diff --git a/components/layout/animation.rs b/components/layout/animation.rs
index dcb860308ef..863b1eb325a 100644
--- a/components/layout/animation.rs
+++ b/components/layout/animation.rs
@@ -49,32 +49,41 @@ pub fn start_transitions_if_applicable(new_animations_sender: &Sender<Animation>
 }
 
 /// Processes any new animations that were discovered after style recalculation.
-pub fn process_new_animations(rw_data: &mut LayoutTaskData, pipeline_id: PipelineId) {
+/// Also expire any old animations that have completed.
+pub fn update_animation_state(rw_data: &mut LayoutTaskData, pipeline_id: PipelineId) {
     let mut new_running_animations = Vec::new();
     while let Ok(animation) = rw_data.new_animations_receiver.try_recv() {
         new_running_animations.push(animation)
     }
-    if !new_running_animations.is_empty() {
-        let mut running_animations = (*rw_data.running_animations).clone();
 
-        // Expire old running animations.
-        let now = clock_ticks::precise_time_s();
-        for (_, running_animations) in &mut running_animations {
-            running_animations.retain(|running_animation| now < running_animation.end_time);
+    let mut running_animations_hash = (*rw_data.running_animations).clone();
+
+    // Expire old running animations.
+    let now = clock_ticks::precise_time_s();
+    let mut keys_to_remove = Vec::new();
+    for (key, running_animations) in &mut running_animations_hash {
+        running_animations.retain(|running_animation| {
+            now < running_animation.end_time
+        });
+        if running_animations.len() == 0 {
+            keys_to_remove.push(*key);
         }
-
-        // Add new running animations.
-        for new_running_animation in new_running_animations {
-            match running_animations.entry(OpaqueNode(new_running_animation.node)) {
-                Entry::Vacant(entry) => {
-                    entry.insert(vec![new_running_animation]);
-                }
-                Entry::Occupied(mut entry) => entry.get_mut().push(new_running_animation),
-            }
-        }
-
-        rw_data.running_animations = Arc::new(running_animations);
     }
+    for key in keys_to_remove {
+        running_animations_hash.remove(&key).unwrap();
+    }
+
+    // Add new running animations.
+    for new_running_animation in new_running_animations {
+        match running_animations_hash.entry(OpaqueNode(new_running_animation.node)) {
+            Entry::Vacant(entry) => {
+                entry.insert(vec![new_running_animation]);
+            }
+            Entry::Occupied(mut entry) => entry.get_mut().push(new_running_animation),
+        }
+    }
+
+    rw_data.running_animations = Arc::new(running_animations_hash);
 
     let animation_state;
     if rw_data.running_animations.is_empty() {
diff --git a/components/layout/layout_task.rs b/components/layout/layout_task.rs
index 17bd99b0749..28eacfd1763 100644
--- a/components/layout/layout_task.rs
+++ b/components/layout/layout_task.rs
@@ -1183,9 +1183,6 @@ impl LayoutTask {
 
             // Retrieve the (possibly rebuilt) root flow.
             rw_data.root_flow = self.try_get_layout_root((*node).clone());
-
-            // Kick off animations if any were triggered.
-            animation::process_new_animations(&mut *rw_data, self.id);
         }
 
         // Send new canvas renderers to the paint task
@@ -1327,6 +1324,9 @@ impl LayoutTask {
                                                    rw_data: &mut LayoutTaskData,
                                                    layout_context: &mut SharedLayoutContext) {
         if let Some(mut root_flow) = rw_data.layout_root() {
+            // Kick off animations if any were triggered, expire completed ones.
+            animation::update_animation_state(&mut *rw_data, self.id);
+
             profile(time::ProfilerCategory::LayoutRestyleDamagePropagation,
                     self.profiler_metadata(),
                     self.time_profiler_chan.clone(),
diff --git a/components/msg/constellation_msg.rs b/components/msg/constellation_msg.rs
index c43a5566a1c..ab244707c28 100644
--- a/components/msg/constellation_msg.rs
+++ b/components/msg/constellation_msg.rs
@@ -275,7 +275,7 @@ pub enum Msg {
     NodeStatus(Option<String>),
 }
 
-#[derive(Clone, Eq, PartialEq, Deserialize, Serialize)]
+#[derive(Clone, Eq, PartialEq, Deserialize, Serialize, Debug)]
 pub enum AnimationState {
     AnimationsPresent,
     AnimationCallbacksPresent,