Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/page/scrolling/ViewportConstraints.h

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
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  * modification, are permitted provided that the following conditions
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  * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
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-#ifndef ScrollingConstraints_h
-#define ScrollingConstraints_h

[chrishtr, 2016/01/30 01:49:03]

Rename to ScrollingConstraints? Sorry for going in circles on this name with
you. :(

+#ifndef ViewportConstraints_h
+#define ViewportConstraints_h
 
 #include "platform/geometry/FloatRect.h"
 #include "wtf/Allocator.h"
 
 namespace blink {
 
 // ViewportConstraints classes encapsulate data and logic required to reposition elements whose layout
-// depends on the viewport rect (i.e., position fixed), when scrolling and zooming.
+// depends on the viewport rect (positioned fixed and sticky), when scrolling and zooming.
 class ViewportConstraints {
     STACK_ALLOCATED();
 public:
-    // FIXME: Simplify this code now that position: sticky doesn't exist.
     enum ConstraintType {
         FixedPositionConstaint,
+        StickyPositionConstraint,
     };
 
     enum AnchorEdgeFlags {
         AnchorEdgeLeft = 1 << 0,
         AnchorEdgeRight = 1 << 1,
         AnchorEdgeTop = 1 << 2,
         AnchorEdgeBottom = 1 << 3
     };
     typedef unsigned AnchorEdges;
 
@@ skipping 25 matching lines @@
 
 class FixedPositionViewportConstraints final : public ViewportConstraints {
     STACK_ALLOCATED();
 public:
     FixedPositionViewportConstraints()
         : ViewportConstraints()
     { }
 
     FixedPositionViewportConstraints(const FixedPositionViewportConstraints& other)
         : ViewportConstraints(other)
-        , m_viewportRectAtLastLayout(other.m_viewportRectAtLastLayout)
-        , m_layerPositionAtLastLayout(other.m_layerPositionAtLastLayout)
     { }
 
-    FloatPoint layerPositionForViewportRect(const FloatRect& viewportRect) const;
-
-    const FloatRect& viewportRectAtLastLayout() const { return m_viewportRectAtLastLayout; }
-    void setViewportRectAtLastLayout(const FloatRect& rect) { m_viewportRectAtLastLayout = rect; }
-
-    const FloatPoint& layerPositionAtLastLayout() const { return m_layerPositionAtLastLayout; }
-    void setLayerPositionAtLastLayout(const FloatPoint& point) { m_layerPositionAtLastLayout = point; }
-
     bool operator==(const FixedPositionViewportConstraints& other) const
     {
         return m_alignmentOffset == other.m_alignmentOffset
-            && m_anchorEdges == other.m_anchorEdges
-            && m_viewportRectAtLastLayout == other.m_viewportRectAtLastLayout
-            && m_layerPositionAtLastLayout == other.m_layerPositionAtLastLayout;
+            && m_anchorEdges == other.m_anchorEdges;
     }
 
     bool operator!=(const FixedPositionViewportConstraints& other) const { return !(*this == other); }
 
 private:
     ConstraintType constraintType() const override { return FixedPositionConstaint; }
-
-    FloatRect m_viewportRectAtLastLayout;
-    FloatPoint m_layerPositionAtLastLayout;
 };
 
 } // namespace blink
 
-#endif // ScrollingConstraints_h
+#endif // ViewportConstraints_h