4 files changed, 10 insertions, 11 deletions
diff --git a/components/layout/animation.rs b/components/layout/animation.rs
index 27bf118bfda..7b58e435681 100644
--- a/components/layout/animation.rs
+++ b/components/layout/animation.rs
@@ -14,14 +14,14 @@ use script::layout_interface::Animation;
 use script_traits::ConstellationControlMsg;
 use std::collections::HashMap;
 use std::collections::hash_map::Entry;
-use std::sync::Arc;
 use std::sync::mpsc::Sender;
+use std::sync::{Arc, Mutex};
 use style::animation::{GetMod, PropertyAnimation};
 use style::properties::ComputedValues;
 
 /// Inserts transitions into the queue of running animations as applicable for the given style
 /// difference. This is called from the layout worker threads.
-pub fn start_transitions_if_applicable(new_animations_sender: &Sender<Animation>,
+pub fn start_transitions_if_applicable(new_animations_sender: &Mutex<Sender<Animation>>,
                                        node: OpaqueNode,
                                        old_style: &ComputedValues,
                                        new_style: &mut ComputedValues) {
@@ -37,7 +37,7 @@ pub fn start_transitions_if_applicable(new_animations_sender: &Sender<Animation>
             let animation_style = new_style.get_animation();
             let start_time =
                 now + (animation_style.transition_delay.0.get_mod(i).seconds() as f64);
-            new_animations_sender.send(Animation {
+            new_animations_sender.lock().unwrap().send(Animation {
                 node: node.id(),
                 property_animation: property_animation,
                 start_time: start_time,
diff --git a/components/layout/context.rs b/components/layout/context.rs
index 5a7bbdfed8f..eb5c19cc596 100644
--- a/components/layout/context.rs
+++ b/components/layout/context.rs
@@ -105,7 +105,7 @@ pub struct SharedLayoutContext {
 
     /// A channel on which new animations that have been triggered by style recalculation can be
     /// sent.
-    pub new_animations_sender: Sender<Animation>,
+    pub new_animations_sender: Mutex<Sender<Animation>>,
 
     /// A channel to send canvas renderers to paint task, in order to correctly paint the layers
     pub canvas_layers_sender: Sender<(LayerId, IpcSender<CanvasMsg>)>,
@@ -123,7 +123,6 @@ pub struct SharedLayoutContext {
 // FIXME(#6569) This implementations is unsound:
 // XXX UNSOUND!!! for image_cache_task
 // XXX UNSOUND!!! for stylist
-// XXX UNSOUND!!! for new_animations_sender
 // XXX UNSOUND!!! for canvas_layers_sender
 #[allow(unsafe_code)]
 unsafe impl Sync for SharedLayoutContext {}
diff --git a/components/layout/css/matching.rs b/components/layout/css/matching.rs
index 456f8cbe88f..d0260fa5c98 100644
--- a/components/layout/css/matching.rs
+++ b/components/layout/css/matching.rs
@@ -21,8 +21,8 @@ use smallvec::SmallVec;
 use std::borrow::ToOwned;
 use std::hash::{Hash, Hasher};
 use std::slice::Iter;
-use std::sync::Arc;
 use std::sync::mpsc::Sender;
+use std::sync::{Arc, Mutex};
 use string_cache::{Atom, Namespace};
 use style::node::TElementAttributes;
 use style::properties::{ComputedValues, PropertyDeclaration, cascade};
@@ -406,7 +406,7 @@ pub trait MatchMethods {
                            parent: Option<LayoutNode>,
                            applicable_declarations: &ApplicableDeclarations,
                            applicable_declarations_cache: &mut ApplicableDeclarationsCache,
-                           new_animations_sender: &Sender<Animation>);
+                           new_animations_sender: &Mutex<Sender<Animation>>);
 }
 
 trait PrivateMatchMethods {
@@ -417,7 +417,7 @@ trait PrivateMatchMethods {
                                    style: &mut Option<Arc<ComputedValues>>,
                                    applicable_declarations_cache:
                                     &mut ApplicableDeclarationsCache,
-                                   new_animations_sender: &Sender<Animation>,
+                                   new_animations_sender: &Mutex<Sender<Animation>>,
                                    shareable: bool,
                                    animate_properties: bool)
                                    -> RestyleDamage;
@@ -438,7 +438,7 @@ impl<'ln> PrivateMatchMethods for LayoutNode<'ln> {
                                    style: &mut Option<Arc<ComputedValues>>,
                                    applicable_declarations_cache:
                                     &mut ApplicableDeclarationsCache,
-                                   new_animations_sender: &Sender<Animation>,
+                                   new_animations_sender: &Mutex<Sender<Animation>>,
                                    shareable: bool,
                                    animate_properties: bool)
                                    -> RestyleDamage {
@@ -655,7 +655,7 @@ impl<'ln> MatchMethods for LayoutNode<'ln> {
                            parent: Option<LayoutNode>,
                            applicable_declarations: &ApplicableDeclarations,
                            applicable_declarations_cache: &mut ApplicableDeclarationsCache,
-                           new_animations_sender: &Sender<Animation>) {
+                           new_animations_sender: &Mutex<Sender<Animation>>) {
         // Get our parent's style. This must be unsafe so that we don't touch the parent's
         // borrow flags.
         //
diff --git a/components/layout/layout_task.rs b/components/layout/layout_task.rs
index edf408cf937..604a0743d34 100644
--- a/components/layout/layout_task.rs
+++ b/components/layout/layout_task.rs
@@ -461,7 +461,7 @@ impl LayoutTask {
             url: (*url).clone(),
             visible_rects: rw_data.visible_rects.clone(),
             generation: rw_data.generation,
-            new_animations_sender: rw_data.new_animations_sender.clone(),
+            new_animations_sender: Mutex::new(rw_data.new_animations_sender.clone()),
             goal: goal,
             running_animations: rw_data.running_animations.clone(),
         }