Handle nested position:sticky elements correctly (main thread)

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

 // 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.
 
 #ifndef StickyPositionScrollingConstraints_h
 #define StickyPositionScrollingConstraints_h
 
 #include "platform/geometry/FloatRect.h"
+#include "platform/geometry/FloatSize.h"
 
 namespace blink {
 
+class LayoutBoxModelObject;
+
 class StickyPositionScrollingConstraints final {
  public:
   enum AnchorEdgeFlags {
     AnchorEdgeLeft = 1 << 0,
     AnchorEdgeRight = 1 << 1,
     AnchorEdgeTop = 1 << 2,
     AnchorEdgeBottom = 1 << 3
   };
   typedef unsigned AnchorEdges;
 
   StickyPositionScrollingConstraints()
       : m_anchorEdges(0),
         m_leftOffset(0),
         m_rightOffset(0),
         m_topOffset(0),
-        m_bottomOffset(0) {}
+        m_bottomOffset(0),
+        m_nearestStickyElementShiftingStickyBox(nullptr),
+        m_nearestStickyElementShiftingContainingBlock(nullptr) {}
 
   StickyPositionScrollingConstraints(
       const StickyPositionScrollingConstraints& other)
       : m_anchorEdges(other.m_anchorEdges),
         m_leftOffset(other.m_leftOffset),
         m_rightOffset(other.m_rightOffset),
         m_topOffset(other.m_topOffset),
         m_bottomOffset(other.m_bottomOffset),
         m_scrollContainerRelativeContainingBlockRect(
             other.m_scrollContainerRelativeContainingBlockRect),
         m_scrollContainerRelativeStickyBoxRect(
-            other.m_scrollContainerRelativeStickyBoxRect) {}
+            other.m_scrollContainerRelativeStickyBoxRect),
+        m_nearestStickyElementShiftingStickyBox(
+            other.m_nearestStickyElementShiftingStickyBox),
+        m_nearestStickyElementShiftingContainingBlock(
+            other.m_nearestStickyElementShiftingContainingBlock),
+        m_totalStickyBoxStickyOffset(other.m_totalStickyBoxStickyOffset),
+        m_totalContainingBlockStickyOffset(
+            other.m_totalContainingBlockStickyOffset),
+        m_cachedComputedStickyOffset(other.m_cachedComputedStickyOffset) {}
 
-  FloatSize computeStickyOffset(const FloatRect& viewportRect) const;
+  FloatSize computeStickyOffset(
+      const FloatRect& viewportRect,
+      StickyPositionScrollingConstraints* ancestorStickyBoxConstraints,

[flackr, 2017/02/22 22:40:15]

const StickyPositionScrollingConstraints*

[smcgruer, 2017/02/23 20:14:17]

Done.

+      StickyPositionScrollingConstraints* ancestorContainingBlockConstraints);
+
+  bool hasAncestorStickyElement() const {
+    return m_nearestStickyElementShiftingStickyBox ||
+           m_nearestStickyElementShiftingContainingBlock;
+  }
 
   AnchorEdges anchorEdges() const { return m_anchorEdges; }
   bool hasAnchorEdge(AnchorEdgeFlags flag) const {
     return m_anchorEdges & flag;
   }
   void addAnchorEdge(AnchorEdgeFlags edgeFlag) { m_anchorEdges |= edgeFlag; }
   void setAnchorEdges(AnchorEdges edges) { m_anchorEdges = edges; }
 
   float leftOffset() const { return m_leftOffset; }
   float rightOffset() const { return m_rightOffset; }
@@ skipping 12 matching lines @@
     return m_scrollContainerRelativeContainingBlockRect;
   }
 
   void setScrollContainerRelativeStickyBoxRect(const FloatRect& rect) {
     m_scrollContainerRelativeStickyBoxRect = rect;
   }
   const FloatRect& scrollContainerRelativeStickyBoxRect() const {
     return m_scrollContainerRelativeStickyBoxRect;
   }
 
