--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -109,16 +109,18 @@
 #include "HTMLImageElement.h"
 #include "mozilla/css/ImageLoader.h"
 #include "mozilla/layers/APZCTreeManager.h" // for layers::ZoomToRectBehavior
 #include "mozilla/dom/Promise.h"
 #include "mozilla/StyleSheetInlines.h"
 #include "mozilla/gfx/GPUProcessManager.h"
 #include "mozilla/dom/TimeoutManager.h"
 #include "mozilla/PreloadedStyleSheet.h"
+#include "mozilla/layers/WebRenderBridgeChild.h"
+#include "mozilla/layers/WebRenderLayerManager.h"
 
 #ifdef XP_WIN
 #undef GetClassName
 #endif
 
 using namespace mozilla;
 using namespace mozilla::dom;
 using namespace mozilla::ipc;
@@ -2517,16 +2519,24 @@ nsDOMWindowUtils::SetAsyncScrollOffset(n
   nsIWidget* widget = GetWidget();
   if (!widget) {
     return NS_ERROR_FAILURE;
   }
   LayerManager* manager = widget->GetLayerManager();
   if (!manager) {
     return NS_ERROR_FAILURE;
   }
+  if (WebRenderLayerManager* wrlm = manager->AsWebRenderLayerManager()) {
+    WebRenderBridgeChild* wrbc = wrlm->WrBridge();
+    if (!wrbc) {
+      return NS_ERROR_UNEXPECTED;
+    }
+    wrbc->SendSetAsyncScrollOffset(viewId, aX, aY);
+    return NS_OK;
+  }
   ShadowLayerForwarder* forwarder = manager->AsShadowForwarder();
   if (!forwarder || !forwarder->HasShadowManager()) {
     return NS_ERROR_UNEXPECTED;
   }
   forwarder->GetShadowManager()->SendSetAsyncScrollOffset(viewId, aX, aY);
   return NS_OK;
 }
 
@@ -2544,16 +2554,24 @@ nsDOMWindowUtils::SetAsyncZoom(nsIDOMNod
   nsIWidget* widget = GetWidget();
   if (!widget) {
     return NS_ERROR_FAILURE;
   }
   LayerManager* manager = widget->GetLayerManager();
   if (!manager) {
     return NS_ERROR_FAILURE;
   }
+  if (WebRenderLayerManager* wrlm = manager->AsWebRenderLayerManager()) {
+    WebRenderBridgeChild* wrbc = wrlm->WrBridge();
+    if (!wrbc) {
+      return NS_ERROR_UNEXPECTED;
+    }
+    wrbc->SendSetAsyncZoom(viewId, aValue);
+    return NS_OK;
+  }
   ShadowLayerForwarder* forwarder = manager->AsShadowForwarder();
   if (!forwarder || !forwarder->HasShadowManager()) {
     return NS_ERROR_UNEXPECTED;
   }
   forwarder->GetShadowManager()->SendSetAsyncZoom(viewId, aValue);
   return NS_OK;
 }
 
@@ -2570,16 +2588,24 @@ nsDOMWindowUtils::FlushApzRepaints(bool*
     *aOutResult = false;
     return NS_OK;
   }
   LayerManager* manager = widget->GetLayerManager();
   if (!manager) {
     *aOutResult = false;
     return NS_OK;
   }
+  if (WebRenderLayerManager* wrlm = manager->AsWebRenderLayerManager()) {
+    WebRenderBridgeChild* wrbc = wrlm->WrBridge();
+    if (!wrbc) {
+      return NS_ERROR_UNEXPECTED;
+    }
+    wrbc->SendFlushApzRepaints();
+    return NS_OK;
+  }
   ShadowLayerForwarder* forwarder = manager->AsShadowForwarder();
   if (!forwarder || !forwarder->HasShadowManager()) {
     *aOutResult = false;
     return NS_OK;
   }
   forwarder->GetShadowManager()->SendFlushApzRepaints();
   *aOutResult = true;
   return NS_OK;

--- a/gfx/layers/ipc/PWebRenderBridge.ipdl
+++ b/gfx/layers/ipc/PWebRenderBridge.ipdl
@@ -62,15 +62,21 @@ parent:
   async AddExternalImageId(ExternalImageId aImageId, CompositableHandle aHandle);
   async AddExternalImageIdForCompositable(ExternalImageId aImageId, CompositableHandle aHandle);
   async RemoveExternalImageId(ExternalImageId aImageId);
   async SetLayerObserverEpoch(uint64_t layerObserverEpoch);
   async ClearCachedResources();
   // Schedule a composite if one isn't already scheduled.
   async ForceComposite();
 
+  // Testing APIs. These correspond exactly to the equivalent APIs in
+  // PLayerTransaction - see those for documentation.
+  sync SetAsyncScrollOffset(ViewID scrollId, float x, float y);
+  sync SetAsyncZoom(ViewID scrollId, float zoom);
+  async FlushApzRepaints();
+
   async Shutdown();
 child:
   async __delete__();
 };
 
 } // layers
 } // mozilla

--- a/gfx/layers/wr/WebRenderBridgeParent.cpp
+++ b/gfx/layers/wr/WebRenderBridgeParent.cpp
@@ -1,16 +1,17 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set sw=4 ts=8 et tw=80 : */
 /* 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/. */
 
