--- a/gfx/layers/wr/WebRenderBridgeParent.cpp
+++ b/gfx/layers/wr/WebRenderBridgeParent.cpp
@@ -302,20 +302,28 @@ WebRenderBridgeParent::HandleDPEnd(const
   // to early-return from RecvDPEnd without doing so.
   AutoWebRenderBridgeParentAsyncMessageSender autoAsyncMessageSender(this, &aToDestroy);
 
   ++mWrEpoch; // Update webrender epoch
   ProcessWebRenderCommands(aSize, aCommands, wr::NewEpoch(mWrEpoch),
                            dl, dlDesc, aux, auxDesc);
   HoldPendingTransactionId(mWrEpoch, aTransactionId);
 
+  mScrollData = aScrollData;
   // TODO: pass the WebRenderScrollData to APZ (this will happen in a future
   // patch)
 }
 
+const WebRenderScrollData&
+WebRenderBridgeParent::GetScrollData() const
+{
+  MOZ_ASSERT(mozilla::layers::CompositorThreadHolder::IsInCompositorThread());
+  return mScrollData;
+}
+
 mozilla::ipc::IPCResult
 WebRenderBridgeParent::RecvDPEnd(const gfx::IntSize& aSize,
                                  InfallibleTArray<WebRenderParentCommand>&& aCommands,
                                  InfallibleTArray<OpDestroy>&& aToDestroy,
                                  const uint64_t& aFwdTransactionId,
                                  const uint64_t& aTransactionId,
                                  const ByteBuffer& dl,
                                  const WrBuiltDisplayListDescriptor& dlDesc,

--- a/gfx/layers/wr/WebRenderBridgeParent.h
+++ b/gfx/layers/wr/WebRenderBridgeParent.h
@@ -155,16 +155,18 @@ public:
     aNotifications->AppendElements(Move(mImageCompositeNotifications));
   }
 
   uint32_t GetIdNameSpace()
   {
     return mIdNameSpace;
   }
 
+  const WebRenderScrollData& GetScrollData() const;
+
 private:
   virtual ~WebRenderBridgeParent();
 
   void DeleteOldImages();
   void ProcessWebRenderCommands(const gfx::IntSize &aSize, InfallibleTArray<WebRenderParentCommand>& commands, const wr::Epoch& aEpoch,
                                     const ByteBuffer& dl,
                                     const WrBuiltDisplayListDescriptor& dlDesc,
                                     const ByteBuffer& aux,
@@ -218,15 +220,18 @@ private:
 
   std::queue<PendingTransactionId> mPendingTransactionIds;
   uint32_t mWrEpoch;
   uint32_t mIdNameSpace;
 
   bool mPaused;
   bool mDestroyed;
 
+  // Can only be accessed on the compositor thread.
+  WebRenderScrollData mScrollData;
+
   static uint32_t sIdNameSpace;
 };
 
 } // namespace layers
 } // namespace mozilla
 
 #endif // mozilla_layers_WebRenderBridgeParent_h

--- a/gfx/layers/wr/WebRenderScrollDataWrapper.h
+++ b/gfx/layers/wr/WebRenderScrollDataWrapper.h
@@ -2,16 +2,18 @@
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef GFX_WEBRENDERSCROLLDATAWRAPPER_H
 #define GFX_WEBRENDERSCROLLDATAWRAPPER_H
 
 #include "FrameMetrics.h"
+#include "mozilla/layers/CompositorBridgeParent.h"
+#include "mozilla/layers/WebRenderBridgeParent.h"
 #include "mozilla/layers/WebRenderScrollData.h"
 
 namespace mozilla {
 namespace layers {
 
 /*
  * This class is a wrapper to walk through a WebRenderScrollData object, with
  * an exposed API that is template-compatible to LayerMetricsWrapper. This allows
@@ -152,16 +154,29 @@ public:
     // we compute the first index outside the subtree rooted at this node
     // (in |subtreeLastIndex|) and pass that in to the child wrapper to use as
     // its mContainingSubtreeLastIndex.
     if (mLayer->GetDescendantCount() > 0) {
       size_t prevSiblingIndex = mLayerIndex + 1 + mLayer->GetDescendantCount();
       size_t subtreeLastIndex = std::min(mContainingSubtreeLastIndex, prevSiblingIndex);
       return WebRenderScrollDataWrapper(mData, mLayerIndex + 1, subtreeLastIndex);
     }
+
+    // We've run out of descendants. But! If the original layer was a RefLayer,
+    // then it connects to another layer tree and we need to traverse that too.
+    // So return a WebRenderScrollDataWrapper for the root of the child layer
+    // tree.
+    if (mLayer->GetReferentId()) {
+      CompositorBridgeParent::LayerTreeState* lts =
+          CompositorBridgeParent::GetIndirectShadowTree(mLayer->GetReferentId().value());
+      if (lts && lts->mWrBridge) {
+        return WebRenderScrollDataWrapper(&(lts->mWrBridge->GetScrollData()));
+      }
+    }
+
     return WebRenderScrollDataWrapper();
   }
 
   WebRenderScrollDataWrapper GetPrevSibling() const
   {
     MOZ_ASSERT(IsValid());
 
     if (!AtTopLayer()) {