Fix slow repaint object registration

Source/core/layout/LayoutBox.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, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. 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 227 matching lines @@
 
     if (isDocumentElement() || isBody()) {
         document().view()->recalculateScrollbarOverlayStyle();
         document().view()->recalculateCustomScrollbarStyle();
     }
     updateShapeOutsideInfoAfterStyleChange(*style(), oldStyle);
     updateGridPositionAfterStyleChange(oldStyle);
 
     if (LayoutMultiColumnSpannerPlaceholder* placeholder = this->spannerPlaceholder())
         placeholder->layoutObjectInFlowThreadStyleDidChange(oldStyle);
+
+    updateSlowRepaintStatusAfterStyleChange();
+}
+
+void LayoutBox::updateSlowRepaintStatusAfterStyleChange()
+{
+    if (!frameView())
+        return;
+
+    // On low-powered/mobile devices, preventing blitting on a scroll can cause noticeable delays
+    // when scrolling a page with a fixed background image. As an optimization, assuming there are
+    // no fixed positoned elements on the page, we can acclerate scrolling (via blitting) if we
+    // ignore the CSS property "background-attachment: fixed".
+    bool ignoreFixedBackgroundAttachment = RuntimeEnabledFeatures::fastMobileScrollingEnabled();
+    if (ignoreFixedBackgroundAttachment)
+        return;
+
+    // An object needs to be repainted on frame scroll when it has background-attachment:fixed.
+    // LayoutObject is responsible for painting root background, thus the root element (and the
+    // body element if html element has no background) skips painting backgrounds.
+    bool isSlowRepaintObject = !isDocumentElement() && !backgroundStolenForBeingBody() && styleRef().hasFixedBackgroundImage();
+    if (isLayoutView() && view()->compositor()->supportsFixedRootBackgroundCompositing()) {
+        if (styleRef().hasEntirelyFixedBackground())
+            isSlowRepaintObject = false;
+    }
+
+    setIsSlowRepaintObject(isSlowRepaintObject);
 }
 
 void LayoutBox::updateShapeOutsideInfoAfterStyleChange(const ComputedStyle& style, const ComputedStyle* oldStyle)
 {
     const ShapeValue* shapeOutside = style.shapeOutside();
     const ShapeValue* oldShapeOutside = oldStyle ? oldStyle->shapeOutside() : ComputedStyle::initialShapeOutside();
 
     Length shapeMargin = style.shapeMargin();
     Length oldShapeMargin = oldStyle ? oldStyle->shapeMargin() : ComputedStyle::initialShapeMargin();
 
@@ skipping 4498 matching lines @@
     StyleImage* borderImage = style()->borderImage().image();
     return borderImage && borderImage->canRender(*this, style()->effectiveZoom()) && borderImage->isLoaded();
 }
 
 ShapeOutsideInfo* LayoutBox::shapeOutsideInfo() const
 {
     return ShapeOutsideInfo::isEnabledFor(*this) ? ShapeOutsideInfo::info(*this) : nullptr;
 }
 
 } // namespace blink