WIP: Prune the prepaint tree walk

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"
@@ skipping 80 matching lines @@
                            bounds, userScrollableHorizontal,
                            userScrollableVertical, mainThreadScrollingReasons);
   } else {
     frameView.setScroll(ScrollPaintPropertyNode::create(
         std::move(parent), std::move(scrollOffset), clip, bounds,
         userScrollableHorizontal, userScrollableVertical,
         mainThreadScrollingReasons));
   }
 }
 
-void PaintPropertyTreeBuilder::updateProperties(
+TreeStructureChange PaintPropertyTreeBuilder::updateProperties(
     FrameView& frameView,
     PaintPropertyTreeBuilderContext& context) {
   if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
     // With root layer scrolling, the LayoutView (a LayoutObject) properties are
     // updated like other objects (see updatePropertiesAndContextForSelf and
     // updatePropertiesAndContextForChildren) instead of needing LayoutView-
     // specific property updates here.
     context.current.paintOffset.moveBy(frameView.location());
     context.current.renderingContextID = 0;
     context.current.shouldFlattenInheritedTransform = true;
     context.absolutePosition = context.current;
     context.containerForAbsolutePosition = nullptr;
     context.fixedPosition = context.current;
-    return;
+    return StructureNotChanged;
   }
 
 #if DCHECK_IS_ON()
   FindFrameViewPropertiesNeedingUpdateScope checkNeedsUpdateScope(&frameView);
 #endif
 
   if (frameView.needsPaintPropertyUpdate()) {
     TransformationMatrix frameTranslate;
     frameTranslate.translate(frameView.x() + context.current.paintOffset.x(),
                              frameView.y() + context.current.paintOffset.y());
@@ skipping 58 matching lines @@
   context.absolutePosition = context.current;
   context.containerForAbsolutePosition = nullptr;
   context.fixedPosition = context.current;
   context.fixedPosition.transform = fixedTransformNode;
   context.fixedPosition.scroll = fixedScrollNode;
 
   std::unique_ptr<PropertyTreeState> contentsState(
       new PropertyTreeState(context.current.transform, context.current.clip,
                             context.currentEffect, context.current.scroll));
   frameView.setTotalPropertyTreeStateForContents(std::move(contentsState));
+
+  // TODO(pdr): Do not return StructureChanged when the FrameView update does
+  // not actually cause a property tree node to be created/removed.
+  return frameView.needsPaintPropertyUpdate() ? StructureChanged
+                                              : StructureNotChanged;
 }
 
 void PaintPropertyTreeBuilder::updatePaintOffsetTranslation(
     const LayoutObject& object,
-    PaintPropertyTreeBuilderContext& context) {
+    PaintPropertyTreeBuilderContext& context,
+    bool& treeStructureChanged) {
   bool usesPaintOffsetTranslation = false;
   if (RuntimeEnabledFeatures::rootLayerScrollingEnabled() &&
       object.isLayoutView()) {
     // Root layer scrolling always creates a translation node for LayoutView to
     // ensure fixed and absolute contexts use the correct transform space.
     usesPaintOffsetTranslation = true;
   } else if (object.isBoxModelObject() &&
              context.current.paintOffset != LayoutPoint()) {
     // TODO(trchen): Eliminate PaintLayer dependency.
     PaintLayer* layer = toLayoutBoxModelObject(object).layer();
     if (layer && layer->paintsWithTransform(GlobalPaintNormalPhase))
       usesPaintOffsetTranslation = true;
   }
 
