Ensure baseComputedStyle optimisation is cleared during registered custom property animations

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
@@ skipping 594 matching lines @@
     // presence of custom property animations that don't affect pre-animated
     // computed values.
     return nullptr;
   }
 
   return elementAnimations->baseComputedStyle();
 }
 
 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(
     Element* element,
     const ComputedStyle* defaultParent,
     const ComputedStyle* defaultLayoutParent,
     StyleSharingBehavior sharingBehavior,
     RuleMatchingBehavior matchingBehavior) {
   DCHECK(document().frame());
   DCHECK(document().settings());
@@ skipping 1315 matching lines @@
 
 DEFINE_TRACE(StyleResolver) {
   visitor->trace(m_matchedPropertiesCache);
   visitor->trace(m_selectorFilter);
   visitor->trace(m_styleSharingLists);
   visitor->trace(m_document);
   visitor->trace(m_tracker);
 }
 
 }  // namespace blink