From 84c4eb7aa9353a3ad77b10b27b2bc9ef6af2b1ec Mon Sep 17 00:00:00 2001
From: Jelle Raaijmakers <jelle@ladybird.org>
Date: Tue, 26 Aug 2025 14:23:30 +0200
Subject: [PATCH] LibWeb: Always update computed properties for finished
 animations

If an animation got to its finished state before its target's computed
properties could be updated, we would end up with invalid styles. Do not
skip finished animations, but prevent effect invalidation on timeline
updates if the animation is already finished.

This fixes the CI flake on WPT test
`css/css-transitions/inherit-height-transition.html`.
---
 Libraries/LibWeb/Animations/Animation.cpp             | 11 ++++++-----
 Libraries/LibWeb/DOM/Document.cpp                     |  5 ++---
 .../css/css-transitions/transitioncancel-003.txt      |  4 ++--
 3 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/Libraries/LibWeb/Animations/Animation.cpp b/Libraries/LibWeb/Animations/Animation.cpp
index 8f69d371c93..75caac5b52e 100644
--- a/Libraries/LibWeb/Animations/Animation.cpp
+++ b/Libraries/LibWeb/Animations/Animation.cpp
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2023-2024, Matthew Olsson <mattco@serenityos.org>.
+ * Copyright (c) 2025, Jelle Raaijmakers <jelle@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -934,7 +935,9 @@ GC::Ptr<DOM::Document> Animation::document_for_timing() const
 
 void Animation::notify_timeline_time_did_change()
 {
-    update_finished_state(DidSeek::No, SynchronouslyNotify::Yes);
+    // Update finished state if not already finished; prevents recurring invalidation when the timeline updates.
+    if (!m_is_finished)
+        update_finished_state(DidSeek::No, SynchronouslyNotify::Yes);
 
     // Act on the pending play or pause task
     if (m_pending_play_task == TaskState::Scheduled) {
@@ -1330,13 +1333,11 @@ GC::Ref<WebIDL::Promise> Animation::current_finished_promise() const
 
 void Animation::invalidate_effect()
 {
-    if (!m_effect) {
+    if (!m_effect)
         return;
-    }
 
-    if (auto* target = m_effect->target(); target) {
+    if (auto* target = m_effect->target())
         target->document().set_needs_animated_style_update();
-    }
 }
 
 Animation::Animation(JS::Realm& realm)
diff --git a/Libraries/LibWeb/DOM/Document.cpp b/Libraries/LibWeb/DOM/Document.cpp
index f02077a11d8..d9d6ec2a425 100644
--- a/Libraries/LibWeb/DOM/Document.cpp
+++ b/Libraries/LibWeb/DOM/Document.cpp
@@ -1574,11 +1574,10 @@ void Document::update_animated_style_if_needed()
 
     for (auto& timeline : m_associated_animation_timelines) {
         for (auto& animation : timeline->associated_animations()) {
-            if (animation->is_idle() || animation->is_finished())
+            if (animation->is_idle())
                 continue;
-            if (auto effect = animation->effect()) {
+            if (auto effect = animation->effect())
                 effect->update_computed_properties(context);
-            }
         }
     }
 
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-transitions/transitioncancel-003.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-transitions/transitioncancel-003.txt
index 93cbe377682..cbb9ab615dd 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/css/css-transitions/transitioncancel-003.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-transitions/transitioncancel-003.txt
@@ -2,5 +2,5 @@ Harness status: OK
 
 Found 1 tests
 
-1 Fail
-Fail	Mid-transition transition changes affect subsequent transitions
\ No newline at end of file
+1 Pass
+Pass	Mid-transition transition changes affect subsequent transitions
\ No newline at end of file