LibWeb: Do not invalidate elements with animations in the CSS cascade

See the note added to Animation::cancel for more info

Userland/Libraries/LibWeb/Animations/Animation.cpp

@@ -396,8 +396,14 @@ void Animation::set_onremove(JS::GCPtr<WebIDL::CallbackType> event_handler)
 }
 
 // https://www.w3.org/TR/web-animations-1/#dom-animation-cancel
-void Animation::cancel()
+void Animation::cancel(ShouldInvalidate should_invalidate)
 {
+    // Note: When called from JS, we always want to invalidate the animation target's style. However, this method is
+    //       also called from the StyleComputer when the animation-name CSS property changes. That happens in the
+    //       middle of a cascade, and importantly, _before_ computing the animation effect stack, so there is no
+    //       need for another invalidation. And in fact, if we did invalidate, it would lead to a crash, as the element
+    //       would not have it's "m_needs_style_update" flag cleared.
+
     auto& realm = this->realm();
 
     // 1. If animation’s play state is not idle, perform the following steps:
@@ -452,7 +458,8 @@ void Animation::cancel()
     // 3. Make animation’s start time unresolved.
     m_start_time = {};
 
-    invalidate_effect();
+    if (should_invalidate == ShouldInvalidate::Yes)
+        invalidate_effect();
 }
 
 // https://www.w3.org/TR/web-animations-1/#dom-animation-finish

Userland/Libraries/LibWeb/Animations/Animation.h

@@ -77,7 +77,11 @@ public:
         Yes,
         No,
     };
-    void cancel();
+    enum class ShouldInvalidate {
+        Yes,
+        No,
+    };
+    void cancel(ShouldInvalidate = ShouldInvalidate::Yes);
     WebIDL::ExceptionOr<void> finish();
     WebIDL::ExceptionOr<void> play();
     WebIDL::ExceptionOr<void> play_an_animation(AutoRewind);

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -1395,7 +1395,7 @@ ErrorOr<void> StyleComputer::compute_cascaded_values(StyleProperties& style, DOM
             if (source_declaration != element.cached_animation_name_source()) {
                 // This animation name is new, so we need to create a new animation for it.
                 if (auto existing_animation = element.cached_animation_name_animation())
-                    existing_animation->cancel();
+                    existing_animation->cancel(Animations::Animation::ShouldInvalidate::No);
                 element.set_cached_animation_name_source(source_declaration);
 
                 auto effect = Animations::KeyframeEffect::create(realm);
@@ -1422,7 +1422,7 @@ ErrorOr<void> StyleComputer::compute_cascaded_values(StyleProperties& style, DOM
     } else {
         // If the element had an existing animation, cancel it
         if (auto existing_animation = element.cached_animation_name_animation()) {
-            existing_animation->cancel();
+            existing_animation->cancel(Animations::Animation::ShouldInvalidate::No);
             element.set_cached_animation_name_animation({});
             element.set_cached_animation_name_source({});
         }