--- a/gfx/ipc/GPUProcessManager.cpp
+++ b/gfx/ipc/GPUProcessManager.cpp
@@ -994,22 +994,16 @@ GPUProcessManager::CreateContentVideoDec
     return;
   }
 
   mGPUChild->SendNewContentVideoDecoderManager(Move(parentPipe));
 
   *aOutEndpoint = Move(childPipe);
 }
 
-already_AddRefed<IAPZCTreeManager>
-GPUProcessManager::GetAPZCTreeManagerForLayers(uint64_t aLayersId)
-{
-  return CompositorBridgeParent::GetAPZCTreeManager(aLayersId);
-}
-
 void
 GPUProcessManager::MapLayerTreeId(uint64_t aLayersId, base::ProcessId aOwningId)
 {
   LayerTreeOwnerTracker::Get()->Map(aLayersId, aOwningId);
 
   if (EnsureGPUReady()) {
     mGPUChild->SendAddLayerTreeIdMapping(LayerTreeIdMapping(aLayersId, aOwningId));
   }

--- a/gfx/ipc/GPUProcessManager.h
+++ b/gfx/ipc/GPUProcessManager.h
@@ -102,20 +102,16 @@ public:
   bool CreateContentBridges(
     base::ProcessId aOtherProcess,
     mozilla::ipc::Endpoint<PCompositorManagerChild>* aOutCompositor,
     mozilla::ipc::Endpoint<PImageBridgeChild>* aOutImageBridge,
     mozilla::ipc::Endpoint<PVRManagerChild>* aOutVRBridge,
     mozilla::ipc::Endpoint<dom::PVideoDecoderManagerChild>* aOutVideoManager,
     nsTArray<uint32_t>* aNamespaces);
 
-  // This returns a reference to the APZCTreeManager to which
-  // pan/zoom-related events can be sent.
-  already_AddRefed<IAPZCTreeManager> GetAPZCTreeManagerForLayers(uint64_t aLayersId);
-
   // Maps the layer tree and process together so that aOwningPID is allowed
   // to access aLayersId across process.
   void MapLayerTreeId(uint64_t aLayersId, base::ProcessId aOwningId);
 
   // Release compositor-thread resources referred to by |aID|.
   //
   // Must run on the content main thread.
   void UnmapLayerTreeId(uint64_t aLayersId, base::ProcessId aOwningId);

--- a/gfx/ipc/InProcessCompositorSession.cpp
+++ b/gfx/ipc/InProcessCompositorSession.cpp
@@ -1,18 +1,16 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=8 sts=2 et sw=2 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 "InProcessCompositorSession.h"
 
-// so we can cast an APZCTreeManager to an IAPZCTreeManager
-#include "mozilla/layers/APZCTreeManager.h"
 #include "mozilla/layers/IAPZCTreeManager.h"
 #include "nsBaseWidget.h"
 
 namespace mozilla {
 namespace layers {
 
 InProcessCompositorSession::InProcessCompositorSession(widget::CompositorWidget* aWidget,
                                                        nsBaseWidget* baseWidget,

--- a/gfx/layers/ipc/CompositorBridgeParent.cpp
+++ b/gfx/layers/ipc/CompositorBridgeParent.cpp
@@ -1853,28 +1853,28 @@ CompositorBridgeParent::SetControllerFor
   // This ref is adopted by UpdateControllerForLayersId().
   aController->AddRef();
   CompositorLoop()->PostTask(NewRunnableFunction("UpdateControllerForLayersIdRunnable",
                                                  &UpdateControllerForLayersId,
                                                  aLayersId,
                                                  aController));
 }
 
-/*static*/ already_AddRefed<APZCTreeManager>
+/*static*/ already_AddRefed<IAPZCTreeManager>
 CompositorBridgeParent::GetAPZCTreeManager(uint64_t aLayersId)
 {
   EnsureLayerTreeMapReady();
   MonitorAutoLock lock(*sIndirectLayerTreesLock);
   LayerTreeMap::iterator cit = sIndirectLayerTrees.find(aLayersId);
   if (sIndirectLayerTrees.end() == cit) {
     return nullptr;
   }
   LayerTreeState* lts = &cit->second;
 
-  RefPtr<APZCTreeManager> apzctm = lts->mParent
+  RefPtr<IAPZCTreeManager> apzctm = lts->mParent
                                    ? lts->mParent->mApzcTreeManager.get()
                                    : nullptr;
   return apzctm.forget();
 }
 
 #if defined(MOZ_GECKO_PROFILER)
 static void
 InsertVsyncProfilerMarker(TimeStamp aVsyncTimestamp)

--- a/gfx/layers/ipc/CompositorBridgeParent.h
+++ b/gfx/layers/ipc/CompositorBridgeParent.h
@@ -67,16 +67,17 @@ class APZCTreeManagerParent;
 class AsyncCompositionManager;
 class AsyncImagePipelineManager;
 class Compositor;
 class CompositorAnimationStorage;
 class CompositorBridgeParent;
 class CompositorManagerParent;
 class CompositorVsyncScheduler;
 class HostLayerManager;
+class IAPZCTreeManager;
 class LayerTransactionParent;
 class PAPZParent;
 class CrossProcessCompositorBridgeParent;
 class CompositorThreadHolder;
 class InProcessCompositorSession;
 class WebRenderBridgeParent;
 
 struct ScopedLayerTreeRegistration
@@ -480,20 +481,21 @@ private:
   void Initialize();
 
   /**
    * Called during destruction in order to release resources as early as possible.
    */
   void StopAndClearResources();
 
   /**
-   * This returns a reference to the APZCTreeManager to which
-   * pan/zoom-related events can be sent.
+   * This returns a reference to the IAPZCTreeManager "controller subinterface"
+   * to which pan/zoom-related events can be sent. The controller subinterface
+   * doesn't expose any sampler-thread APZCTreeManager methods.
    */
-  static already_AddRefed<APZCTreeManager> GetAPZCTreeManager(uint64_t aLayersId);
+  static already_AddRefed<IAPZCTreeManager> GetAPZCTreeManager(uint64_t aLayersId);
 
   /**
    * Release compositor-thread resources referred to by |aID|.
    *
    * Must run on the content main thread.
    */
   static void DeallocateLayerTreeId(uint64_t aId);
 

--- a/gfx/layers/ipc/CompositorThread.cpp
+++ b/gfx/layers/ipc/CompositorThread.cpp
@@ -2,16 +2,17 @@
 /* vim: set ts=8 sts=2 et sw=2 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 "CompositorThread.h"
 #include "MainThreadUtils.h"
 #include "nsThreadUtils.h"
 #include "CompositorBridgeParent.h"
+#include "mozilla/layers/CompositorManagerParent.h"
 #include "mozilla/layers/ImageBridgeParent.h"
 #include "mozilla/media/MediaSystemResourceService.h"
 
 namespace mozilla {
 
 namespace gfx {
 // See VRManagerChild.cpp
 void ReleaseVRManagerParentSingleton();