Reland recording box shadow as main thread scrolling reason with conflict fixed

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 23 matching lines @@
 #include "core/layout/LayoutGeometryMap.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutView.h"
 #include "core/layout/compositing/CompositedLayerMapping.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
 #include "core/paint/ObjectPaintInvalidator.h"
 #include "core/paint/PaintLayer.h"
 #include "core/style/ShadowList.h"
 #include "platform/LengthFunctions.h"
 #include "platform/geometry/TransformState.h"
+#include "platform/scroll/MainThreadScrollingReason.h"
 #include "wtf/PtrUtil.h"
 
 namespace blink {
 
 class FloatStateForStyleChange {
  public:
   static void setWasFloating(LayoutBoxModelObject* boxModelObject,
                              bool wasFloating) {
     s_wasFloating = wasFloating;
     s_boxModelObject = boxModelObject;
@@ skipping 50 matching lines @@
 }
 
 LayoutBoxModelObject::LayoutBoxModelObject(ContainerNode* node)
     : LayoutObject(node) {}
 
 bool LayoutBoxModelObject::usesCompositedScrolling() const {
   return hasOverflowClip() && hasLayer() &&
          layer()->getScrollableArea()->usesCompositedScrolling();
 }
 
-BackgroundPaintLocation LayoutBoxModelObject::backgroundPaintLocation() const {
+BackgroundPaintLocation LayoutBoxModelObject::backgroundPaintLocation(
+    uint32_t* reasons) const {
   bool hasCustomScrollbars = false;
   // TODO(flackr): Detect opaque custom scrollbars which would cover up a
   // border-box background.
   if (PaintLayerScrollableArea* scrollableArea = getScrollableArea()) {
     if ((scrollableArea->horizontalScrollbar() &&
          scrollableArea->horizontalScrollbar()->isCustomScrollbar()) ||
         (scrollableArea->verticalScrollbar() &&
          scrollableArea->verticalScrollbar()->isCustomScrollbar())) {
       hasCustomScrollbars = true;
     }
   }
 
   // TODO(flackr): When we correctly clip the scrolling contents layer we can
   // paint locally equivalent backgrounds into it. https://crbug.com/645957
   if (!style()->hasAutoClip())
     return BackgroundPaintInGraphicsLayer;
 
   // TODO(flackr): Remove this when box shadows are still painted correctly when
   // painting into the composited scrolling contents layer.
   // https://crbug.com/646464
-  if (style()->boxShadow())
+  if (style()->boxShadow()) {
+    if (reasons)
+      *reasons |= MainThreadScrollingReason::kHasBoxShadowFromNonRootLayer;
     return BackgroundPaintInGraphicsLayer;
+  }
 
   // Assume optimistically that the background can be painted in the scrolling
   // contents until we find otherwise.
   BackgroundPaintLocation paintLocation = BackgroundPaintInScrollingContents;
   const FillLayer* layer = &(style()->backgroundLayers());
   for (; layer; layer = layer->next()) {
     if (layer->attachment() == LocalBackgroundAttachment)
       continue;
 
     // Solid color layers with an effective background clip of the padding box
@@ skipping 1215 matching lines @@
   if (rootElementStyle->hasBackground())
     return false;
 
   if (node() != document().firstBodyElement())
     return false;
 
   return true;
 }
 
 }  // namespace blink