Handle nested position:sticky elements

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..f2ec9ed594c1abc856819eef983441814aba0f1a 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_nearestStickyElementToContainingBlock(nullptr),
+        m_nearestStickyElementFromContainingBlockToViewport(nullptr) {}
 
   StickyPositionScrollingConstraints(
       const StickyPositionScrollingConstraints& other)
@@ -36,9 +41,22 @@ class StickyPositionScrollingConstraints final {
         m_scrollContainerRelativeContainingBlockRect(
             other.m_scrollContainerRelativeContainingBlockRect),
         m_scrollContainerRelativeStickyBoxRect(
-            other.m_scrollContainerRelativeStickyBoxRect) {}
+            other.m_scrollContainerRelativeStickyBoxRect),
+        m_nearestStickyElementToContainingBlock(
+            other.m_nearestStickyElementToContainingBlock),
+        m_nearestStickyElementFromContainingBlockToViewport(
+            other.m_nearestStickyElementFromContainingBlockToViewport),
+        m_cachedAccumulatedStickyOffset(other.m_cachedAccumulatedStickyOffset) {
+  }
 
-  FloatSize computeStickyOffset(const FloatRect& viewportRect) const;
+  FloatSize computeStickyOffset(const FloatRect& viewportRect,
+                                const FloatSize& accumulatedToCB,
+                                const FloatSize& accumulatedToVP);
+
+  bool hasAncestorStickyElement() {
+    return m_nearestStickyElementToContainingBlock ||
+           m_nearestStickyElementFromContainingBlockToViewport;
+  }
 
   AnchorEdges anchorEdges() const { return m_anchorEdges; }
   bool hasAnchorEdge(AnchorEdgeFlags flag) const {
@@ -71,6 +89,26 @@ class StickyPositionScrollingConstraints final {
     return m_scrollContainerRelativeStickyBoxRect;
   }
 
+  void setNearestStickyElementToContainingBlock(LayoutBoxModelObject* layer) {
+    m_nearestStickyElementToContainingBlock = layer;
+  }
+  const LayoutBoxModelObject* nearestStickyElementToContainingBlock() {
+    return m_nearestStickyElementToContainingBlock;
+  }
+
+  void setNearestStickyElementFromContainingBlockToViewport(
+      LayoutBoxModelObject* layer) {
+    m_nearestStickyElementFromContainingBlockToViewport = layer;
+  }
+  const LayoutBoxModelObject*
+  nearestStickyElementFromContainingBlockToViewport() {
+    return m_nearestStickyElementFromContainingBlockToViewport;
+  }
+
+  const FloatSize& getCachedAccumulatedStickyOffset() {
+    return m_cachedAccumulatedStickyOffset;
+  }
+
   bool operator==(const StickyPositionScrollingConstraints& other) const {
     return m_leftOffset == other.m_leftOffset &&
            m_rightOffset == other.m_rightOffset &&
@@ -79,7 +117,13 @@ class StickyPositionScrollingConstraints final {
            m_scrollContainerRelativeContainingBlockRect ==
                other.m_scrollContainerRelativeContainingBlockRect &&
            m_scrollContainerRelativeStickyBoxRect ==
-               other.m_scrollContainerRelativeStickyBoxRect;
+               other.m_scrollContainerRelativeStickyBoxRect &&
+           m_nearestStickyElementToContainingBlock ==
+               other.m_nearestStickyElementToContainingBlock &&
+           m_nearestStickyElementFromContainingBlockToViewport ==
+               other.m_nearestStickyElementFromContainingBlockToViewport &&
+           m_cachedAccumulatedStickyOffset ==
+               other.m_cachedAccumulatedStickyOffset;
   }
 
   bool operator!=(const StickyPositionScrollingConstraints& other) const {
@@ -94,6 +138,17 @@ 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 containingBlock and
+  // between our containingBlock and the viewport.
+  //
+  // Any such ancestors are computed at layout time, and then the accumulated
+  // offset from them is recomputed after each scroll update.
+  LayoutBoxModelObject* m_nearestStickyElementToContainingBlock;
+  LayoutBoxModelObject* m_nearestStickyElementFromContainingBlockToViewport;
+
+  FloatSize m_cachedAccumulatedStickyOffset;
 };
 
 }  // namespace blink