Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/page/scrolling/ScrollingConstraints.cpp

 /*
  * Copyright (C) 2012 Apple 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
@@ skipping 10 matching lines @@
  * 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.
  */
 
 #include "config.h"
 #include "core/page/scrolling/ScrollingConstraints.h"
 
 namespace blink {
 
-FloatPoint FixedPositionViewportConstraints::layerPositionForViewportRect(const FloatRect& viewportRect) const
+LayoutSize StickyPositionViewportConstraints::computeStickyOffset(const LayoutRect& viewportRect) const
 {
-    FloatSize offset;
+    LayoutRect boxRect = m_absoluteStickyBoxRect;
 
-    if (hasAnchorEdge(AnchorEdgeLeft))
-        offset.setWidth(viewportRect.x() - m_viewportRectAtLastLayout.x());
-    else if (hasAnchorEdge(AnchorEdgeRight))
-        offset.setWidth(viewportRect.maxX() - m_viewportRectAtLastLayout.maxX());
+    if (hasAnchorEdge(AnchorEdgeRight)) {
+        LayoutUnit rightLimit = viewportRect.maxX() - m_rightOffset;
+        LayoutUnit rightDelta = std::min<LayoutUnit>(0, rightLimit - m_absoluteStickyBoxRect.maxX());
+        LayoutUnit availableSpace = std::min<LayoutUnit>(0, m_absoluteContainingBlockRect.x() - m_absoluteStickyBoxRect.x());
+        if (rightDelta < availableSpace)
+            rightDelta = availableSpace;
 
-    if (hasAnchorEdge(AnchorEdgeTop))
-        offset.setHeight(viewportRect.y() - m_viewportRectAtLastLayout.y());
-    else if (hasAnchorEdge(AnchorEdgeBottom))
-        offset.setHeight(viewportRect.maxY() - m_viewportRectAtLastLayout.maxY());
+        boxRect.move(rightDelta, 0);
+    }
 
-    return m_layerPositionAtLastLayout + offset;
+    if (hasAnchorEdge(AnchorEdgeLeft)) {
+        LayoutUnit leftLimit = viewportRect.x() + m_leftOffset;
+        LayoutUnit leftDelta = std::max<LayoutUnit>(0, leftLimit - m_absoluteStickyBoxRect.x());
+        LayoutUnit availableSpace = std::max<LayoutUnit>(0, m_absoluteContainingBlockRect.maxX() - m_absoluteStickyBoxRect.maxX());
+        if (leftDelta > availableSpace)
+            leftDelta = availableSpace;
+
+        boxRect.move(leftDelta, 0);
+    }
+
+    if (hasAnchorEdge(AnchorEdgeBottom)) {
+        LayoutUnit bottomLimit = viewportRect.maxY() - m_bottomOffset;
+        LayoutUnit bottomDelta = std::min<LayoutUnit>(0, bottomLimit - m_absoluteStickyBoxRect.maxY());
+        LayoutUnit availableSpace = std::min<LayoutUnit>(0, m_absoluteContainingBlockRect.y() - m_absoluteStickyBoxRect.y());
+        if (bottomDelta < availableSpace)
+            bottomDelta = availableSpace;
+
+        boxRect.move(0, bottomDelta);
+    }
+
+    if (hasAnchorEdge(AnchorEdgeTop)) {
+        LayoutUnit topLimit = viewportRect.y() + m_topOffset;
+        LayoutUnit topDelta = std::max<LayoutUnit>(0, topLimit - m_absoluteStickyBoxRect.y());
+        LayoutUnit availableSpace = std::max<LayoutUnit>(0, m_absoluteContainingBlockRect.maxY() - m_absoluteStickyBoxRect.maxY());
+        if (topDelta > availableSpace)
+            topDelta = availableSpace;
+
+        boxRect.move(0, topDelta);
+    }
+
+    return boxRect.location() - m_absoluteStickyBoxRect.location();
 }
 
 } // namespace blink