Record non-stacking-context as main thread scrolling reasons

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 2773 matching lines @@
 }
 
 bool PaintLayer::CompositesWithTransform() const {
   return TransformAncestor() || Transform();
 }
 
 bool PaintLayer::CompositesWithOpacity() const {
   return OpacityAncestor() || GetLayoutObject().Style()->HasOpacity();
 }
 
-bool PaintLayer::BackgroundIsKnownToBeOpaqueInRect(
-    const LayoutRect& local_rect) const {
+bool PaintLayer::BackgroundIsKnownToBeOpaqueInRect(const LayoutRect& local_rect,
+                                                   uint32_t* reasons) const {
   if (PaintsWithTransparency(kGlobalPaintNormalPhase))
     return false;
 
   // We can't use hasVisibleContent(), because that will be true if our
   // layoutObject is hidden, but some child is visible and that child doesn't
   // cover the entire rect.
   if (GetLayoutObject().Style()->Visibility() != EVisibility::kVisible)
     return false;
 
   if (PaintsWithFilters() &&
@@ skipping 17 matching lines @@
     return false;
 
   // FIXME: We currently only check the immediate layoutObject,
   // which will miss many cases where additional layout objects paint
   // into this layer.
   if (GetLayoutObject().BackgroundIsKnownToBeOpaqueInRect(local_rect))
     return true;
 
   // We can't consult child layers if we clip, since they might cover
   // parts of the rect that are clipped out.
-  if (GetLayoutObject().HasClipRelatedProperty())
+  if (GetLayoutObject().HasClipRelatedProperty()) {
+    if (reasons && GetLayoutObject().HasOverflowClip()) {
+      *reasons |= MainThreadScrollingReason::
+          kHasOverflowClipAndLayoutObjectBackgroundNotOpaqueAndLCDText;
+    }
     return false;
+  }
 
   // TODO(schenney): This could be improved by unioning the opaque regions of
   // all the children.  That would require a refactoring because currently
   // children just check they at least cover the given rect, but a unioning
   // method would require children to compute and report their rects.
   return ChildBackgroundIsKnownToBeOpaqueInRect(local_rect);
 }
 
 bool PaintLayer::ChildBackgroundIsKnownToBeOpaqueInRect(
     const LayoutRect& local_rect) const {
@@ skipping 471 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->EnclosingLayer());
 }
 #endif