--- a/gfx/layers/apz/src/HitTestingTreeNode.h
+++ b/gfx/layers/apz/src/HitTestingTreeNode.h
@@ -90,17 +90,16 @@ public:
   bool IsOutsideClip(const ParentLayerPoint& aPoint) const;
 
   /* Scrollbar info */
 
   void SetScrollbarData(FrameMetrics::ViewID aScrollViewId,
                         ScrollDirection aDir,
                         bool aIsScrollContainer);
   bool MatchesScrollDragMetrics(const AsyncDragMetrics& aDragMetrics) const;
-  LayerIntCoord GetScrollThumbLength() const;
   bool IsScrollbarNode() const;
   FrameMetrics::ViewID GetScrollTargetId() const;
 
   /* Fixed pos info */
 
   void SetFixedPosData(FrameMetrics::ViewID aFixedPosTarget);
   FrameMetrics::ViewID GetFixedPosTarget() const;
 

--- a/gfx/layers/wr/WebRenderScrollData.cpp
+++ b/gfx/layers/wr/WebRenderScrollData.cpp
@@ -40,19 +40,16 @@ WebRenderLayerScrollData::Initialize(Web
   mReferentId = aLayer->AsRefLayer()
       ? Some(aLayer->AsRefLayer()->GetReferentId())
       : Nothing();
   mEventRegionsOverride = aLayer->AsContainerLayer()
       ? aLayer->AsContainerLayer()->GetEventRegionsOverride()
       : EventRegionsOverride::NoOverride;
   mScrollbarDirection = aLayer->GetScrollThumbData().mDirection;
   mScrollbarTargetContainerId = aLayer->GetScrollbarTargetContainerId();
-  mScrollThumbLength = mScrollbarDirection == ScrollDirection::VERTICAL
-      ? aLayer->GetVisibleRegion().GetBounds().height
-      : aLayer->GetVisibleRegion().GetBounds().width;
   mIsScrollbarContainer = aLayer->IsScrollbarContainer();
   mFixedPosScrollContainerId = aLayer->GetFixedPositionScrollContainerId();
 }
 
 int32_t
 WebRenderLayerScrollData::GetDescendantCount() const
 {
   MOZ_ASSERT(mDescendantCount >= 0); // check that it was set

--- a/gfx/layers/wr/WebRenderScrollData.h
+++ b/gfx/layers/wr/WebRenderScrollData.h
@@ -50,17 +50,16 @@ public:
   bool IsScrollInfoLayer() const { return mIsScrollInfoLayer; }
   gfx::Matrix4x4 GetTransform() const { return mTransform; }
   bool GetTransformIsPerspective() const { return mTransformIsPerspective; }
   EventRegions GetEventRegions() const { return mEventRegions; }
   Maybe<uint64_t> GetReferentId() const { return mReferentId; }
   EventRegionsOverride GetEventRegionsOverride() const { return mEventRegionsOverride; }
   ScrollDirection GetScrollbarDirection() const { return mScrollbarDirection; }
   FrameMetrics::ViewID GetScrollbarTargetContainerId() const { return mScrollbarTargetContainerId; }
-  int32_t GetScrollThumbLength() const { return mScrollThumbLength; }
   bool IsScrollbarContainer() const { return mIsScrollbarContainer; }
   FrameMetrics::ViewID GetFixedPositionScrollContainerId() const { return mFixedPosScrollContainerId; }
 
   friend struct IPC::ParamTraits<WebRenderLayerScrollData>;
 
 private:
   // The number of descendants this layer has (not including the layer itself).
   // This is needed to reconstruct the depth-first layer tree traversal
@@ -80,17 +79,16 @@ private:
   bool mIsScrollInfoLayer;
   gfx::Matrix4x4 mTransform;
   bool mTransformIsPerspective;
   EventRegions mEventRegions;
   Maybe<uint64_t> mReferentId;
   EventRegionsOverride mEventRegionsOverride;
   ScrollDirection mScrollbarDirection;
   FrameMetrics::ViewID mScrollbarTargetContainerId;
-  int32_t mScrollThumbLength;
   bool mIsScrollbarContainer;
   FrameMetrics::ViewID mFixedPosScrollContainerId;
 };
 
 // Data needed by APZ, for the whole layer tree. One instance of this class
 // is created for each transaction sent over PWebRenderBridge. It is populated
 // with information from the WebRender layer tree on the client side and the
 // information is used by APZ on the parent side.
@@ -163,17 +161,16 @@ struct ParamTraits<mozilla::layers::WebR
     WriteParam(aMsg, aParam.mIsScrollInfoLayer);
     WriteParam(aMsg, aParam.mTransform);
     WriteParam(aMsg, aParam.mTransformIsPerspective);
     WriteParam(aMsg, aParam.mEventRegions);
     WriteParam(aMsg, aParam.mReferentId);
     WriteParam(aMsg, aParam.mEventRegionsOverride);
     WriteParam(aMsg, aParam.mScrollbarDirection);
     WriteParam(aMsg, aParam.mScrollbarTargetContainerId);
-    WriteParam(aMsg, aParam.mScrollThumbLength);
     WriteParam(aMsg, aParam.mIsScrollbarContainer);
     WriteParam(aMsg, aParam.mFixedPosScrollContainerId);
   }
 
   static bool
   Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
   {
     return ReadParam(aMsg, aIter, &aResult->mDescendantCount)
@@ -181,17 +178,16 @@ struct ParamTraits<mozilla::layers::WebR
         && ReadParam(aMsg, aIter, &aResult->mIsScrollInfoLayer)
         && ReadParam(aMsg, aIter, &aResult->mTransform)
         && ReadParam(aMsg, aIter, &aResult->mTransformIsPerspective)
         && ReadParam(aMsg, aIter, &aResult->mEventRegions)
         && ReadParam(aMsg, aIter, &aResult->mReferentId)
         && ReadParam(aMsg, aIter, &aResult->mEventRegionsOverride)
         && ReadParam(aMsg, aIter, &aResult->mScrollbarDirection)
         && ReadParam(aMsg, aIter, &aResult->mScrollbarTargetContainerId)
-        && ReadParam(aMsg, aIter, &aResult->mScrollThumbLength)
         && ReadParam(aMsg, aIter, &aResult->mIsScrollbarContainer)
         && ReadParam(aMsg, aIter, &aResult->mFixedPosScrollContainerId);
   }
 };
 
 template<>
 struct ParamTraits<mozilla::layers::WebRenderScrollData>
 {

--- a/gfx/layers/wr/WebRenderScrollDataWrapper.h
+++ b/gfx/layers/wr/WebRenderScrollDataWrapper.h
@@ -286,22 +286,16 @@ public:
   }
 
   FrameMetrics::ViewID GetScrollbarTargetContainerId() const
   {
     MOZ_ASSERT(IsValid());
     return mLayer->GetScrollbarTargetContainerId();
   }
 
-  int32_t GetScrollThumbLength() const
-  {
-    MOZ_ASSERT(IsValid());
-    return mLayer->GetScrollThumbLength();
-  }
-
   bool IsScrollbarContainer() const
   {
     MOZ_ASSERT(IsValid());
     return mLayer->IsScrollbarContainer();
   }
 
   FrameMetrics::ViewID GetFixedPositionScrollContainerId() const
   {