--- a/gfx/layers/LayerMetricsWrapper.h
+++ b/gfx/layers/LayerMetricsWrapper.h
@@ -406,25 +406,16 @@ public:
 
   FrameMetrics::ViewID GetScrollbarTargetContainerId() const
   {
     MOZ_ASSERT(IsValid());
 
     return mLayer->GetScrollbarTargetContainerId();
   }
 
-  int32_t GetScrollThumbLength() const
-  {
-    if (GetScrollbarDirection() == ScrollDirection::VERTICAL) {
-      return mLayer->GetVisibleRegion().GetBounds().height;
-    } else {
-      return mLayer->GetVisibleRegion().GetBounds().width;
-    }
-  }
-
   bool IsScrollbarContainer() const
   {
     MOZ_ASSERT(IsValid());
     return mLayer->IsScrollbarContainer();
   }
 
   FrameMetrics::ViewID GetFixedPositionScrollContainerId() const
   {

--- a/gfx/layers/apz/src/APZCTreeManager.cpp
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -490,17 +490,16 @@ APZCTreeManager::PrepareNodeForLayer(con
     AttachNodeToTree(node, aParent, aNextSibling);
     node->SetHitTestData(
         GetEventRegions(aLayer),
         aLayer.GetTransformTyped(),
         aLayer.GetClipRect() ? Some(ParentLayerIntRegion(*aLayer.GetClipRect())) : Nothing(),
         GetEventRegionsOverride(aParent, aLayer));
     node->SetScrollbarData(aLayer.GetScrollbarTargetContainerId(),
                            aLayer.GetScrollbarDirection(),
-                           aLayer.GetScrollThumbLength(),
                            aLayer.IsScrollbarContainer());
     node->SetFixedPosData(aLayer.GetFixedPositionScrollContainerId());
     return node;
   }
 
   AsyncPanZoomController* apzc = nullptr;
   // If we get here, aLayer is a scrollable layer and somebody
   // has registered a GeckoContentController for it, so we need to ensure
@@ -680,17 +679,16 @@ APZCTreeManager::PrepareNodeForLayer(con
         GetEventRegionsOverride(aParent, aLayer));
   }
 
   // Note: if layer properties must be propagated to nodes, RecvUpdate in
   // LayerTransactionParent.cpp must ensure that APZ will be notified
   // when those properties change.
   node->SetScrollbarData(aLayer.GetScrollbarTargetContainerId(),
                          aLayer.GetScrollbarDirection(),
-                         aLayer.GetScrollThumbLength(),
                          aLayer.IsScrollbarContainer());
   node->SetFixedPosData(aLayer.GetFixedPositionScrollContainerId());
   return node;
 }
 
 template<typename PanGestureOrScrollWheelInput>
 static bool
 WillHandleInput(const PanGestureOrScrollWheelInput& aPanInput)

--- a/gfx/layers/apz/src/HitTestingTreeNode.cpp
+++ b/gfx/layers/apz/src/HitTestingTreeNode.cpp
@@ -23,17 +23,16 @@ namespace layers {
 HitTestingTreeNode::HitTestingTreeNode(AsyncPanZoomController* aApzc,
                                        bool aIsPrimaryHolder,
                                        uint64_t aLayersId)
   : mApzc(aApzc)
   , mIsPrimaryApzcHolder(aIsPrimaryHolder)
   , mLayersId(aLayersId)
   , mScrollViewId(FrameMetrics::NULL_SCROLL_ID)
   , mScrollDir(ScrollDirection::NONE)
-  , mScrollThumbLength(0)
   , mIsScrollbarContainer(false)
   , mFixedPosTarget(FrameMetrics::NULL_SCROLL_ID)
   , mOverride(EventRegionsOverride::NoOverride)
 {
 if (mIsPrimaryApzcHolder) {
     MOZ_ASSERT(mApzc);
   }
   MOZ_ASSERT(!mApzc || mApzc->GetLayersId() == mLayersId);
@@ -92,41 +91,33 @@ HitTestingTreeNode::SetLastChild(HitTest
       aChild->SetApzcParent(parent);
     }
   }
 }
 
 void
 HitTestingTreeNode::SetScrollbarData(FrameMetrics::ViewID aScrollViewId,
                                      ScrollDirection aDir,
-                                     int32_t aScrollThumbLength,
                                      bool aIsScrollContainer)
 {
   mScrollViewId = aScrollViewId;
   mScrollDir = aDir;
-  mScrollThumbLength = aScrollThumbLength;
   mIsScrollbarContainer = aIsScrollContainer;
 }
 
 bool
 HitTestingTreeNode::MatchesScrollDragMetrics(const AsyncDragMetrics& aDragMetrics) const
 {
   return ((mScrollDir == ScrollDirection::HORIZONTAL &&
            aDragMetrics.mDirection == AsyncDragMetrics::HORIZONTAL) ||
           (mScrollDir == ScrollDirection::VERTICAL &&
            aDragMetrics.mDirection == AsyncDragMetrics::VERTICAL)) &&
          mScrollViewId == aDragMetrics.mViewId;
 }
 
-LayerIntCoord
-HitTestingTreeNode::GetScrollThumbLength() const
-{
-  return mScrollThumbLength;
-}
-
 bool
 HitTestingTreeNode::IsScrollbarNode() const
 {
   return mIsScrollbarContainer || (mScrollDir != ScrollDirection::NONE);
 }
 
 FrameMetrics::ViewID
 HitTestingTreeNode::GetScrollTargetId() const

--- a/gfx/layers/apz/src/HitTestingTreeNode.h
+++ b/gfx/layers/apz/src/HitTestingTreeNode.h
@@ -88,17 +88,16 @@ public:
                       const Maybe<ParentLayerIntRegion>& aClipRegion,
                       const EventRegionsOverride& aOverride);
   bool IsOutsideClip(const ParentLayerPoint& aPoint) const;
 
   /* Scrollbar info */
 
   void SetScrollbarData(FrameMetrics::ViewID aScrollViewId,
                         ScrollDirection aDir,
-                        int32_t aScrollThumbLength,
                         bool aIsScrollContainer);
   bool MatchesScrollDragMetrics(const AsyncDragMetrics& aDragMetrics) const;
   LayerIntCoord GetScrollThumbLength() const;
   bool IsScrollbarNode() const;
   FrameMetrics::ViewID GetScrollTargetId() const;
 
   /* Fixed pos info */
 
@@ -130,17 +129,16 @@ private:
   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