Repaint when background switches painting from/to scrolling contents layer.

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 416 matching lines @@
     // FIXME: generalize accessors to backing GraphicsLayers so that this code is squashing-agnostic.
     if (layer()->groupedMapping()) {
         LayoutRect paintInvalidationRect = r;
         if (GraphicsLayer* squashingLayer = layer()->groupedMapping()->squashingLayer()) {
             // Note: the subpixel accumulation of layer() does not need to be added here. It is already taken into account.
             squashingLayer->setNeedsDisplayInRect(enclosingIntRect(paintInvalidationRect), invalidationReason, object);
         }
     } else if (object.compositedScrollsWithRespectTo(*this)) {
         layer()->compositedLayerMapping()->setScrollingContentsNeedDisplayInRect(r, invalidationReason, object);
     } else if (usesCompositedScrolling()) {
-        if (layer()->compositedLayerMapping()->shouldPaintBackgroundOntoScrollingContentsLayer()) {
+        if (layer()->compositedLayerMapping()->backgroundPaintsOntoScrollingContentsLayer()) {
             // TODO(flackr): Get a correct rect in the context of the scrolling contents layer to update
             // rather than updating the entire rect.
             const LayoutRect& scrollingContentsRect = toLayoutBox(this)->layoutOverflowRect();
             layer()->compositedLayerMapping()->setScrollingContentsNeedDisplayInRect(scrollingContentsRect, invalidationReason, object);
             layer()->setNeedsRepaint();

[Stephen Chennney, 2016/08/22 19:51:47]

I'm concerned that we are missing a layer()->setNeedsRepaint() in the case where
we stop painting the background into the scrolling contents. But if seems the
test catches that. Do you know why it's not necessary?

             invalidateDisplayItemClient(*layer()->compositedLayerMapping()->scrollingContentsLayer(), invalidationReason);
         }
         layer()->compositedLayerMapping()->setNonScrollingContentsNeedDisplayInRect(r, invalidationReason, object);
     } else {
         // Otherwise invalidate everything.
         layer()->compositedLayerMapping()->setContentsNeedDisplayInRect(r, invalidationReason, object);
     }
 }
 
 void LayoutBoxModelObject::addOutlineRectsForNormalChildren(Vector<LayoutRect>& rects, const LayoutPoint& additionalOffset, IncludeBlockVisualOverflowOrNot includeBlockOverflows) const
@@ skipping 685 matching lines @@
     if (rootElementStyle->hasBackground())
         return false;
 
     if (node() != document().firstBodyElement())
         return false;
 
     return true;
 }
 
 } // namespace blink