryApzcHolder;
 
   uint64_t mLayersId;
 
+  // This is set for both scroll track and scroll thumb Container layers, and
+  // represents the scroll id of the scroll frame scrolled by the scrollbar.
   FrameMetrics::ViewID mScrollViewId;
+
+  // This is set for scroll thumb Container layers only.
   ScrollDirection mScrollDir;
   int32_t mScrollThumbLength;
+
+  // This is set for scroll track Container layers only.
   bool mIsScrollbarContainer;
 
   FrameMetrics::ViewID mFixedPosTarget;
 
   /* Let {L,M} be the {layer, scrollable metrics} pair that this node
    * corresponds to in the layer tree. mEventRegions contains the event regions
    * from L, in the case where event-regions are enabled. If event-regions are
    * disabled, it will contain the visible region of L, which we use as an