[view-transitions] Spinner animation on https://www.kvin.me/posts/but…

…ton-interaction jumps

https://bugs.webkit.org/show_bug.cgi?id=274400
<rdar://128405857>

Reviewed by Tim Nguyen.

When an element is captured in a view transition, its 'transform' gets applied to the
::view-transition-group pseudo element, and we stop it being applied to the layer
for the element itself.
This makes sure we don't accidentally put it back again with an async animation.

* Source/WebCore/rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::startAnimation):

Canonical link: https://commits.webkit.org/279170@main

Source/WebCore/rendering/RenderLayerBacking.cpp

@@ -3997,6 +3997,9 @@ void RenderLayerBacking::verifyNotPainting()
 
 bool RenderLayerBacking::startAnimation(double timeOffset, const Animation& animation, const BlendingKeyframes& keyframes)
 {
+    if (renderer().capturedInViewTransition())
+        return false;
+
     bool shouldApplyAnimationsToTargetRenderer = renderer().isRenderBox() || renderer().isSVGLayerAwareRenderer();
 
     bool hasOpacity = keyframes.containsProperty(CSSPropertyOpacity);