Adapt and reland old position sticky implementation

Source/core/layout/LayoutBoxModelObject.h

Index: Source/core/layout/LayoutBoxModelObject.h
diff --git a/Source/core/layout/LayoutBoxModelObject.h b/Source/core/layout/LayoutBoxModelObject.h
index edcf78af70dd276cbf2efcafe373c7ec25676abd..062acc6be6e7a4bc1a47e5927910da8db1ade07a 100644
--- a/Source/core/layout/LayoutBoxModelObject.h
+++ b/Source/core/layout/LayoutBoxModelObject.h
@@ -62,6 +62,7 @@ enum ContentChangeType {
 };
 
 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
@@ -77,6 +78,10 @@ public:
     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)
@@ -270,6 +275,8 @@ private:
     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;
 };