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..8809b13572177cb2dac53d96a149b349059ac2a7 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_nearestStickyElementFromContainingBlockToScrollContainer(nullptr) {}
 
   StickyPositionScrollingConstraints(
       const StickyPositionScrollingConstraints& other)
@@ -36,9 +41,26 @@ class StickyPositionScrollingConstraints final {
         m_scrollContainerRelativeContainingBlockRect(
             other.m_scrollContainerRelativeContainingBlockRect),
         m_scrollContainerRelativeStickyBoxRect(
-            other.m_scrollContainerRelativeStickyBoxRect) {}
+            other.m_scrollContainerRelativeStickyBoxRect),
+        m_nearestStickyElementToContainingBlock(
+            other.m_nearestStickyElementToContainingBlock),
+        m_nearestStickyElementFromContainingBlockToScrollContainer(
+            other.m_nearestStickyElementFromContainingBlockToScrollContainer),
+        m_accumulatedStickyOffsetToContainingBlock(
+            other.m_accumulatedStickyOffsetToContainingBlock),
+        m_accumulatedStickyOffsetFromContainingBlockToScrollContainer(
+            other
+                .m_accumulatedStickyOffsetFromContainingBlockToScrollContainer) {
+  }
 
-  FloatSize computeStickyOffset(const FloatRect& viewportRect) const;
+  FloatSize computeStickyOffset(const FloatRect& viewportRect,
+                                const FloatSize& accumulatedToCB,
+                                const FloatSize& accumulatedToVP);
+
+  bool hasAncestorStickyElement() {
+    return m_nearestStickyElementToContainingBlock ||
+           m_nearestStickyElementFromContainingBlockToScrollContainer;
+  }
 
   AnchorEdges anchorEdges() const { return m_anchorEdges; }
   bool hasAnchorEdge(AnchorEdgeFlags flag) const {
@@ -71,6 +93,30 @@ class StickyPositionScrollingConstraints final {
     return m_scrollContainerRelativeStickyBoxRect;
   }
 
+  void setNearestStickyElementToContainingBlock(LayoutBoxModelObject* layer) {
+    m_nearestStickyElementToContainingBlock = layer;
+  }
+  const LayoutBoxModelObject* nearestStickyElementToContainingBlock() {
+    return m_nearestStickyElementToContainingBlock;
+  }
+
+  void setNearestStickyElementFromContainingBlockToScrollContainer(
+      LayoutBoxModelObject* layer) {
+    m_nearestStickyElementFromContainingBlockToScrollContainer = layer;
+  }
+  const LayoutBoxModelObject*
+  nearestStickyElementFromContainingBlockToScrollContainer() {
+    return m_nearestStickyElementFromContainingBlockToScrollContainer;
+  }
+
+  const FloatSize& getAccumulatedStickyOffsetToContainingBlock() {
+    return m_accumulatedStickyOffsetToContainingBlock;
+  }
+  const FloatSize&
+  getAccumulatedStickyOffsetFromContainingBlockToScrollContainer() {
+    return m_accumulatedStickyOffsetFromContainingBlockToScrollContainer;
+  }
+
   bool operator==(const StickyPositionScrollingConstraints& other) const {
     return m_leftOffset == other.m_leftOffset &&
            m_rightOffset == other.m_rightOffset &&
@@ -79,7 +125,17 @@ class StickyPositionScrollingConstraints final {
            m_scrollContainerRelativeContainingBlockRect ==
                other.m_scrollContainerRelativeContainingBlockRect &&
            m_scrollContainerRelativeStickyBoxRect ==
-               other.m_scrollContainerRelativeStickyBoxRect;
+               other.m_scrollContainerRelativeStickyBoxRect &&
+           m_nearestStickyElementToContainingBlock ==
+               other.m_nearestStickyElementToContainingBlock &&
+           m_nearestStickyElementFromContainingBlockToScrollContainer ==
+               other
+                   .m_nearestStickyElementFromContainingBlockToScrollContainer &&
+           m_accumulatedStickyOffsetToContainingBlock ==
+               other.m_accumulatedStickyOffsetToContainingBlock &&
+           m_accumulatedStickyOffsetFromContainingBlockToScrollContainer ==
+               other
+                   .m_accumulatedStickyOffsetFromContainingBlockToScrollContainer;
   }
 
   bool operator!=(const StickyPositionScrollingConstraints& other) const {
@@ -94,6 +150,27 @@ 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_nearestStickyElementFromContainingBlockToScrollContainer;

[flackr, 2017/01/20 02:42:44]

Maybe m_nearestStickyElementShifting(ContainingBlock|StickyBox)?

[smcgruer, 2017/01/24 15:45:22]

Done.

+
+  // For performance, we cache our accumulated sticky offset for descendant
+  // sticky elements to use. Because we can either be the "to containing block"

[flackr, 2017/01/20 02:42:44]

nit: explain that we offset the constraint rects of descendants by these.

[smcgruer, 2017/01/24 15:45:22]

Done.

+  // or the "to scroll container" ancestor for a given sticky element, we need
+  // to accumulate both.
+  //
+  // NOTE(smcgruer): The case where we can be both a "to containing block"
+  // and a "to scroll container" ancestor for different sticky descendants is
+  // quite complex. See the NAME_ME test in LayoutBoxModelObjectTest.cpp.
+  FloatSize m_accumulatedStickyOffsetToContainingBlock;
+  FloatSize m_accumulatedStickyOffsetFromContainingBlockToScrollContainer;

[flackr, 2017/01/20 02:42:44]

How about m_totalStickyBoxStickyOffset and m_totalContainingBlockStickyOffset?

[smcgruer, 2017/01/24 15:45:22]

Done.

 };
 
 }  // namespace blink