Ensure baseComputedStyle optimisation is cleared during registered custom property animations

third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp

Index: third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
index e24d082b9a7b9a1162313d500b848fe9c182c6a9..844f7eee7ff1bc31047cc829eb29e8e0d5ef26f4 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
@@ -612,12 +612,17 @@ static const ComputedStyle* calculateBaseComputedStyle(
 
 static void updateBaseComputedStyle(StyleResolverState& state,
                                     Element* animatingElement) {
-  if (!animatingElement || state.isAnimatingCustomProperties())
+  if (!animatingElement)
     return;
 
   ElementAnimations* elementAnimations = animatingElement->elementAnimations();
-  if (elementAnimations)
-    elementAnimations->updateBaseComputedStyle(state.style());
+  if (elementAnimations) {
+    if (state.isAnimatingCustomProperties()) {
+      elementAnimations->clearBaseComputedStyle();

[suzyh_UTC10 (ex-contributor), 2017/03/17 03:03:13]

Sanity-check question: Is it possible instead to not set the baseComputedStyle
in the first place, rather than set it and clear it later?

[alancutter (OOO until 2018), 2017/03/17 03:36:50]

I did think of that, to not set the baseComputedStyle on the exit frame of an
animation and make this a non-issue.
I think you'd need to plumb through more data and logic to make that happen.
Much more complexity that it'd be worth despite it making sense to do.

+    } else {
+      elementAnimations->updateBaseComputedStyle(state.style());
+    }
+  }
 }
 
 PassRefPtr<ComputedStyle> StyleResolver::styleForElement(