Skip update_animations if we have no running animations and the element becomes display:none.

components/script/layout_wrapper.rs

@@ -450,6 +450,10 @@ impl<'le> TElement for ServoLayoutElement<'le> {
     fn update_animations(&self, _pseudo: Option<&PseudoElement>) {
         panic!("this should be only called on gecko");
     }
+
+    fn has_css_animations(&self, _pseudo: Option<&PseudoElement>) -> bool {
+        panic!("this should be only called on gecko");
+    }
 }
 
 impl<'le> PartialEq for ServoLayoutElement<'le> {

components/style/dom.rs

@@ -339,6 +339,9 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
     /// Creates a task to update CSS Animations on a given (pseudo-)element.
     /// Note: Gecko only.
     fn update_animations(&self, _pseudo: Option<&PseudoElement>);
+
+    /// Returns true if the element has a CSS animation.
+    fn has_css_animations(&self, _pseudo: Option<&PseudoElement>) -> bool;
 }
 
 /// TNode and TElement aren't Send because we want to be careful and explicit

components/style/gecko/wrapper.rs

@@ -29,6 +29,7 @@ use gecko_bindings::bindings::{Gecko_IsLink, Gecko_IsRootElement, Gecko_MatchesE
 use gecko_bindings::bindings::{Gecko_IsUnvisitedLink, Gecko_IsVisitedLink, Gecko_Namespace};
 use gecko_bindings::bindings::{Gecko_SetNodeFlags, Gecko_UnsetNodeFlags};
 use gecko_bindings::bindings::Gecko_ClassOrClassList;
+use gecko_bindings::bindings::Gecko_ElementHasCSSAnimations;
 use gecko_bindings::bindings::Gecko_GetAnimationRule;
 use gecko_bindings::bindings::Gecko_GetHTMLPresentationAttrDeclarationBlock;
 use gecko_bindings::bindings::Gecko_GetStyleAttrDeclarationBlock;
@@ -526,6 +527,11 @@ impl<'le> TElement for GeckoElement<'le> {
                                    parent_values_opt);
         }
     }
+
+    fn has_css_animations(&self, pseudo: Option<&PseudoElement>) -> bool {
+        let atom_ptr = PseudoElement::ns_atom_or_null_from_opt(pseudo);
+        unsafe { Gecko_ElementHasCSSAnimations(self.0, atom_ptr) }
+    }
 }
 
 impl<'le> PartialEq for GeckoElement<'le> {

components/style/gecko_bindings/bindings.rs

@@ -549,6 +549,11 @@ extern "C" {
                                   aParentComputedValues:
                                       ServoComputedValuesBorrowedOrNull);
 }
+extern "C" {
+    pub fn Gecko_ElementHasCSSAnimations(aElement: RawGeckoElementBorrowed,
+                                         aPseudoTagOrNull: *mut nsIAtom)
+     -> bool;
+}
 extern "C" {
     pub fn Gecko_Atomize(aString: *const ::std::os::raw::c_char, aLength: u32)
      -> *mut nsIAtom;

components/style/matching.rs

@@ -605,21 +605,21 @@ trait PrivateMatchMethods: TElement {
         let ref new_box_style = new_values.get_box();
         let has_new_animation_style = new_box_style.animation_name_count() >= 1 &&
                                       new_box_style.animation_name_at(0).0.len() != 0;
+        let has_animations = self.has_css_animations(pseudo);
+
         let needs_update_animations =
             old_values.as_ref().map_or(has_new_animation_style, |ref old| {
                 let ref old_box_style = old.get_box();
                 let old_display_style = old_box_style.clone_display();
                 let new_display_style = new_box_style.clone_display();
-                // FIXME: If we know that the element has no running CSS animations,
-                // we can also skip the case with checking no_animations.
-                //
                 // FIXME: Bug 1344581: We still need to compare keyframe rules.
                 !old_box_style.animations_equals(&new_box_style) ||
                  (old_display_style == display::T::none &&
                   new_display_style != display::T::none &&
                   has_new_animation_style) ||
                  (old_display_style != display::T::none &&
-                  new_display_style == display::T::none)
+                  new_display_style == display::T::none &&
+                  has_animations)
             });
         if needs_update_animations {
             let task = SequentialTask::update_animations(self.as_node().as_element().unwrap(),