Add fast-path for propagated variable changes (WIP)

third_party/WebKit/Source/core/dom/Element.cpp

Index: third_party/WebKit/Source/core/dom/Element.cpp
diff --git a/third_party/WebKit/Source/core/dom/Element.cpp b/third_party/WebKit/Source/core/dom/Element.cpp
index b7cc35fd88b08689c815799321c8a9e47c565e40..5f89773399a8113b7c57402448419a37c7cbc2d7 100644
--- a/third_party/WebKit/Source/core/dom/Element.cpp
+++ b/third_party/WebKit/Source/core/dom/Element.cpp
@@ -1897,7 +1897,7 @@ void Element::recalcStyle(StyleRecalcChange change, Text* nextTextSibling) {
 
 PassRefPtr<ComputedStyle> Element::propagateInheritedProperties(
     StyleRecalcChange change) {
-  if (change != IndependentInherit)
+  if (change != IndependentInherit && change != IndependentInheritWithVariables)
     return nullptr;
   if (isPseudoElement())
     return nullptr;
@@ -1910,6 +1910,16 @@ PassRefPtr<ComputedStyle> Element::propagateInheritedProperties(
   const ComputedStyle* style = computedStyle();
   if (!style || style->animations() || style->transitions())
     return nullptr;
+  if (change == IndependentInheritWithVariables &&
+      (style->hasVariableReferenceFromInheritedProperty() ||
+       style->hasVariableReferenceFromNonInheritedProperty()
+       // If this element sets any variables, they may refer to other variables.
+       // To be safe, recalc the element's style.
+       // TODO(sashab): Set the hasVariableReferenceFromInheritedProperty flag
+       // on ComputedStyle when a variable references another variable and
+       // remove this additional check.
+       || style->hasAnyNonInheritedVariableDefinitions()))
+    return nullptr;
   RefPtr<ComputedStyle> newStyle = ComputedStyle::clone(*style);
   newStyle->propagateIndependentInheritedProperties(*parentStyle);
   INCREMENT_STYLE_STATS_COUNTER(document().styleEngine(),
@@ -1942,6 +1952,13 @@ StyleRecalcChange Element::recalcOwnStyle(StyleRecalcChange change,
     INCREMENT_STYLE_STATS_COUNTER(document().styleEngine(), stylesChanged, 1);
   }
 
+  if (change == IndependentInheritWithVariables && localChange == NoChange) {
+    // If we are in a propagation fast-path, but we have changed variables,
+    // there
+    // might still be properties that need to be updated below. Force continue.
+    localChange = IndependentInheritWithVariables;
+  }
+
   if (localChange == Reattach) {
     StyleReattachData styleReattachData;
     styleReattachData.computedStyle = std::move(newStyle);