   // We should use the same subpixel paint offset values for snapping
   // regardless of whether a transform is present. If there is a transform
   // we round the paint offset but keep around the residual fractional
   // component for the transformed content to paint with.  In spv1 this was
   // called "subpixel accumulation". For more information, see
   // PaintLayer::subpixelAccumulation() and
   // PaintLayerPainter::paintFragmentByApplyingTransform.
   IntPoint roundedPaintOffset = roundedIntPoint(context.current.paintOffset);
   LayoutPoint fractionalPaintOffset =
       LayoutPoint(context.current.paintOffset - roundedPaintOffset);
 
   if (object.needsPaintPropertyUpdate()) {
     if (usesPaintOffsetTranslation) {
-      object.getMutableForPainting()
-          .ensurePaintProperties()
-          .updatePaintOffsetTranslation(
-              context.current.transform,
-              TransformationMatrix().translate(roundedPaintOffset.x(),
-                                               roundedPaintOffset.y()),
-              FloatPoint3D(), context.current.shouldFlattenInheritedTransform,
-              context.current.renderingContextID);
+      treeStructureChanged |=
+          object.getMutableForPainting()
+              .ensurePaintProperties()
+              .updatePaintOffsetTranslation(
+                  context.current.transform,
+                  TransformationMatrix().translate(roundedPaintOffset.x(),
+                                                   roundedPaintOffset.y()),
+                  FloatPoint3D(),
+                  context.current.shouldFlattenInheritedTransform,
+                  context.current.renderingContextID);
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
-        properties->clearPaintOffsetTranslation();
+        treeStructureChanged |= properties->clearPaintOffsetTranslation();
     }
   }
 
   const auto* properties = object.paintProperties();
   if (properties && properties->paintOffsetTranslation()) {
     context.current.transform = properties->paintOffsetTranslation();
     context.current.paintOffset = fractionalPaintOffset;
     if (RuntimeEnabledFeatures::rootLayerScrollingEnabled() &&
         object.isLayoutView()) {
       context.absolutePosition.transform = properties->paintOffsetTranslation();
       context.fixedPosition.transform = properties->paintOffsetTranslation();
       context.absolutePosition.paintOffset = LayoutPoint();
       context.fixedPosition.paintOffset = LayoutPoint();
     }
   }
 }
 
 static FloatPoint3D transformOrigin(const LayoutBox& box) {
   const ComputedStyle& style = box.styleRef();
   FloatSize borderBoxSize(box.size());
   return FloatPoint3D(
       floatValueForLength(style.transformOriginX(), borderBoxSize.width()),
       floatValueForLength(style.transformOriginY(), borderBoxSize.height()),
       style.transformOriginZ());
 }
 
 // SVG does not use the general transform update of |updateTransform|, instead
 // creating a transform node for SVG-specific transforms without 3D.
-void PaintPropertyTreeBuilder::updateTransformForNonRootSVG(
+TreeStructureChange PaintPropertyTreeBuilder::updateTransformForNonRootSVG(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   DCHECK(object.isSVG() && !object.isSVGRoot());
   // SVG does not use paint offset internally.
   DCHECK(context.current.paintOffset == LayoutPoint());
 
+  bool treeStructureChanged = false;
   if (object.needsPaintPropertyUpdate()) {
     const AffineTransform& transform = object.localToSVGParentTransform();
     // TODO(pdr): Check for the presence of a transform instead of the value.
     // Checking for an identity matrix will cause the property tree structure
     // to change during animations if the animation passes through the
     // identity matrix.
     if (!transform.isIdentity()) {
       // The origin is included in the local transform, so leave origin empty.
-      object.getMutableForPainting().ensurePaintProperties().updateTransform(
-          context.current.transform, TransformationMatrix(transform),
-          FloatPoint3D());
+      treeStructureChanged |=
+          object.getMutableForPainting()
+              .ensurePaintProperties()
+              .updateTransform(context.current.transform,
+                               TransformationMatrix(transform), FloatPoint3D());
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
-        properties->clearTransform();
+        treeStructureChanged |= properties->clearTransform();
     }
   }
 
   if (object.paintProperties() && object.paintProperties()->transform()) {
     context.current.transform = object.paintProperties()->transform();
     context.current.shouldFlattenInheritedTransform = false;
     context.current.renderingContextID = 0;
   }
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updateTransform(
+TreeStructureChange PaintPropertyTreeBuilder::updateTransform(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
-  if (object.isSVG() && !object.isSVGRoot()) {
-    updateTransformForNonRootSVG(object, context);
-    return;
-  }
+  if (object.isSVG() && !object.isSVGRoot())
+    return updateTransformForNonRootSVG(object, context);
 
