Handle nested position:sticky elements correctly (main thread)

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

Index: third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
index e37687262edcd4a096f127a3a5054eeb78330e66..84dab3cc2f76ad7170d18bed9235375e5dc16c62 100644
--- a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
+++ b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
@@ -6,9 +6,12 @@
 #define StickyPositionScrollingConstraints_h
 
 #include "platform/geometry/FloatRect.h"
+#include "platform/geometry/FloatSize.h"
 
 namespace blink {
 
+class LayoutBoxModelObject;
+
 class StickyPositionScrollingConstraints final {
  public:
   enum AnchorEdgeFlags {
@@ -24,7 +27,9 @@ class StickyPositionScrollingConstraints final {
         m_leftOffset(0),
         m_rightOffset(0),
         m_topOffset(0),
-        m_bottomOffset(0) {}
+        m_bottomOffset(0),
+        m_nearestStickyBoxShiftingStickyBox(nullptr),
+        m_nearestStickyBoxShiftingContainingBlock(nullptr) {}
 
   StickyPositionScrollingConstraints(
       const StickyPositionScrollingConstraints& other)
@@ -36,9 +41,25 @@ class StickyPositionScrollingConstraints final {
         m_scrollContainerRelativeContainingBlockRect(
             other.m_scrollContainerRelativeContainingBlockRect),
         m_scrollContainerRelativeStickyBoxRect(
-            other.m_scrollContainerRelativeStickyBoxRect) {}
-
-  FloatSize computeStickyOffset(const FloatRect& viewportRect) const;
+            other.m_scrollContainerRelativeStickyBoxRect),
+        m_nearestStickyBoxShiftingStickyBox(
+            other.m_nearestStickyBoxShiftingStickyBox),
+        m_nearestStickyBoxShiftingContainingBlock(
+            other.m_nearestStickyBoxShiftingContainingBlock),
+        m_totalStickyBoxStickyOffset(other.m_totalStickyBoxStickyOffset),
+        m_totalContainingBlockStickyOffset(
+            other.m_totalContainingBlockStickyOffset) {}
+
+  FloatSize computeStickyOffset(
+      const FloatRect& viewportRect,
+      const StickyPositionScrollingConstraints* ancestorStickyBoxConstraints,
+      const StickyPositionScrollingConstraints*
+          ancestorContainingBlockConstraints);
+
+  bool hasAncestorStickyElement() const {
+    return m_nearestStickyBoxShiftingStickyBox ||
+           m_nearestStickyBoxShiftingContainingBlock;
+  }
 
   AnchorEdges anchorEdges() const { return m_anchorEdges; }
   bool hasAnchorEdge(AnchorEdgeFlags flag) const {
@@ -71,6 +92,27 @@ class StickyPositionScrollingConstraints final {
     return m_scrollContainerRelativeStickyBoxRect;
   }
 
+  void setNearestStickyBoxShiftingStickyBox(LayoutBoxModelObject* layer) {
+    m_nearestStickyBoxShiftingStickyBox = layer;
+  }
+  LayoutBoxModelObject* nearestStickyBoxShiftingStickyBox() const {
+    return m_nearestStickyBoxShiftingStickyBox;
+  }
+
+  void setNearestStickyBoxShiftingContainingBlock(LayoutBoxModelObject* layer) {
+    m_nearestStickyBoxShiftingContainingBlock = layer;
+  }
+  LayoutBoxModelObject* nearestStickyBoxShiftingContainingBlock() const {
+    return m_nearestStickyBoxShiftingContainingBlock;
+  }
+
+  const FloatSize& getTotalStickyBoxStickyOffset() const {
+    return m_totalStickyBoxStickyOffset;
+  }
+  const FloatSize& getTotalContainingBlockStickyOffset() const {
+    return m_totalContainingBlockStickyOffset;
+  }
+
   bool operator==(const StickyPositionScrollingConstraints& other) const {
     return m_leftOffset == other.m_leftOffset &&
            m_rightOffset == other.m_rightOffset &&
@@ -79,7 +121,14 @@ class StickyPositionScrollingConstraints final {
            m_scrollContainerRelativeContainingBlockRect ==
                other.m_scrollContainerRelativeContainingBlockRect &&
            m_scrollContainerRelativeStickyBoxRect ==
-               other.m_scrollContainerRelativeStickyBoxRect;
+               other.m_scrollContainerRelativeStickyBoxRect &&
+           m_nearestStickyBoxShiftingStickyBox ==
+               other.m_nearestStickyBoxShiftingStickyBox &&
+           m_nearestStickyBoxShiftingContainingBlock ==
+               other.m_nearestStickyBoxShiftingContainingBlock &&
+           m_totalStickyBoxStickyOffset == other.m_totalStickyBoxStickyOffset &&
+           m_totalContainingBlockStickyOffset ==
+               other.m_totalContainingBlockStickyOffset;
   }
 
   bool operator!=(const StickyPositionScrollingConstraints& other) const {
@@ -94,6 +143,26 @@ class StickyPositionScrollingConstraints final {
   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_nearestStickyBoxShiftingStickyBox;
+  LayoutBoxModelObject* m_nearestStickyBoxShiftingContainingBlock;
+
+  // 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;
 };
 
 }  // namespace blink