diff --git a/dom/animation/AnimationTimeline.h b/dom/animation/AnimationTimeline.h
index 85b37125569..1cac868b630 100644
--- a/dom/animation/AnimationTimeline.h
+++ b/dom/animation/AnimationTimeline.h
@@ -65,9 +65,10 @@ public:
   // be a no-op.
   void FastForward(const TimeStamp& aTimeStamp);
 
+  nsRefreshDriver* GetRefreshDriver() const;
+
 protected:
   TimeStamp GetCurrentTimeStamp() const;
-  nsRefreshDriver* GetRefreshDriver() const;
 
   nsCOMPtr<nsIDocument> mDocument;
 
diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index fba378ad0ff..6a4b2a12859 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -424,6 +424,23 @@ AddAnimationsForProperty(nsIFrame* aFrame, nsCSSProperty aProperty,
           player->IsRunning())) {
       continue;
     }
+
+    // Don't add animations that are pending when their corresponding
+    // refresh driver is under test control. This is because any pending
+    // animations on layers will have their start time updated with the
+    // current timestamp but when the refresh driver is under test control
+    // its refresh times are unrelated to timestamp values.
+    //
+    // Instead we leave the animation running on the main thread and the
+    // next time the refresh driver is advanced it will trigger any pending
+    // animations.
+    if (player->PlayState() == AnimationPlayState::Pending) {
+      nsRefreshDriver* driver = player->Timeline()->GetRefreshDriver();
+      if (driver && driver->IsTestControllingRefreshesEnabled()) {
+        continue;
+      }
+    }
+
     AddAnimationForProperty(aFrame, aProperty, player, aLayer, aData, aPending);
     player->SetIsRunningOnCompositor();
   }