+  bool treeStructureChanged = false;
   if (object.needsPaintPropertyUpdate()) {
     const ComputedStyle& style = object.styleRef();
     if (object.isBox() && (style.hasTransform() || style.preserves3D())) {
       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);
 
-      object.getMutableForPainting().ensurePaintProperties().updateTransform(
-          context.current.transform, matrix,
-          transformOrigin(toLayoutBox(object)),
-          context.current.shouldFlattenInheritedTransform, renderingContextID);
+      treeStructureChanged |=
+          object.getMutableForPainting()
+              .ensurePaintProperties()
+              .updateTransform(context.current.transform, matrix,
+                               transformOrigin(toLayoutBox(object)),
+                               context.current.shouldFlattenInheritedTransform,
+                               renderingContextID);
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
-        properties->clearTransform();
+        treeStructureChanged |= properties->clearTransform();
     }
   }
 
   const auto* properties = object.paintProperties();
   if (properties && properties->transform()) {
     context.current.transform = properties->transform();
     if (object.styleRef().preserves3D()) {
       context.current.renderingContextID =
           properties->transform()->renderingContextID();
       context.current.shouldFlattenInheritedTransform = false;
     } else {
       context.current.renderingContextID = 0;
       context.current.shouldFlattenInheritedTransform = true;
     }
   }
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updateEffect(
+TreeStructureChange PaintPropertyTreeBuilder::updateEffect(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
+  bool treeStructureChanged = false;
   const ComputedStyle& style = object.styleRef();
-
   if (!style.isStackingContext()) {
     if (object.needsPaintPropertyUpdate()) {
       if (auto* properties = object.getMutableForPainting().paintProperties())
-        properties->clearEffect();
+        treeStructureChanged |= properties->clearEffect();
     }
-    return;
+    return static_cast<TreeStructureChange>(treeStructureChanged);
   }
 
   // TODO(trchen): Can't omit effect node if we have 3D children.
   // TODO(trchen): Can't omit effect node if we have blending children.
   if (object.needsPaintPropertyUpdate()) {
     bool effectNodeNeeded = false;
 
     float opacity = style.opacity();
     if (opacity != 1.0f)
       effectNodeNeeded = true;
@@ skipping 28 matching lines @@
     if (!filter.isEmpty()) {
       effectNodeNeeded = true;
       outputClip = context.current.clip;
 
       // TODO(trchen): A filter may contain spatial operations such that an
       // output pixel may depend on an input pixel outside of the output clip.
       // We should generate a special clip node to represent this expansion.
     }
 
     if (effectNodeNeeded) {
-      object.getMutableForPainting().ensurePaintProperties().updateEffect(
-          context.currentEffect, context.current.transform, outputClip,
-          std::move(filter), opacity);
+      treeStructureChanged |=
+          object.getMutableForPainting().ensurePaintProperties().updateEffect(
+              context.currentEffect, context.current.transform, outputClip,
+              std::move(filter), opacity);
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
-        properties->clearEffect();
+        treeStructureChanged |= properties->clearEffect();
     }
   }
 
