Adapt and reland old position sticky implementation

public/platform/WebLayerPositionConstraint.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
  *     documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE AND ITS CONTRIBUTORS "AS IS" AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL APPLE OR ITS CONTRIBUTORS BE LIABLE FOR ANY
  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
  * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef WebLayerPositionConstraint_h
 #define WebLayerPositionConstraint_h
 
+#include "public/platform/WebRect.h"
+
 namespace blink {
 
 struct WebLayerPositionConstraint {
     bool isFixedPosition     : 1;
     bool isFixedToRightEdge  : 1;
     bool isFixedToBottomEdge : 1;
 
     WebLayerPositionConstraint()
         : isFixedPosition(false)
         , isFixedToRightEdge(false)
         , isFixedToBottomEdge(false)
     {
     }
 
     static WebLayerPositionConstraint fixedPosition(bool fixedToRightEdge, bool fixedToBottomEdge)
     {
         WebLayerPositionConstraint constraint;
         constraint.isFixedPosition = true;
         constraint.isFixedToRightEdge = fixedToRightEdge;
         constraint.isFixedToBottomEdge = fixedToBottomEdge;
         return constraint;
     }
 };
 
+// TODO(flackr): Combine with the fixed position constraint above?

[enne (OOO), 2015/09/17 16:57:40]

Yeah, I think this should probably be merged with the other constraints.

+struct WebLayerStickyPositionConstraint {
+    bool isSticky         : 1;
+    bool isAnchoredLeft   : 1;
+    bool isAnchoredRight  : 1;
+    bool isAnchoredTop    : 1;
+    bool isAnchoredBottom : 1;
+
+    float leftOffset;
+    float rightOffset;
+    float topOffset;
+    float bottomOffset;
+
+    WebRect absoluteStickyBoxRect;

[enne (OOO), 2015/09/17 16:57:40]

Are these values redundant with WebLayer's bounds and the offsets above? Or can
the sticky part be some subrect of a layer?

[flackr, 2015/09/22 22:25:32]

I don't think so, but please correct me if I am mistaken about the layer's
bounds. The sticky position depends on the element's in flow position. Is it
possible to determine where the layer was before it was transformed /
positioned? (e.g. if it's anchored to the top it must not move above it's in
flow position)

+    WebRect absoluteContainingBlockRect;
+
+    // TODO(flackr): Add fancy ctor and static creation sugar.
+    WebLayerStickyPositionConstraint()
+        : isSticky(false)
+        , isAnchoredLeft(false)
+        , isAnchoredRight(false)
+        , isAnchoredTop(false)
+        , isAnchoredBottom(false)
+        , leftOffset(0.f)
+        , rightOffset(0.f)
+        , topOffset(0.f)
+        , bottomOffset(0.f)
+    {
+    }
+};
+
 } // namespace blink
 
 #endif // WebLayerPositionConstraint_h