[view-transitions] Async animation of snapshot position via transform…

… looks jumpy with sync animation of size

https://bugs.webkit.org/show_bug.cgi?id=274437
<rdar://128441578>

Reviewed by Antoine Quint.

The default pseudo element structure results in an asynchronous animation of the snapshot position via
transform, and a sync animation of the width/height.

If there's any slowness at all on the main thread, then this looks super jumpy.

* Source/WebCore/animation/KeyframeEffect.cpp:
(WebCore::KeyframeEffect::computeHasSizeDependentTransform):

Canonical link: https://commits.webkit.org/279093@main

Source/WebCore/animation/KeyframeEffect.cpp

@@ -2551,6 +2551,14 @@ void KeyframeEffect::computeHasSizeDependentTransform()
 {
     m_animatesSizeAndSizeDependentTransform = (m_blendingKeyframes.hasWidthDependentTransform() && m_blendingKeyframes.containsProperty(CSSPropertyWidth))
         || (m_blendingKeyframes.hasHeightDependentTransform() && m_blendingKeyframes.containsProperty(CSSPropertyHeight));
+
+    // If this is a ::view-transition-group pseudo element with the UA-generated transform
+    // and width/height animations, then prevent the transform component from being applied
+    // asynchronously.
+    if (auto target = targetStyleable()) {
+        if (target->pseudoElementIdentifier && target->pseudoElementIdentifier->pseudoId == PseudoId::ViewTransitionGroup)
+            m_animatesSizeAndSizeDependentTransform |= ((m_blendingKeyframes.containsProperty(CSSPropertyWidth) || m_blendingKeyframes.containsProperty(CSSPropertyHeight)) && m_blendingKeyframes.containsProperty(CSSPropertyTransform));
+    }
 }
 
 void KeyframeEffect::effectStackNoLongerPreventsAcceleration()