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diff --git a/Source/WebCore/rendering/style/WillChangeData.cpp b/Source/WebCore/rendering/style/WillChangeData.cpp
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+++ b/Source/WebCore/rendering/style/WillChangeData.cpp
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+/*
+ * Copyright (C) 2015 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "WillChangeData.h"
+
+namespace WebCore {
+
+bool WillChangeData::operator==(const WillChangeData& other) const
+{
+    return m_animatableFeatures == other.m_animatableFeatures;
+}
+
+bool WillChangeData::containsScrollPosition() const
+{
+    for (const auto& feature : m_animatableFeatures) {
+        if (feature.feature() == ScrollPosition)
+            return true;
+    }
+    return false;
+}
+
+bool WillChangeData::containsContents() const
+{
+    for (const auto& feature : m_animatableFeatures) {
+        if (feature.feature() == Contents)
+            return true;
+    }
+    return false;
+}
+
+bool WillChangeData::containsProperty(CSSPropertyID property) const
+{
+    for (const auto& feature : m_animatableFeatures) {
+        if (feature.property() == property)
+            return true;
+    }
+    return false;
+}
+
+// "If any non-initial value of a property would create a stacking context on the element,
+// specifying that property in will-change must create a stacking context on the element."
+bool WillChangeData::propertyCreatesStackingContext(CSSPropertyID property)
+{
+    switch (property) {
+    case CSSPropertyPerspective:
+    case CSSPropertyTransform:
+    case CSSPropertyTransformStyle:
+    case CSSPropertyWebkitTransformStyle:
+    case CSSPropertyClipPath:
+    case CSSPropertyWebkitClipPath:
+    case CSSPropertyMask:
+    case CSSPropertyOpacity:
+    case CSSPropertyPosition:
+    case CSSPropertyZIndex:
+    case CSSPropertyWebkitBoxReflect:
+#if ENABLE(CSS_COMPOSITING)
+    case CSSPropertyMixBlendMode:
+    case CSSPropertyIsolation:
+#endif
+    case CSSPropertyFilter:
+#if ENABLE(FILTERS_LEVEL_2)
+    case CSSPropertyWebkitBackdropFilter:
+#endif
+    case CSSPropertyWebkitMask:
+    case CSSPropertyWebkitMaskImage:
+    case CSSPropertyWebkitMaskBoxImage:
+#if ENABLE(CSS_REGIONS)
+    case CSSPropertyWebkitFlowFrom:
+#endif
+#if ENABLE(ACCELERATED_OVERFLOW_SCROLLING)
+    case CSSPropertyWebkitOverflowScrolling:
+#endif
+        return true;
+    default:
+        return false;
+    }
+}
+
+static bool propertyTriggersCompositing(CSSPropertyID property)
+{
+    switch (property) {
+    case CSSPropertyOpacity:
+    case CSSPropertyFilter:
+#if ENABLE(FILTERS_LEVEL_2)
+    case CSSPropertyWebkitBackdropFilter:
+#endif
+        return true;
+    default:
+        return false;
+    }
+}
+
+static bool propertyTriggersCompositingOnBoxesOnly(CSSPropertyID property)
+{
+    // Don't trigger for perspective and transform-style, because those
+    // only do compositing if they have a 3d-transformed descendant and
+    // we don't want to do compositing all the time.
+    // Similarly, we don't want -webkit-overflow-scrolling-touch to
+    // always composite if there's no scrollable overflow.
+    switch (property) {
+    case CSSPropertyTransform:
+        return true;
+    default:
+        return false;
+    }
+}
+
+void WillChangeData::addFeature(Feature feature, CSSPropertyID propertyID)
+{
+    ASSERT(feature == Property || propertyID == CSSPropertyInvalid);
+    m_animatableFeatures.append(AnimatableFeature(feature, propertyID));
+
+    m_canCreateStackingContext |= propertyCreatesStackingContext(propertyID);
+
+    m_canTriggerCompositingOnInline |= propertyTriggersCompositing(propertyID);
+    m_canTriggerCompositing |= m_canTriggerCompositingOnInline | propertyTriggersCompositingOnBoxesOnly(propertyID);
+}
+
+WillChangeData::FeaturePropertyPair WillChangeData::featureAt(size_t index) const
+{
+    if (index >= m_animatableFeatures.size())
+        return FeaturePropertyPair(Invalid, CSSPropertyInvalid);
+
+    return m_animatableFeatures[index].featurePropertyPair();
+}
+
+} // namespace WebCore