Adapt and reland old position sticky implementation

Source/core/layout/LayoutBoxModelObject.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007, 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
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 44 matching lines @@
     BackgroundBleedClipLayer,
 };
 
 enum ContentChangeType {
     ImageChanged,
     CanvasChanged,
     CanvasContextChanged
 };
 
 class InlineFlowBox;
+class StickyPositionViewportConstraints;
 
 // This class is the base for all objects that adhere to the CSS box model as described
 // at http://www.w3.org/TR/CSS21/box.html
 
 class CORE_EXPORT LayoutBoxModelObject : public LayoutObject {
 public:
     LayoutBoxModelObject(ContainerNode*);
     ~LayoutBoxModelObject() override;
 
     // This is the only way layers should ever be destroyed.
     void destroyLayer();
 
     LayoutSize relativePositionOffset() const;
     LayoutSize relativePositionLogicalOffset() const { return style()->isHorizontalWritingMode() ? relativePositionOffset() : relativePositionOffset().transposedSize(); }
 
+    void computeStickyPositionConstraints(StickyPositionViewportConstraints&, const LayoutRect& constrainingRect) const;

[chrishtr, 2015/09/17 01:18:56]

What are the return values from this? What space is the rect in?

[flackr, 2015/10/07 20:38:11]

Added a comment.

+    LayoutRect computeStickyConstrainingRect() const;
+    LayoutSize stickyPositionOffset() const;
+
     LayoutSize offsetForInFlowPosition() const;
 
     // IE extensions. Used to calculate offsetWidth/Height.  Overridden by inlines (LayoutFlow)
     // to return the remaining width on a given line (and the height of a single line).
     virtual LayoutUnit offsetLeft() const;
     virtual LayoutUnit offsetTop() const;
     virtual LayoutUnit offsetWidth() const = 0;
     virtual LayoutUnit offsetHeight() const = 0;
 
     int pixelSnappedOffsetLeft() const { return roundToInt(offsetLeft()); }
@@ skipping 173 matching lines @@
 
     enum ScaleByEffectiveZoomOrNot { ScaleByEffectiveZoom, DoNotScaleByEffectiveZoom };
     IntSize calculateImageIntrinsicDimensions(StyleImage*, const IntSize& scaledPositioningAreaSize, ScaleByEffectiveZoomOrNot) const;
 
 private:
     void createLayer(DeprecatedPaintLayerType);
 
     LayoutUnit computedCSSPadding(const Length&) const;
     bool isBoxModelObject() const final { return true; }
 
+    virtual LayoutRect frameRectForStickyPositioning() const = 0;

[skobes, 2015/09/16 00:07:57]

Can you add a comment explaining what this is and why it is different from
frameRect()?  (Why doesn't frameRect() simply return the correct position for
sticky-positioned elements?)

[flackr, 2015/10/07 20:38:11]

Done.

+
     OwnPtr<DeprecatedPaintLayer> m_layer;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBoxModelObject, isBoxModelObject());
 
 } // namespace blink
 
 #endif // LayoutBoxModelObject_h