   const auto* properties = object.paintProperties();
   if (properties && properties->effect()) {
     context.currentEffect = properties->effect();
     // TODO(pdr): Once the expansion clip node is created above, it should be
     // used here to update all current clip nodes;
     const ClipPaintPropertyNode* expansionHint = context.current.clip;
     context.current.clip = context.absolutePosition.clip =
         context.fixedPosition.clip = expansionHint;
   }
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updateCssClip(
+TreeStructureChange PaintPropertyTreeBuilder::updateCssClip(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
+  bool treeStructureChanged = false;
   if (object.needsPaintPropertyUpdate()) {
     if (object.hasClip()) {
       // Create clip node for descendants that are not fixed position.
       // We don't have to setup context.absolutePosition.clip here because this
       // object must be a container for absolute position descendants, and will
       // copy from in-flow context later at updateOutOfFlowContext() step.
       DCHECK(object.canContainAbsolutePositionObjects());
       LayoutRect clipRect =
           toLayoutBox(object).clipRect(context.current.paintOffset);
-      object.getMutableForPainting().ensurePaintProperties().updateCssClip(
-          context.current.clip, context.current.transform,
-          FloatRoundedRect(FloatRect(clipRect)));
+      treeStructureChanged |=
+          object.getMutableForPainting().ensurePaintProperties().updateCssClip(
+              context.current.clip, context.current.transform,
+              FloatRoundedRect(FloatRect(clipRect)));
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
-        properties->clearCssClip();
+        treeStructureChanged |= properties->clearCssClip();
     }
   }
 
   const auto* properties = object.paintProperties();
   if (properties && properties->cssClip())
     context.current.clip = properties->cssClip();
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updateLocalBorderBoxContext(
+TreeStructureChange PaintPropertyTreeBuilder::updateLocalBorderBoxContext(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   if (!object.needsPaintPropertyUpdate())
-    return;
+    return StructureNotChanged;
 
+  bool treeStructureChanged = false;
   // Avoid adding an ObjectPaintProperties for non-boxes to save memory, since
   // we don't need them at the moment.
   if (!object.isBox() && !object.hasLayer()) {
     if (auto* properties = object.getMutableForPainting().paintProperties())
-      properties->clearLocalBorderBoxProperties();
+      treeStructureChanged |= properties->clearLocalBorderBoxProperties();
   } else {
     std::unique_ptr<ObjectPaintProperties::PropertyTreeStateWithOffset>
         borderBoxContext =
             wrapUnique(new ObjectPaintProperties::PropertyTreeStateWithOffset(
                 context.current.paintOffset,
                 PropertyTreeState(context.current.transform,
                                   context.current.clip, context.currentEffect,
                                   context.current.scroll)));
-    object.getMutableForPainting()
-        .ensurePaintProperties()
-        .setLocalBorderBoxProperties(std::move(borderBoxContext));
+    treeStructureChanged |=
+        object.getMutableForPainting()
+            .ensurePaintProperties()
+            .setLocalBorderBoxProperties(std::move(borderBoxContext));
   }
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
 // TODO(trchen): Remove this once we bake the paint offset into frameRect.
-void PaintPropertyTreeBuilder::updateScrollbarPaintOffset(
+TreeStructureChange PaintPropertyTreeBuilder::updateScrollbarPaintOffset(
     const LayoutObject& object,
     const PaintPropertyTreeBuilderContext& context) {
   if (!object.needsPaintPropertyUpdate())
-    return;
+    return StructureNotChanged;
 
+  bool treeStructureChanged = false;
   bool needsScrollbarPaintOffset = false;
   IntPoint roundedPaintOffset = roundedIntPoint(context.current.paintOffset);
   if (roundedPaintOffset != IntPoint() && object.isBoxModelObject()) {
     if (auto* area = toLayoutBoxModelObject(object).getScrollableArea()) {
       if (area->horizontalScrollbar() || area->verticalScrollbar()) {
         auto paintOffset = TransformationMatrix().translate(
             roundedPaintOffset.x(), roundedPaintOffset.y());
-        object.getMutableForPainting()
-            .ensurePaintProperties()
-            .updateScrollbarPaintOffset(context.current.transform, paintOffset,
-                                        FloatPoint3D());
+        treeStructureChanged |=
+            object.getMutableForPainting()
+                .ensurePaintProperties()
+                .updateScrollbarPaintOffset(context.current.transform,
+                                            paintOffset, FloatPoint3D());
         needsScrollbarPaintOffset = true;
       }
     }
   }
 
   auto* properties = object.getMutableForPainting().paintProperties();
   if (!needsScrollbarPaintOffset && properties)
-    properties->clearScrollbarPaintOffset();
+    treeStructureChanged |= properties->clearScrollbarPaintOffset();
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updateOverflowClip(
+TreeStructureChange PaintPropertyTreeBuilder::updateOverflowClip(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   if (!object.isBox())
-    return;
+    return StructureNotChanged;
 
