Set a direct compositing reason for 3D transform & will-change property tree nodes

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintPropertyTreeBuilder.h"
 
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/Settings.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutView.h"
+#include "core/layout/compositing/CompositingReasonFinder.h"
 #include "core/layout/svg/LayoutSVGRoot.h"
 #include "core/paint/FindPropertiesNeedingUpdate.h"
 #include "core/paint/ObjectPaintProperties.h"
 #include "core/paint/PaintLayer.h"
 #include "core/paint/SVGRootPainter.h"
 #include "platform/transforms/TransformationMatrix.h"
 #include "wtf/PtrUtil.h"
 #include <memory>
 
 namespace blink {
@@ skipping 279 matching lines @@
     }
   }
 
   if (object.paintProperties() && object.paintProperties()->transform()) {
     context.current.transform = object.paintProperties()->transform();
     context.current.shouldFlattenInheritedTransform = false;
     context.current.renderingContextID = 0;
   }
 }
 
+static CompositingReasons compositingReasonsForTransform(
+    const LayoutObject& object) {
+  CompositingReasons compositingReasons = CompositingReasonNone;
+  if (CompositingReasonFinder::requiresCompositingForTransform(object))
+    compositingReasons |= CompositingReason3DTransform;
+
+  if (object.styleRef().hasWillChangeCompositingHint() &&
+      !object.styleRef().subtreeWillChangeContents())
+    compositingReasons |= CompositingReasonWillChangeCompositingHint;
+
+  return compositingReasons;
+}
+
 void PaintPropertyTreeBuilder::updateTransform(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   if (object.isSVG() && !object.isSVGRoot()) {
     updateTransformForNonRootSVG(object, context);
     return;
   }
 
   if (object.needsPaintPropertyUpdate() || context.forceSubtreeUpdate) {
     const ComputedStyle& style = object.styleRef();
-    if (object.isBox() && (style.hasTransform() || style.preserves3D())) {
+
+    CompositingReasons compositingReasons =
+        compositingReasonsForTransform(object);
+
+    // A transform node is allocated for transforms, preserves-3d and any
+    // direct compositing reason. The latter is required because this is the
+    // only way to represent compositing both an element and its stacking
+    // descendants.
+    if (object.isBox() && (style.hasTransform() || style.preserves3D() ||
+                           compositingReasons != CompositingReasonNone)) {
       TransformationMatrix matrix;
       style.applyTransform(
           matrix, toLayoutBox(object).size(),
           ComputedStyle::ExcludeTransformOrigin,
           ComputedStyle::IncludeMotionPath,
           ComputedStyle::IncludeIndependentTransformProperties);
 
       // TODO(trchen): transform-style should only be respected if a PaintLayer
       // is created.
       // If a node with transform-style: preserve-3d does not exist in an
       // existing rendering context, it establishes a new one.
       unsigned renderingContextID = context.current.renderingContextID;
       if (style.preserves3D() && !renderingContextID)
         renderingContextID = PtrHash<const LayoutObject>::hash(&object);
 
       auto& properties = object.getMutableForPainting().ensurePaintProperties();
       context.forceSubtreeUpdate |= properties.updateTransform(
           context.current.transform, matrix,
           transformOrigin(toLayoutBox(object)),
-          context.current.shouldFlattenInheritedTransform, renderingContextID);
+          context.current.shouldFlattenInheritedTransform, renderingContextID,
+          compositingReasons);
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
         context.forceSubtreeUpdate |= properties->clearTransform();
     }
   }
 
   const auto* properties = object.paintProperties();
   if (properties && properties->transform()) {
     context.current.transform = properties->transform();
     if (object.styleRef().preserves3D()) {
@@ skipping 553 matching lines @@
 #endif
 
   updateOverflowClip(object, context);
   updatePerspective(object, context);
   updateSvgLocalToBorderBoxTransform(object, context);
   updateScrollAndScrollTranslation(object, context);
   updateOutOfFlowContext(object, context);
 }
 
 }  // namespace blink