servo: Merge #17526 - Check has_new_animation_style along with for_css_rule_changes() in ne… (from hiikezoe:check-has_new_animation_style); r=birtles

…eds_animations_update().

<!-- Please describe your changes on the following line: -->
https://bugzilla.mozilla.org/show_bug.cgi?id=1375787

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

Source-Repo: https://github.com/servo/servo
Source-Revision: 4b99a75508e1c84ef007a16a80d9ba513eafe22c

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : e1a3a4bfb2a2f53cb5271e5a628168c8b5e11662

servo/components/style/matching.rs

@@ -653,9 +653,14 @@ trait PrivateMatchMethods: TElement {
             let old_display_style = old_box_style.clone_display();
             let new_display_style = new_box_style.clone_display();
 
-            // If the traverse is triggered by CSS rule changes,
-            // we need to try to update all CSS animations.
-            context.shared.traversal_flags.for_css_rule_changes() ||
+            // If the traverse is triggered by CSS rule changes, we need to
+            // try to update all CSS animations on the element if the element
+            // has CSS animation style regardless of whether the animation is
+            // running or not.
+            // TODO: We should check which @keyframes changed/added/deleted
+            // and update only animations corresponding to those @keyframes.
+            (context.shared.traversal_flags.for_css_rule_changes() &&
+             has_new_animation_style) ||
             !old_box_style.animations_equals(&new_box_style) ||
              (old_display_style == display::T::none &&
               new_display_style != display::T::none &&