+  bool treeStructureChanged = false;
   if (object.needsPaintPropertyUpdate()) {
     const LayoutBox& box = toLayoutBox(object);
     // The <input> elements can't have contents thus CSS overflow property
     // doesn't apply.  However for layout purposes we do generate child layout
     // objects for them, e.g. button label.  We should clip the overflow from
     // those children. This is called control clip and we technically treat them
     // like overflow clip.
     LayoutRect clipRect;
     if (box.hasControlClip()) {
       clipRect = box.controlClipRect(context.current.paintOffset);
     } else if (box.hasOverflowClip() || box.styleRef().containsPaint() ||
                (box.isSVGRoot() &&
                 toLayoutSVGRoot(box).shouldApplyViewportClip())) {
       clipRect = LayoutRect(pixelSnappedIntRect(
           box.overflowClipRect(context.current.paintOffset)));
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties()) {
-        properties->clearInnerBorderRadiusClip();
-        properties->clearOverflowClip();
+        treeStructureChanged |= properties->clearInnerBorderRadiusClip();
+        treeStructureChanged |= properties->clearOverflowClip();
       }
-      return;
+      return static_cast<TreeStructureChange>(treeStructureChanged);
     }
 
     const auto* currentClip = context.current.clip;
     if (box.styleRef().hasBorderRadius()) {
       auto innerBorder = box.styleRef().getRoundedInnerBorderFor(
           LayoutRect(context.current.paintOffset, box.size()));
-      object.getMutableForPainting()
-          .ensurePaintProperties()
-          .updateInnerBorderRadiusClip(context.current.clip,
-                                       context.current.transform, innerBorder);
+      treeStructureChanged |=
+          object.getMutableForPainting()
+              .ensurePaintProperties()
+              .updateInnerBorderRadiusClip(
+                  context.current.clip, context.current.transform, innerBorder);
       currentClip = object.paintProperties()->innerBorderRadiusClip();
     } else if (auto* properties =
                    object.getMutableForPainting().paintProperties()) {
-      properties->clearInnerBorderRadiusClip();
+      treeStructureChanged |= properties->clearInnerBorderRadiusClip();
     }
 
-    object.getMutableForPainting().ensurePaintProperties().updateOverflowClip(
-        currentClip, context.current.transform,
-        FloatRoundedRect(FloatRect(clipRect)));
+    treeStructureChanged |=
+        object.getMutableForPainting()
+            .ensurePaintProperties()
+            .updateOverflowClip(currentClip, context.current.transform,
+                                FloatRoundedRect(FloatRect(clipRect)));
   }
 
   const auto* properties = object.paintProperties();
   if (properties && properties->overflowClip())
     context.current.clip = properties->overflowClip();
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
 static FloatPoint perspectiveOrigin(const LayoutBox& box) {
   const ComputedStyle& style = box.styleRef();
   FloatSize borderBoxSize(box.size());
   return FloatPoint(
       floatValueForLength(style.perspectiveOriginX(), borderBoxSize.width()),
       floatValueForLength(style.perspectiveOriginY(), borderBoxSize.height()));
 }
 
