Adapt and reland old position sticky implementation

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

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/page/scrolling/StickyPositionScrollingConstraints.h"
+
+namespace blink {
+
+FloatSize StickyPositionScrollingConstraints::computeStickyOffset(const FloatRect& viewportRect) const
+{
+    FloatRect boxRect = m_scrollContainerRelativeStickyBoxRect;
+
+    if (hasAnchorEdge(AnchorEdgeRight)) {
+        float rightLimit = viewportRect.maxX() - m_rightOffset;
+        float rightDelta = std::min<float>(0, rightLimit - m_scrollContainerRelativeStickyBoxRect.maxX());
+        float availableSpace = std::min<float>(0, m_scrollContainerRelativeContainingBlockRect.x() - m_scrollContainerRelativeStickyBoxRect.x());
+        if (rightDelta < availableSpace)
+            rightDelta = availableSpace;
+
+        boxRect.move(rightDelta, 0);
+    }
+
+    if (hasAnchorEdge(AnchorEdgeLeft)) {
+        float leftLimit = viewportRect.x() + m_leftOffset;
+        float leftDelta = std::max<float>(0, leftLimit - m_scrollContainerRelativeStickyBoxRect.x());
+        float availableSpace = std::max<float>(0, m_scrollContainerRelativeContainingBlockRect.maxX() - m_scrollContainerRelativeStickyBoxRect.maxX());
+        if (leftDelta > availableSpace)
+            leftDelta = availableSpace;
+
+        boxRect.move(leftDelta, 0);
+    }
+
+    if (hasAnchorEdge(AnchorEdgeBottom)) {
+        float bottomLimit = viewportRect.maxY() - m_bottomOffset;
+        float bottomDelta = std::min<float>(0, bottomLimit - m_scrollContainerRelativeStickyBoxRect.maxY());
+        float availableSpace = std::min<float>(0, m_scrollContainerRelativeContainingBlockRect.y() - m_scrollContainerRelativeStickyBoxRect.y());
+        if (bottomDelta < availableSpace)
+            bottomDelta = availableSpace;
+
+        boxRect.move(0, bottomDelta);
+    }
+
+    if (hasAnchorEdge(AnchorEdgeTop)) {
+        float topLimit = viewportRect.y() + m_topOffset;
+        float topDelta = std::max<float>(0, topLimit - m_scrollContainerRelativeStickyBoxRect.y());
+        float availableSpace = std::max<float>(0, m_scrollContainerRelativeContainingBlockRect.maxY() - m_scrollContainerRelativeStickyBoxRect.maxY());
+        if (topDelta > availableSpace)
+            topDelta = availableSpace;
+
+        boxRect.move(0, topDelta);
+    }
+
+    return boxRect.location() - m_scrollContainerRelativeStickyBoxRect.location();
+}
+
+} // namespace blink