3 files changed, 34 insertions, 7 deletions
diff --git a/components/script/animations.rs b/components/script/animations.rs
index 73131763430..49dfe77e1b4 100644
--- a/components/script/animations.rs
+++ b/components/script/animations.rs
@@ -2,8 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-#![deny(missing_docs)]
-
 //! The set of animations for a document.
 
 use crate::dom::animationevent::AnimationEvent;
@@ -47,6 +45,11 @@ pub(crate) struct Animations {
 
     /// A list of pending animation-related events.
     pending_events: DomRefCell<Vec<TransitionOrAnimationEvent>>,
+
+    /// The timeline value at the last time all animations were marked dirty.
+    /// This is used to prevent marking animations dirty when the timeline
+    /// has not changed.
+    timeline_value_at_last_dirty: Cell<f64>,
 }
 
 impl Animations {
@@ -56,6 +59,7 @@ impl Animations {
             has_running_animations: Cell::new(false),
             rooted_nodes: Default::default(),
             pending_events: Default::default(),
+            timeline_value_at_last_dirty: Cell::new(0.0),
         }
     }
 
@@ -65,12 +69,23 @@ impl Animations {
         self.pending_events.borrow_mut().clear();
     }
 
-    pub(crate) fn mark_animating_nodes_as_dirty(&self) {
+    // Mark all animations dirty, if they haven't been marked dirty since the
+    // specified `current_timeline_value`. Returns true if animations were marked
+    // dirty or false otherwise.
+    pub(crate) fn mark_animating_nodes_as_dirty(&self, current_timeline_value: f64) -> bool {
+        if current_timeline_value <= self.timeline_value_at_last_dirty.get() {
+            return false;
+        }
+        self.timeline_value_at_last_dirty
+            .set(current_timeline_value);
+
         let sets = self.sets.sets.read();
         let rooted_nodes = self.rooted_nodes.borrow();
         for node in sets.keys().filter_map(|key| rooted_nodes.get(&key.node)) {
             node.dirty(NodeDamage::NodeStyleDamaged);
         }
+
+        true
     }
 
     pub(crate) fn update_for_new_timeline_value(&self, window: &Window, now: f64) {
@@ -97,7 +112,6 @@ impl Animations {
         }
 
         self.unroot_unused_nodes(&sets);
-        //self.update_running_animations_presence(window);
     }
 
     /// Cancel animations for the given node, if any exist.
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index d8a01e68b4d..f636e11fc51 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -3852,6 +3852,18 @@ impl Document {
             .update_for_new_timeline_value(&self.window, current_timeline_value);
     }
 
+    pub(crate) fn maybe_mark_animating_nodes_as_dirty(&self) {
+        let current_timeline_value = self.current_animation_timeline_value();
+        let marked_dirty = self
+            .animations
+            .borrow()
+            .mark_animating_nodes_as_dirty(current_timeline_value);
+
+        if marked_dirty {
+            self.window().add_pending_reflow();
+        }
+    }
+
     pub(crate) fn current_animation_timeline_value(&self) -> f64 {
         self.animation_timeline.borrow().current_value()
     }
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index b417f453cdf..f52fc0d777a 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1702,10 +1702,12 @@ impl ScriptThread {
         true
     }
 
-    // Perform step 11.10 from https://html.spec.whatwg.org/multipage/#event-loops.
+    // Perform step 7.10 from https://html.spec.whatwg.org/multipage/#event-loop-processing-model.
+    // Described at: https://drafts.csswg.org/web-animations-1/#update-animations-and-send-events
     fn update_animations_and_send_events(&self) {
         for (_, document) in self.documents.borrow().iter() {
             document.update_animation_timeline();
+            document.maybe_mark_animating_nodes_as_dirty();
         }
 
         for (_, document) in self.documents.borrow().iter() {
@@ -3012,8 +3014,7 @@ impl ScriptThread {
             document.run_the_animation_frame_callbacks();
         }
         if tick_type.contains(AnimationTickType::CSS_ANIMATIONS_AND_TRANSITIONS) {
-            document.animations().mark_animating_nodes_as_dirty();
-            document.window().add_pending_reflow();
+            document.maybe_mark_animating_nodes_as_dirty();
         }
     }