layout: Don't restart a transition if the end value for the new

transition is the same as the end value for a running transition per
CSS-TRANSITIONS § 3.

Besides being contrary to spec, the old behavior could cause a cascade
of CSS transitions incorrectly triggering themselves when calls to
`getComputedStyle()` were intermingled with them.

Improves performance of nytimes.com.

components/style/animation.rs

@@ -335,6 +335,11 @@ impl PropertyAnimation {
     fn does_animate(&self) -> bool {
         self.property.does_animate() && self.duration != Time(0.0)
     }
+
+    #[inline]
+    pub fn has_the_same_end_value_as(&self, other: &PropertyAnimation) -> bool {
+        self.property.has_the_same_end_value_as(&other.property)
+    }
 }
 
 /// Inserts transitions into the queue of running animations as applicable for
@@ -348,13 +353,24 @@ pub fn start_transitions_if_applicable(new_animations_sender: &Sender<Animation>
                                        unsafe_node: UnsafeNode,
                                        old_style: &ComputedValues,
                                        new_style: &mut Arc<ComputedValues>,
-                                       timer: &Timer)
+                                       timer: &Timer,
+                                       possibly_expired_animations: &[PropertyAnimation])
                                        -> bool {
     let mut had_animations = false;
     for i in 0..new_style.get_box().transition_property_count() {
         // Create any property animations, if applicable.
         let property_animations = PropertyAnimation::from_transition(i, old_style, Arc::make_mut(new_style));
         for property_animation in property_animations {
+            // Per [1], don't trigger a new transition if the end state for that transition is
+            // the same as that of a transition that's already running on the same node.
+            //
+            // [1]: https://drafts.csswg.org/css-transitions/#starting
+            if possibly_expired_animations.iter().any(|animation| {
+                    animation.has_the_same_end_value_as(&property_animation)
+               }) {
+                continue
+            }
+
             // Set the property to the initial value.
             // NB: get_mut is guaranteed to succeed since we called make_mut()
             // above.