Paint local attachment backgrounds into composited scrolling contents layer

third_party/WebKit/Source/core/layout/LayoutBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 Apple 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
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 27 matching lines @@
 class ShapeOutsideInfo;
 
 struct PaintInfo;
 
 enum SizeType { MainOrPreferredSize, MinSize, MaxSize };
 enum AvailableLogicalHeightType { ExcludeMarginBorderPadding, IncludeMarginBorderPadding };
 // When painting, overlay scrollbars do not take up space and should not affect
 // clipping behavior. During hit testing, overlay scrollbars behave like regular
 // scrollbars and should change how hit testing is clipped.
 enum MarginDirection { BlockDirection, InlineDirection };
+enum BackgroundRectType { BackgroundClipRect, BackgroundKnownOpaqueRect };
 
 enum ShouldComputePreferred { ComputeActual, ComputePreferred };
 
 enum ApplyOverflowClipFlag {
     ApplyOverflowClip,
     // Don't apply overflow clipping or scrolling.
     ApplyNonScrollOverflowClip
 };
 
 using SnapAreaSet = HashSet<const LayoutBox*>;
@@ skipping 283 matching lines @@
 
     // The content area of the box (excludes padding - and intrinsic padding for table cells, etc... - and border).
     LayoutRect contentBoxRect() const { return LayoutRect(borderLeft() + paddingLeft(), borderTop() + paddingTop(), contentWidth(), contentHeight()); }
     LayoutSize contentBoxOffset() const { return LayoutSize(borderLeft() + paddingLeft(), borderTop() + paddingTop()); }
     // The content box in absolute coords. Ignores transforms.
     IntRect absoluteContentBox() const;
     // The offset of the content box in absolute coords, ignoring transforms.
     IntSize absoluteContentBoxOffset() const;
     // The content box converted to absolute coords (taking transforms into account).
     FloatQuad absoluteContentQuad() const;
-    // The clip rect of the background.
-    LayoutRect backgroundClipRect() const;
+    // The enclosing rectangle of the background with given opacity requirement.

[trchen, 2016/08/17 01:22:55]

I think this comment needs to be revised too, as for BackgroundKnownOpaqueRect
it is in fact an enclosed rect.

+    LayoutRect backgroundRect(BackgroundRectType) const;
 
     // This returns the content area of the box (excluding padding and border). The only difference with contentBoxRect is that computedCSSContentBoxRect
     // does include the intrinsic padding in the content box as this is what some callers expect (like getComputedStyle).
     LayoutRect computedCSSContentBoxRect() const { return LayoutRect(borderLeft() + computedCSSPaddingLeft(), borderTop() + computedCSSPaddingTop(), clientWidth() - computedCSSPaddingLeft() - computedCSSPaddingRight(), clientHeight() - computedCSSPaddingTop() - computedCSSPaddingBottom()); }
 
     void addOutlineRects(Vector<LayoutRect>&, const LayoutPoint& additionalOffset, IncludeBlockVisualOverflowOrNot) const override;
 
     // Use this with caution! No type checking is done!
     LayoutBox* previousSiblingBox() const;
     LayoutBox* previousInFlowSiblingBox() const;
@@ skipping 854 matching lines @@
         || breakValue == BreakLeft
         || breakValue == BreakPage
         || breakValue == BreakRecto
         || breakValue == BreakRight
         || breakValue == BreakVerso;
 }
 
 } // namespace blink
 
 #endif // LayoutBox_h