Make all PaintLayers that scroll overflow self-painting.

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 2885 matching lines @@
   return false;
 }
 
 bool PaintLayer::ShouldBeSelfPaintingLayer() const {
   if (GetLayoutObject().IsLayoutPart() &&
       ToLayoutPart(GetLayoutObject()).RequiresAcceleratedCompositing())
     return true;
 
   return GetLayoutObject().LayerTypeRequired() == kNormalPaintLayer ||
          (scrollable_area_ && scrollable_area_->HasOverlayScrollbars()) ||
-         NeedsCompositedScrolling();

[Xianzhu, 2017/06/03 04:10:21]

Was there anything preventing us from letting all scrolling layers be
self-painting?

+         ScrollsOverflow();
 }
 
 void PaintLayer::UpdateSelfPaintingLayer() {
   bool is_self_painting_layer = ShouldBeSelfPaintingLayer();
   if (this->IsSelfPaintingLayer() == is_self_painting_layer)
     return;
 
   is_self_painting_layer_ = is_self_painting_layer;
   self_painting_status_changed_ = true;
 
@@ skipping 431 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->EnclosingLayer());
 }
 #endif