--- a/gfx/layers/apz/src/HitTestingTreeNode.cpp
+++ b/gfx/layers/apz/src/HitTestingTreeNode.cpp
@@ -108,19 +108,25 @@ HitTestingTreeNode::MatchesScrollDragMet
   return ((mScrollThumbData.mDirection == ScrollDirection::HORIZONTAL &&
            aDragMetrics.mDirection == AsyncDragMetrics::HORIZONTAL) ||
           (mScrollThumbData.mDirection == ScrollDirection::VERTICAL &&
            aDragMetrics.mDirection == AsyncDragMetrics::VERTICAL)) &&
          mScrollViewId == aDragMetrics.mViewId;
 }
 
 bool
+HitTestingTreeNode::IsScrollThumbNode() const
+{
+  return mScrollThumbData.mDirection != ScrollDirection::NONE;
+}
+
+bool
 HitTestingTreeNode::IsScrollbarNode() const
 {
-  return mIsScrollbarContainer || (mScrollThumbData.mDirection != ScrollDirection::NONE);
+  return mIsScrollbarContainer || IsScrollThumbNode();
 }
 
 FrameMetrics::ViewID
 HitTestingTreeNode::GetScrollTargetId() const
 {
   return mScrollViewId;
 }
 

--- a/gfx/layers/apz/src/HitTestingTreeNode.h
+++ b/gfx/layers/apz/src/HitTestingTreeNode.h
@@ -90,17 +90,18 @@ public:
   bool IsOutsideClip(const ParentLayerPoint& aPoint) const;
 
   /* Scrollbar info */
 
   void SetScrollbarData(FrameMetrics::ViewID aScrollViewId,
                         const ScrollThumbData& aThumbData,
                         bool aIsScrollContainer);
   bool MatchesScrollDragMetrics(const AsyncDragMetrics& aDragMetrics) const;
-  bool IsScrollbarNode() const;
+  bool IsScrollbarNode() const;  // Scroll thumb or scrollbar container layer.
+  bool IsScrollThumbNode() const;  // Scroll thumb container layer.
   FrameMetrics::ViewID GetScrollTargetId() const;
   const ScrollThumbData& GetScrollThumbData() const;
 
   /* Fixed pos info */
 
   void SetFixedPosData(FrameMetrics::ViewID aFixedPosTarget);
   FrameMetrics::ViewID GetFixedPosTarget() const;