-void PaintPropertyTreeBuilder::updatePerspective(
+TreeStructureChange PaintPropertyTreeBuilder::updatePerspective(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
+  bool treeStructureChanged = false;
   if (object.needsPaintPropertyUpdate()) {
     const ComputedStyle& style = object.styleRef();
     if (object.isBox() && style.hasPerspective()) {
       // The perspective node must not flatten (else nothing will get
       // perspective), but it should still extend the rendering context as
       // most transform nodes do.
       TransformationMatrix matrix =
           TransformationMatrix().applyPerspective(style.perspective());
       FloatPoint3D origin = perspectiveOrigin(toLayoutBox(object)) +
                             toLayoutSize(context.current.paintOffset);
-      object.getMutableForPainting().ensurePaintProperties().updatePerspective(
-          context.current.transform, matrix, origin,
-          context.current.shouldFlattenInheritedTransform,
-          context.current.renderingContextID);
+      treeStructureChanged |=
+          object.getMutableForPainting()
+              .ensurePaintProperties()
+              .updatePerspective(
+                  context.current.transform, matrix, origin,
+                  context.current.shouldFlattenInheritedTransform,
+                  context.current.renderingContextID);
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
-        properties->clearPerspective();
+        treeStructureChanged |= properties->clearPerspective();
     }
   }
 
   const auto* properties = object.paintProperties();
   if (properties && properties->perspective()) {
     context.current.transform = properties->perspective();
     context.current.shouldFlattenInheritedTransform = false;
   }
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updateSvgLocalToBorderBoxTransform(
+TreeStructureChange
+PaintPropertyTreeBuilder::updateSvgLocalToBorderBoxTransform(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   if (!object.isSVGRoot())
-    return;
+    return StructureNotChanged;
 
+  bool treeStructureChanged = false;
   if (object.needsPaintPropertyUpdate()) {
     AffineTransform transformToBorderBox =
         SVGRootPainter(toLayoutSVGRoot(object))
             .transformToPixelSnappedBorderBox(context.current.paintOffset);
     if (!transformToBorderBox.isIdentity()) {
-      object.getMutableForPainting()
-          .ensurePaintProperties()
-          .updateSvgLocalToBorderBoxTransform(
-              context.current.transform, transformToBorderBox, FloatPoint3D());
+      treeStructureChanged |= object.getMutableForPainting()
+                                  .ensurePaintProperties()
+                                  .updateSvgLocalToBorderBoxTransform(
+                                      context.current.transform,
+                                      transformToBorderBox, FloatPoint3D());
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
-        properties->clearSvgLocalToBorderBoxTransform();
+        treeStructureChanged |= properties->clearSvgLocalToBorderBoxTransform();
     }
   }
 
   const auto* properties = object.paintProperties();
   if (properties && properties->svgLocalToBorderBoxTransform()) {
     context.current.transform = properties->svgLocalToBorderBoxTransform();
     context.current.shouldFlattenInheritedTransform = false;
     context.current.renderingContextID = 0;
   }
   // The paint offset is included in |transformToBorderBox| so SVG does not need
   // to handle paint offset internally.
   context.current.paintOffset = LayoutPoint();
+
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updateScrollAndScrollTranslation(
+TreeStructureChange PaintPropertyTreeBuilder::updateScrollAndScrollTranslation(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
+  bool treeStructureChanged = false;
   if (object.needsPaintPropertyUpdate()) {
     if (object.hasOverflowClip()) {
       const LayoutBox& box = toLayoutBox(object);
       const PaintLayerScrollableArea* scrollableArea = box.getScrollableArea();
       IntSize scrollOffset = box.scrolledContentOffset();
       if (!scrollOffset.isZero() || scrollableArea->scrollsOverflow()) {
         TransformationMatrix matrix = TransformationMatrix().translate(
             -scrollOffset.width(), -scrollOffset.height());
-        object.getMutableForPainting()
-            .ensurePaintProperties()
-            .updateScrollTranslation(
-                context.current.transform, matrix, FloatPoint3D(),
-                context.current.shouldFlattenInheritedTransform,
-                context.current.renderingContextID);
+        treeStructureChanged |=
+            object.getMutableForPainting()
+                .ensurePaintProperties()
+                .updateScrollTranslation(
+                    context.current.transform, matrix, FloatPoint3D(),
+                    context.current.shouldFlattenInheritedTransform,
+                    context.current.renderingContextID);
 