+  void setNearestStickyElementShiftingStickyBox(LayoutBoxModelObject* layer) {
+    m_nearestStickyElementShiftingStickyBox = layer;
+  }
+  LayoutBoxModelObject* nearestStickyElementShiftingStickyBox() const {
+    return m_nearestStickyElementShiftingStickyBox;
+  }
+
+  void setNearestStickyElementShiftingContainingBlock(
+      LayoutBoxModelObject* layer) {
+    m_nearestStickyElementShiftingContainingBlock = layer;
+  }
+  LayoutBoxModelObject* nearestStickyElementShiftingContainingBlock() const {
+    return m_nearestStickyElementShiftingContainingBlock;
+  }
+
+  const FloatSize& getTotalStickyBoxStickyOffset() const {
+    return m_totalStickyBoxStickyOffset;
+  }
+  const FloatSize& getTotalContainingBlockStickyOffset() const {
+    return m_totalContainingBlockStickyOffset;
+  }
+
+  const FloatSize& getCachedComputedStickyOffset() const {
+    return m_cachedComputedStickyOffset;
+  }
+
   bool operator==(const StickyPositionScrollingConstraints& other) const {
     return m_leftOffset == other.m_leftOffset &&
            m_rightOffset == other.m_rightOffset &&
            m_topOffset == other.m_topOffset &&
            m_bottomOffset == other.m_bottomOffset &&
            m_scrollContainerRelativeContainingBlockRect ==
                other.m_scrollContainerRelativeContainingBlockRect &&
            m_scrollContainerRelativeStickyBoxRect ==
-               other.m_scrollContainerRelativeStickyBoxRect;
+               other.m_scrollContainerRelativeStickyBoxRect &&
+           m_nearestStickyElementShiftingStickyBox ==
+               other.m_nearestStickyElementShiftingStickyBox &&
+           m_nearestStickyElementShiftingContainingBlock ==
+               other.m_nearestStickyElementShiftingContainingBlock &&
+           m_totalStickyBoxStickyOffset == other.m_totalStickyBoxStickyOffset &&
+           m_totalContainingBlockStickyOffset ==
+               other.m_totalContainingBlockStickyOffset &&
+           m_cachedComputedStickyOffset == other.m_cachedComputedStickyOffset;
   }
 
   bool operator!=(const StickyPositionScrollingConstraints& other) const {
     return !(*this == other);
   }
 
  private:
   AnchorEdges m_anchorEdges;
   float m_leftOffset;
   float m_rightOffset;
   float m_topOffset;
   float m_bottomOffset;
   FloatRect m_scrollContainerRelativeContainingBlockRect;
   FloatRect m_scrollContainerRelativeStickyBoxRect;
+
+  // In order to properly compute the stickyOffset, we need to know if we have
+  // any sticky ancestors both between ourselves and our containing block and
+  // between our containing block and the viewport. These ancestors are needed
+  // to properly shift our constraining rects with regards to the containing
+  // block and viewport.
+  LayoutBoxModelObject* m_nearestStickyElementShiftingStickyBox;
+  LayoutBoxModelObject* m_nearestStickyElementShiftingContainingBlock;

[flackr, 2017/02/22 22:40:15]

bikeshed nit: Can we call these nearestBoxShifting* since they're LayoutBox
pointers rather than elements?

[smcgruer, 2017/02/23 20:14:17]

Done.

+
+  // For performance we cache our accumulated sticky offset to allow descendant
+  // sticky elements to offset their constraint rects. Because we can either
+  // affect the sticky box constraint rect or the containing block constraint
+  // rect, we need to accumulate both.
+  //
+  // The case where we can affect both the sticky box constraint rect and the
+  // constraining block constriant rect for different sticky descendants is
+  // quite complex. See the StickyPositionComplexTableNesting test in
+  // LayoutBoxModelObjectTest.cpp.
+  FloatSize m_totalStickyBoxStickyOffset;
+  FloatSize m_totalContainingBlockStickyOffset;
+
+  FloatSize m_cachedComputedStickyOffset;

[flackr, 2017/02/22 22:40:15]

m_localStickyOffset?

But it looks like the code never uses this as far as I can tell, in which case
we should remove it.

[smcgruer, 2017/02/23 20:14:17]

Ah, this is from the cc side. Removed for now.

 };
 
 }  // namespace blink
 
 #endif  // StickyPositionScrollingConstraints_h