--- a/gfx/layers/Layers.cpp
+++ b/gfx/layers/Layers.cpp
@@ -200,24 +200,30 @@ Layer::Layer(LayerManager* aManager, voi
   mAnimationGeneration(0)
 {
 }
 
 Layer::~Layer()
 {
 }
 
+void
+Layer::EnsureAnimationsId()
+{
+  if (!mCompositorAnimationsId) {
+    mCompositorAnimationsId = AnimationHelper::GetNextCompositorAnimationsId();
+  }
+}
+
 Animation*
 Layer::AddAnimation()
 {
   // Here generates a new id when the first animation is added and
   // this id is used to represent the animations in this layer.
-  if (!mCompositorAnimationsId) {
-    mCompositorAnimationsId = AnimationHelper::GetNextCompositorAnimationsId();
-  }
+  EnsureAnimationsId();
 
   MOZ_LAYERS_LOG_IF_SHADOWABLE(
     this, ("Layer::Mutated(%p) AddAnimation with id=%" PRIu64, this, mCompositorAnimationsId));
 
   MOZ_ASSERT(!mPendingAnimations, "should have called ClearAnimations first");
 
   Animation* anim = mAnimations.AppendElement();
 

--- a/gfx/layers/Layers.h
+++ b/gfx/layers/Layers.h
@@ -1218,16 +1218,19 @@ public:
   void SetTransformIsPerspective(bool aTransformIsPerspective)
   {
     if (mSimpleAttrs.SetTransformIsPerspective(aTransformIsPerspective)) {
       MOZ_LAYERS_LOG_IF_SHADOWABLE(this, ("Layer::Mutated(%p) TransformIsPerspective", this));
       MutatedSimple();
     }
   }
 
+  // Ensure that this layer has a valid (non-zero) animations id. This value is
+  // unique across layers.
+  void EnsureAnimationsId();
   // Call AddAnimation to add a new animation to this layer from layout code.
   // Caller must fill in all the properties of the returned animation.
   // A later animation overrides an earlier one.
   Animation* AddAnimation();
   // ClearAnimations clears animations on this layer.
   virtual void ClearAnimations();
   // This is only called when the layer tree is updated. Do not call this from
   // layout code.  To add an animation to this layer, use AddAnimation.

--- a/gfx/layers/wr/WebRenderContainerLayer.cpp
+++ b/gfx/layers/wr/WebRenderContainerLayer.cpp
@@ -85,16 +85,33 @@ WebRenderContainerLayer::RenderLayer(wr:
 
     animationsId = GetCompositorAnimationsId();
     OpAddCompositorAnimations
       anim(CompositorAnimations(GetAnimations(), animationsId),
            transformForCompositor, opacityForCompositor);
     WrBridge()->AddWebRenderParentCommand(anim);
   }
 
+  // If APZ is enabled and this layer is a scroll thumb, then it might need
+  // to move in the compositor to represent the async scroll position. So we
+  // ensure that there is an animations id set on it, we will use this to give
+  // WebRender updated transforms for composition.
+  if (WrManager()->AsyncPanZoomEnabled() &&
+      GetScrollThumbData().mDirection != ScrollDirection::NONE) {
+    // A scroll thumb better not have a transform animation already or we're
+    // going to end up clobbering it with APZ animating it too.
+    MOZ_ASSERT(transformForSC);
+
+    EnsureAnimationsId();
+    animationsId = GetCompositorAnimationsId();
+    // We need to set the transform in the stacking context to null for it to
+    // pick up and install the animation id.
+    transformForSC = nullptr;
+  }
+
   ScrollingLayersHelper scroller(this, aBuilder, aSc);
   StackingContextHelper sc(aSc, aBuilder, this, animationsId, opacityForSC, transformForSC);
 
   LayerRect rect = Bounds();
   DumpLayerInfo("ContainerLayer", rect);
 
   Maybe<WrImageMask> mask = BuildWrMaskLayer(&sc);
   aBuilder.PushClip(sc.ToRelativeWrRect(rect), mask.ptrOr(nullptr));