         IntSize scrollClip = scrollableArea->visibleContentRect().size();
         IntSize scrollBounds = scrollableArea->contentsSize();
         bool userScrollableHorizontal =
             scrollableArea->userInputScrollable(HorizontalScrollbar);
         bool userScrollableVertical =
             scrollableArea->userInputScrollable(VerticalScrollbar);
         MainThreadScrollingReasons reasons = 0;
         if (!object.document().settings()->threadedScrollingEnabled())
           reasons |= MainThreadScrollingReason::kThreadedScrollingDisabled;
         // Checking for descendants in the layout tree has two downsides:
         // 1) There can be more descendants in layout order than in paint
         //    order (e.g., fixed position objects).
         // 2) Iterating overall all background attachment fixed objects for
         //    every scroll node can be slow, though there will be no objects
         //    in the common case.
         const FrameView& frameView = *object.frameView();
         if (frameView.hasBackgroundAttachmentFixedDescendants(object)) {
           reasons |=
               MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects;
         }
-        object.getMutableForPainting().ensurePaintProperties().updateScroll(
-            context.current.scroll,
-            object.paintProperties()->scrollTranslation(), scrollClip,
-            scrollBounds, userScrollableHorizontal, userScrollableVertical,
-            reasons);
+        treeStructureChanged |=
+            object.getMutableForPainting().ensurePaintProperties().updateScroll(
+                context.current.scroll,
+                object.paintProperties()->scrollTranslation(), scrollClip,
+                scrollBounds, userScrollableHorizontal, userScrollableVertical,
+                reasons);
       } else {
         // Ensure pre-existing properties are cleared when there is no
         // scrolling.
         auto* properties = object.getMutableForPainting().paintProperties();
         if (properties) {
-          properties->clearScrollTranslation();
-          properties->clearScroll();
+          treeStructureChanged |= properties->clearScrollTranslation();
+          treeStructureChanged |= properties->clearScroll();
         }
       }
     }
   }
 
   if (object.paintProperties() && object.paintProperties()->scroll()) {
     context.current.transform = object.paintProperties()->scrollTranslation();
     context.current.scroll = object.paintProperties()->scroll();
     context.current.shouldFlattenInheritedTransform = false;
   }
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updateOutOfFlowContext(
+TreeStructureChange PaintPropertyTreeBuilder::updateOutOfFlowContext(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   if (object.canContainAbsolutePositionObjects()) {
     context.absolutePosition = context.current;
     context.containerForAbsolutePosition = &object;
   }
 
+  bool treeStructureChanged = false;
   if (object.isLayoutView()) {
     if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
       const auto* initialFixedTransform = context.fixedPosition.transform;
       const auto* initialFixedScroll = context.fixedPosition.scroll;
 
       context.fixedPosition = context.current;
 
       // Fixed position transform and scroll nodes should not be affected.
       context.fixedPosition.transform = initialFixedTransform;
       context.fixedPosition.scroll = initialFixedScroll;
     }
   } else if (object.canContainFixedPositionObjects()) {
     context.fixedPosition = context.current;
   } else if (object.getMutableForPainting().paintProperties() &&
              object.paintProperties()->cssClip()) {
     // CSS clip applies to all descendants, even if this object is not a
     // containing block ancestor of the descendant. It is okay for
     // absolute-position descendants because having CSS clip implies being
     // absolute position container. However for fixed-position descendants we
     // need to insert the clip here if we are not a containing block ancestor of
     // them.
     auto* cssClip = object.getMutableForPainting().paintProperties()->cssClip();
 
