Handle nested position:sticky elements correctly (compositor)

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"
 
@@ skipping 95 matching lines @@
     return m_nearestStickyBoxShiftingContainingBlock;
   }
 
   const FloatSize& getTotalStickyBoxStickyOffset() const {
     return m_totalStickyBoxStickyOffset;
   }
   const FloatSize& getTotalContainingBlockStickyOffset() const {
     return m_totalContainingBlockStickyOffset;
   }
 
+  const LayoutBoxModelObject* nearestStickyAncestor() const {
+    // If we have one or more sticky ancestor elements between ourselves and our
+    // containing block, |m_nearestStickyBoxShiftingStickyBox| points to the
+    // closest. Otherwise, |m_nearestStickyBoxShiftingContainingBlock| points
+    // to the the first sticky ancestor between our containing block (inclusive)
+    // and our scroll ancestor (exclusive). Therefore our nearest sticky
+    // ancestor is the former if it exists, or the latter otherwise.
+    //
+    // If both are null, then we have no sticky ancestors before our scroll
+    // ancestor, so the correct action is to return null.
+    return m_nearestStickyBoxShiftingStickyBox
+               ? m_nearestStickyBoxShiftingStickyBox
+               : m_nearestStickyBoxShiftingContainingBlock;
+  }
+
   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 &&
            m_nearestStickyBoxShiftingStickyBox ==
@@ skipping 35 matching lines @@
   // 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
 
 #endif  // StickyPositionScrollingConstraints_h