LibWeb: Don't invalidate style for animations in idle state

Author: https://github.com/kalenikaliaksandr
Commit: https://github.com/LadybirdBrowser/ladybird/commit/80c0a16597c
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3800

Libraries/LibWeb/Animations/Animation.h

@@ -66,6 +66,8 @@ public:
     GC::Ref<WebIDL::Promise> finished() const { return current_finished_promise(); }
     bool is_finished() const { return m_is_finished; }
 
+    bool is_idle() const { return play_state() == Bindings::AnimationPlayState::Idle; }
+
     GC::Ptr<WebIDL::CallbackType> onfinish();
     void set_onfinish(GC::Ptr<WebIDL::CallbackType>);
     GC::Ptr<WebIDL::CallbackType> oncancel();

Libraries/LibWeb/DOM/Document.cpp

@@ -1522,7 +1522,7 @@ void Document::update_animated_style_if_needed()
 
     for (auto& timeline : m_associated_animation_timelines) {
         for (auto& animation : timeline->associated_animations()) {
-            if (animation->is_finished())
+            if (animation->is_idle() || animation->is_finished())
                 continue;
             if (auto effect = animation->effect()) {
                 if (auto* target = effect->target())