     // Before we actually create anything, check whether in-flow context and
     // fixed-position context has exactly the same clip. Reuse if possible.
     if (context.fixedPosition.clip == cssClip->parent()) {
       context.fixedPosition.clip = cssClip;
     } else {
       if (object.needsPaintPropertyUpdate()) {
-        object.getMutableForPainting()
-            .ensurePaintProperties()
-            .updateCssClipFixedPosition(context.fixedPosition.clip,
+        treeStructureChanged |= object.getMutableForPainting()
+                                    .ensurePaintProperties()
+                                    .updateCssClipFixedPosition(
+                                        context.fixedPosition.clip,
                                         const_cast<TransformPaintPropertyNode*>(
                                             cssClip->localTransformSpace()),
                                         cssClip->clipRect());
       }
       const auto* properties = object.paintProperties();
       if (properties && properties->cssClipFixedPosition())
         context.fixedPosition.clip = properties->cssClipFixedPosition();
-      return;
+      return static_cast<TreeStructureChange>(treeStructureChanged);
     }
   }
 
   if (object.needsPaintPropertyUpdate()) {
     if (auto* properties = object.getMutableForPainting().paintProperties())
-      properties->clearCssClipFixedPosition();
+      treeStructureChanged |= properties->clearCssClipFixedPosition();
   }
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
 // Override ContainingBlockContext based on the properties of a containing block
 // that was previously walked in a subtree other than the current subtree being
 // walked. Used for out-of-flow positioned descendants of multi-column spanner
 // when the containing block is not in the normal tree walk order.
 // For example:
 // <div id="columns" style="columns: 2">
 //   <div id="relative" style="position: relative">
 //     <div id="spanner" style="column-span: all">
@@ skipping 99 matching lines @@
     // Similar adjustment is done in LayoutTableCell::offsetFromContainer().
     if (boxModelObject.isTableCell()) {
       LayoutObject* parentRow = boxModelObject.parent();
       DCHECK(parentRow && parentRow->isTableRow());
       context.current.paintOffset.moveBy(
           -toLayoutBox(parentRow)->topLeftLocation());
     }
   }
 }
 
-void PaintPropertyTreeBuilder::updatePropertiesForSelf(
+TreeStructureChange PaintPropertyTreeBuilder::updatePropertiesForSelf(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   if (!object.isBoxModelObject() && !object.isSVG())
-    return;
+    return StructureNotChanged;
 
 #if DCHECK_IS_ON()
   FindObjectPropertiesNeedingUpdateScope checkNeedsUpdateScope(object);
 #endif
 
   deriveBorderBoxFromContainerContext(object, context);
 
-  updatePaintOffsetTranslation(object, context);
-  updateTransform(object, context);
-  updateEffect(object, context);
-  updateCssClip(object, context);
-  updateLocalBorderBoxContext(object, context);
-  updateScrollbarPaintOffset(object, context);
+  bool treeStructureChanged = false;
+  updatePaintOffsetTranslation(object, context, treeStructureChanged);
+  treeStructureChanged |= updateTransform(object, context);
+  treeStructureChanged |= updateEffect(object, context);
+  treeStructureChanged |= updateCssClip(object, context);
+  treeStructureChanged |= updateLocalBorderBoxContext(object, context);
+  treeStructureChanged |= updateScrollbarPaintOffset(object, context);
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
-void PaintPropertyTreeBuilder::updatePropertiesForChildren(
+TreeStructureChange PaintPropertyTreeBuilder::updatePropertiesForChildren(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   if (!object.isBoxModelObject() && !object.isSVG())
-    return;
+    return StructureNotChanged;
 
 #if DCHECK_IS_ON()
   FindObjectPropertiesNeedingUpdateScope checkNeedsUpdateScope(object);
 #endif
 
-  updateOverflowClip(object, context);
-  updatePerspective(object, context);
-  updateSvgLocalToBorderBoxTransform(object, context);
-  updateScrollAndScrollTranslation(object, context);
-  updateOutOfFlowContext(object, context);
+  bool treeStructureChanged = false;
+  treeStructureChanged |= updateOverflowClip(object, context);
+  treeStructureChanged |= updatePerspective(object, context);
+  treeStructureChanged |= updateSvgLocalToBorderBoxTransform(object, context);
+  treeStructureChanged |= updateScrollAndScrollTranslation(object, context);
+  treeStructureChanged |= updateOutOfFlowContext(object, context);
+  return static_cast<TreeStructureChange>(treeStructureChanged);
 }
 
 }  // namespace blink