--- a/gfx/layers/apz/src/AsyncPanZoomAnimation.h
+++ b/gfx/layers/apz/src/AsyncPanZoomAnimation.h
@@ -60,16 +60,22 @@ public:
   virtual SmoothScrollAnimation* AsSmoothScrollAnimation() {
     return nullptr;
   }
 
   virtual bool WantsRepaints() {
     return true;
   }
 
+  // A function that APZC calls prior to calling DoSample(), before acquiring
+  // the APZC lock (which is held while DoSample() is called). This allows
+  // the animation to perform computation that requires the APZC lock not to
+  // be held (like accessing the tree manager).
+  virtual void BeforeSample() {}
+
 protected:
   // Protected destructor, to discourage deletion outside of Release():
   virtual ~AsyncPanZoomAnimation()
   { }
 
   /**
    * Tasks scheduled for execution after the APZC's mMonitor is released.
    * Derived classes can add tasks here in Sample(), and the APZC can call

--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -3292,16 +3292,33 @@ bool AsyncPanZoomController::AdvanceAnim
 {
   APZThreadUtils::AssertOnCompositorThread();
 
   // Don't send any state-change notifications until the end of the function,
   // because we may go through some intermediate states while we finish
   // animations and start new ones.
   StateChangeNotificationBlocker blocker(this);
 
+  // If we have an animation, give it a chance to perform operations
+  // that require |mMonitor| not to be held.
+  {
+    // We still need to hold |mMonitor| while we acquire a ref to the
+    // animation, because otherwise the controller thread could get a
+    // timeslice between the null check and the BeforeSample() call,
+    // and call CancelAnimation().
+    RefPtr<AsyncPanZoomAnimation> animation;
+    {
+      ReentrantMonitorAutoEnter lock(mMonitor);
+      animation = mAnimation;
+    }
+    if (animation) {
+      animation->BeforeSample();
+    }
+  }
+
   // The eventual return value of this function. The compositor needs to know
   // whether or not to advance by a frame as soon as it can. For example, if a
   // fling is happening, it has to keep compositing so that the animation is
   // smooth. If an animation frame is requested, it is the compositor's
   // responsibility to schedule a composite.
   mAsyncTransformAppliedToContent = false;
   bool requestAnimationFrame = false;
   nsTArray<RefPtr<Runnable>> deferredTasks;