 #include "mozilla/layers/WebRenderBridgeParent.h"
 
+#include "apz/src/AsyncPanZoomController.h"
 #include "CompositableHost.h"
 #include "gfxPrefs.h"
 #include "GLContext.h"
 #include "GLContextProvider.h"
 #include "mozilla/Range.h"
 #include "mozilla/layers/AnimationHelper.h"
 #include "mozilla/layers/APZCTreeManager.h"
 #include "mozilla/layers/Compositor.h"
@@ -721,16 +722,60 @@ WebRenderBridgeParent::RecvForceComposit
 {
   if (mDestroyed) {
     return IPC_OK();
   }
   ScheduleComposition();
   return IPC_OK();
 }
 
+already_AddRefed<AsyncPanZoomController>
+WebRenderBridgeParent::GetTargetAPZC(const FrameMetrics::ViewID& aScrollId)
+{
+  RefPtr<AsyncPanZoomController> apzc;
+  if (CompositorBridgeParent* cbp = GetRootCompositorBridgeParent()) {
+    if (RefPtr<APZCTreeManager> apzctm = cbp->GetAPZCTreeManager()) {
+      apzc = apzctm->GetTargetAPZC(wr::AsUint64(mPipelineId), aScrollId);
+    }
+  }
+  return apzc.forget();
+}
+
+mozilla::ipc::IPCResult
+WebRenderBridgeParent::RecvSetAsyncScrollOffset(const FrameMetrics::ViewID& aScrollId,
+                                                const float& aX,
+                                                const float& aY)
+{
+  RefPtr<AsyncPanZoomController> apzc = GetTargetAPZC(aScrollId);
+  if (!apzc) {
+    return IPC_FAIL_NO_REASON(this);
+  }
+  apzc->SetTestAsyncScrollOffset(CSSPoint(aX, aY));
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult
+WebRenderBridgeParent::RecvSetAsyncZoom(const FrameMetrics::ViewID& aScrollId,
+                                        const float& aZoom)
+{
+  RefPtr<AsyncPanZoomController> apzc = GetTargetAPZC(aScrollId);
+  if (!apzc) {
+    return IPC_FAIL_NO_REASON(this);
+  }
+  apzc->SetTestAsyncZoom(LayerToParentLayerScale(aZoom));
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult
+WebRenderBridgeParent::RecvFlushApzRepaints()
+{
+  mCompositorBridge->FlushApzRepaints(wr::AsUint64(mPipelineId));
+  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
@@ -111,16 +111,23 @@ public:
   mozilla::ipc::IPCResult RecvAddExternalImageIdForCompositable(const ExternalImageId& aImageId,
                                                                 const CompositableHandle& aHandle) override;
   mozilla::ipc::IPCResult RecvRemoveExternalImageId(const ExternalImageId& aImageId) override;
   mozilla::ipc::IPCResult RecvSetLayerObserverEpoch(const uint64_t& aLayerObserverEpoch) override;
 
   mozilla::ipc::IPCResult RecvClearCachedResources() override;
   mozilla::ipc::IPCResult RecvForceComposite() 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;
+
   void ActorDestroy(ActorDestroyReason aWhy) override;
   void SetWebRenderProfilerEnabled(bool aEnabled);
 
   void Pause();
   bool Resume();
 
   void Destroy();
 
@@ -198,16 +205,20 @@ private:
   CompositorBridgeParent* GetRootCompositorBridgeParent() const;
 
   // Have APZ push the async scroll state to WR. Returns true if an APZ
   // animation is in effect and we need to schedule another composition.
   // If scrollbars need their transforms updated, the provided aTransformArray
   // is populated with the property update details.
   bool PushAPZStateToWR(nsTArray<WrTransformProperty>& aTransformArray);
 
+  // Helper method to get an APZC reference from a scroll id. Uses the layers
+  // id of this bridge, and may return null if the APZC wasn't found.
+  already_AddRefed<AsyncPanZoomController> GetTargetAPZC(const FrameMetrics::ViewID& aId);
+
 private:
   struct PendingTransactionId {
     PendingTransactionId(wr::Epoch aEpoch, uint64_t aId)
       : mEpoch(aEpoch)
       , mId(aId)
     {}
     wr::Epoch mEpoch;
     uint64_t mId;

--- a/ipc/ipdl/sync-messages.ini
+++ b/ipc/ipdl/sync-messages.ini
@@ -1041,16 +1041,20 @@ description =
 [PWebRenderBridge::UpdateImage]
 description =
 [PWebRenderBridge::DeleteImage]
 description =
 [PWebRenderBridge::DPSyncEnd]
 description =
 [PWebRenderBridge::DPGetSnapshot]
 description =
+[PWebRenderBridge::SetAsyncScrollOffset]
+description = test only
+[PWebRenderBridge::SetAsyncZoom]
+description = test only
 [PVRManager::GetSensorState]
 description =
 [PHal::GetCurrentBatteryInformation]
 description =
 [PHal::GetCurrentNetworkInformation]
 description =
 [PHal::GetScreenEnabled]
 description =