--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -3903,23 +3903,30 @@ NS_IMETHODIMP
 nsDOMWindowUtils::GetCompositorAPZTestData(JSContext* aContext,
                                            JS::MutableHandleValue aOutCompositorTestData)
 {
   if (nsIWidget* widget = GetWidget()) {
     RefPtr<LayerManager> lm = widget->GetLayerManager();
     if (!lm) {
       return NS_OK;
     }
+    APZTestData compositorSideData;
     if (ClientLayerManager* clm = lm->AsClientLayerManager()) {
-      APZTestData compositorSideData;
       clm->GetCompositorSideAPZTestData(&compositorSideData);
-      if (!compositorSideData.ToJS(aOutCompositorTestData, aContext)) {
+    } else if (WebRenderLayerManager* wrlm = lm->AsWebRenderLayerManager()) {
+      if (!wrlm->WrBridge()) {
+        return NS_ERROR_UNEXPECTED;
+      }
+      if (!wrlm->WrBridge()->SendGetAPZTestData(&compositorSideData)) {
         return NS_ERROR_FAILURE;
       }
     }
+    if (!compositorSideData.ToJS(aOutCompositorTestData, aContext)) {
+      return NS_ERROR_FAILURE;
+    }
   }
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDOMWindowUtils::PostRestyleSelfEvent(nsIDOMElement* aElement)
 {

--- a/gfx/layers/ipc/PWebRenderBridge.ipdl
+++ b/gfx/layers/ipc/PWebRenderBridge.ipdl
@@ -9,16 +9,17 @@ include LayersSurfaces;
 include LayersMessages;
 include "mozilla/GfxMessageUtils.h";
 include "mozilla/layers/WebRenderMessageUtils.h";
 
 include WebRenderMessages;
 include protocol PCompositorBridge;
 include protocol PTexture;
 
+using mozilla::layers::APZTestData from "mozilla/layers/APZTestData.h";
 using struct mozilla::layers::ScrollableLayerGuid from "FrameMetrics.h";
 using struct mozilla::layers::TextureInfo from "mozilla/layers/CompositorTypes.h";
 using mozilla::layers::CompositableHandle from "mozilla/layers/LayersTypes.h";
 using mozilla::wr::ByteBuffer from "mozilla/webrender/WebRenderTypes.h";
 using mozilla::wr::ExternalImageId from "mozilla/webrender/WebRenderTypes.h";
 using mozilla::wr::ImageKey from "mozilla/webrender/WebRenderTypes.h";
 using mozilla::wr::FontKey from "mozilla/webrender/WebRenderTypes.h";
 using mozilla::wr::PipelineId from "mozilla/webrender/WebRenderTypes.h";
@@ -71,16 +72,17 @@ parent:
 
   // These correspond exactly to the equivalent APIs in PLayerTransaction -
   // see those for documentation.
   async SetConfirmedTargetAPZC(uint64_t aInputBlockId, ScrollableLayerGuid[] aTargets);
   // More copied from PLayerTransaction, but these are only used for testing.
   sync SetAsyncScrollOffset(ViewID scrollId, float x, float y);
   sync SetAsyncZoom(ViewID scrollId, float zoom);
   async FlushApzRepaints();
+  sync GetAPZTestData() returns (APZTestData data);
 
   async Shutdown();
 child:
   async __delete__();
 };
 
 } // layers
 } // mozilla

--- a/gfx/layers/wr/WebRenderBridgeParent.cpp
+++ b/gfx/layers/wr/WebRenderBridgeParent.cpp
@@ -763,16 +763,23 @@ WebRenderBridgeParent::RecvSetAsyncZoom(
 
 mozilla::ipc::IPCResult
 WebRenderBridgeParent::RecvFlushApzRepaints()
 {
   mCompositorBridge->FlushApzRepaints(GetLayersId());
   return IPC_OK();
 }
 
+mozilla::ipc::IPCResult
+WebRenderBridgeParent::RecvGetAPZTestData(APZTestData* aOutData)
+{
+  mCompositorBridge->GetAPZTestData(GetLayersId(), aOutData);
+  return IPC_OK();
+}
+
 void
 WebRenderBridgeParent::ActorDestroy(ActorDestroyReason aWhy)
 {
   Destroy();
 }
 
 void
 WebRenderBridgeParent::SampleAnimations(nsTArray<WrOpacityProperty>& aOpacityArray,

--- a/gfx/layers/wr/WebRenderBridgeParent.h
+++ b/gfx/layers/wr/WebRenderBridgeParent.h
@@ -121,16 +121,17 @@ public:
   mozilla::ipc::IPCResult RecvSetConfirmedTargetAPZC(const uint64_t& aBlockId,
                                                      nsTArray<ScrollableLayerGuid>&& aTargets) override;
   mozilla::ipc::IPCResult RecvSetAsyncScrollOffset(const FrameMetrics::ViewID& aScrollId,
                                                    const float& aX,
                                                    const float& aY) override;
   mozilla::ipc::IPCResult RecvSetAsyncZoom(const FrameMetrics::ViewID& aScrollId,
                                            const float& aZoom) override;
   mozilla::ipc::IPCResult RecvFlushApzRepaints() override;
+  mozilla::ipc::IPCResult RecvGetAPZTestData(APZTestData* data) override;
 
   void ActorDestroy(ActorDestroyReason aWhy) override;
   void SetWebRenderProfilerEnabled(bool aEnabled);
 
   void Pause();
   bool Resume();
 
   void Destroy();