diff --git a/gfx/layers/apz/public/APZSampler.h b/gfx/layers/apz/public/APZSampler.h
index 53aea769dcc9..e4630ed7dc5a 100644
--- a/gfx/layers/apz/public/APZSampler.h
+++ b/gfx/layers/apz/public/APZSampler.h
@@ -7,7 +7,10 @@
 #ifndef mozilla_layers_APZSampler_h
 #define mozilla_layers_APZSampler_h
 
+#include <unordered_map>
+
 #include "mozilla/layers/AsyncCompositionManager.h" // for AsyncTransform
+#include "mozilla/StaticMutex.h"
 #include "nsTArray.h"
 #include "Units.h"
 
@@ -18,6 +21,7 @@ class TimeStamp;
 namespace wr {
 class TransactionBuilder;
 struct WrTransformProperty;
+struct WrWindowId;
 } // namespace wr
 
 namespace layers {
@@ -37,6 +41,8 @@ class APZSampler {
 public:
   explicit APZSampler(const RefPtr<APZCTreeManager>& aApz);
 
+  void SetWebRenderWindowId(const wr::WindowId& aWindowId);
+
   bool PushStateToWR(wr::TransactionBuilder& aTxn,
                      const TimeStamp& aSampleTime);
 
@@ -84,8 +90,18 @@ public:
 protected:
   virtual ~APZSampler();
 
+  static already_AddRefed<APZSampler> GetSampler(const wr::WrWindowId& aWindowId);
+
 private:
   RefPtr<APZCTreeManager> mApz;
+
+  // Used to manage the mapping from a WR window id to APZSampler. These are only
+  // used if WebRender is enabled. Both sWindowIdMap and mWindowId should only
+  // be used while holding the sWindowIdLock.
+  static StaticMutex sWindowIdLock;
+  static std::unordered_map<uint64_t, APZSampler*> sWindowIdMap;
+  Maybe<wr::WrWindowId> mWindowId;
+
 };
 
 } // namespace layers
diff --git a/gfx/layers/apz/src/APZSampler.cpp b/gfx/layers/apz/src/APZSampler.cpp
index 819e227ac401..56c3956723eb 100644
--- a/gfx/layers/apz/src/APZSampler.cpp
+++ b/gfx/layers/apz/src/APZSampler.cpp
@@ -17,6 +17,10 @@
 namespace mozilla {
 namespace layers {
 
+StaticMutex APZSampler::sWindowIdLock;
+std::unordered_map<uint64_t, APZSampler*> APZSampler::sWindowIdMap;
+
+
 APZSampler::APZSampler(const RefPtr<APZCTreeManager>& aApz)
   : mApz(aApz)
 {
@@ -27,6 +31,20 @@ APZSampler::APZSampler(const RefPtr<APZCTreeManager>& aApz)
 APZSampler::~APZSampler()
 {
   mApz->SetSampler(nullptr);
+
+  StaticMutexAutoLock lock(sWindowIdLock);
+  if (mWindowId) {
+    sWindowIdMap.erase(wr::AsUint64(*mWindowId));
+  }
+}
+
+void
+APZSampler::SetWebRenderWindowId(const wr::WindowId& aWindowId)
+{
+  StaticMutexAutoLock lock(sWindowIdLock);
+  MOZ_ASSERT(!mWindowId);
+  mWindowId = Some(aWindowId);
+  sWindowIdMap[wr::AsUint64(aWindowId)] = this;
 }
 
 bool
@@ -158,5 +176,17 @@ APZSampler::IsSamplerThread() const
   return CompositorThreadHolder::IsInCompositorThread();
 }
 
+/*static*/ already_AddRefed<APZSampler>
+APZSampler::GetSampler(const wr::WrWindowId& aWindowId)
+{
+  RefPtr<APZSampler> sampler;
+  StaticMutexAutoLock lock(sWindowIdLock);
+  auto it = sWindowIdMap.find(wr::AsUint64(aWindowId));
+  if (it != sWindowIdMap.end()) {
+    sampler = it->second;
+  }
+  return sampler.forget();
+}
+
 } // namespace layers
 } // namespace mozilla
diff --git a/gfx/layers/ipc/CompositorBridgeParent.cpp b/gfx/layers/ipc/CompositorBridgeParent.cpp
index 2da570291ef9..3815d199dac4 100644
--- a/gfx/layers/ipc/CompositorBridgeParent.cpp
+++ b/gfx/layers/ipc/CompositorBridgeParent.cpp
@@ -1779,6 +1779,10 @@ CompositorBridgeParent::AllocPWebRenderBridgeParent(const wr::PipelineId& aPipel
     // that the callback from the updater thread can find the right APZUpdater.
     mApzUpdater->SetWebRenderWindowId(windowId);
   }
+  if (mApzSampler) {
+    // Same as for mApzUpdater, but for the sampler thread.
+    mApzSampler->SetWebRenderWindowId(windowId);
+  }
   RefPtr<wr::WebRenderAPI> api = wr::WebRenderAPI::Create(this, Move(widget), windowId, aSize);
   if (!api) {
     mWrBridge = WebRenderBridgeParent::CreateDestroyed(aPipelineId);