IntersectionObserver: use nullptr for implicit root.

third_party/WebKit/Source/core/dom/IntersectionObservation.cpp

 // 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.
 
 #include "core/dom/IntersectionObservation.h"
 
 #include "core/dom/ElementRareData.h"
 #include "core/dom/IntersectionObserver.h"
 #include "core/layout/IntersectionGeometry.h"
 
 namespace blink {
 
 IntersectionObservation::IntersectionObservation(IntersectionObserver& observer,
                                                  Element& target,
                                                  bool shouldReportRootBounds)
     : m_observer(observer),
       m_target(&target),
       m_shouldReportRootBounds(shouldReportRootBounds),
       m_lastThresholdIndex(0) {}
 
 void IntersectionObservation::computeIntersectionObservations(
     DOMHighResTimeStamp timestamp) {
   if (!m_target)
     return;
   Vector<Length> rootMargin(4);
   rootMargin[0] = m_observer->topMargin();
   rootMargin[1] = m_observer->rightMargin();
   rootMargin[2] = m_observer->bottomMargin();
   rootMargin[3] = m_observer->leftMargin();
-  Node* rootNode = m_observer->rootNode();
-  IntersectionGeometry geometry(
-      rootNode && !rootNode->isDocumentNode() ? toElement(rootNode) : nullptr,
-      *target(), rootMargin, m_shouldReportRootBounds);
+  IntersectionGeometry geometry(m_observer->root(), *target(), rootMargin,
+                                m_shouldReportRootBounds);

[kenrb, 2016/12/09 21:01:43]

There is something I don't quite understand here. m_observer->root() can now
return nullptr, but IntersectionGeometry::computeGeometry() on the next line
will always return early if it has a null root. Is this intentional? How do we
get intersection calculations in that case?

[szager1, 2016/12/10 20:51:18]

This patch depends on another patch (https://codereview.chromium.org/2553103004)
that fixes IntersectionGeometry::computeGeometry() to treat null as the implicit
root.

   geometry.computeGeometry();
 
   // Some corner cases for threshold index:
   //   - If target rect is zero area, because it has zero width and/or zero
   //     height,
   //     only two states are recognized:
   //     - 0 means not intersecting.
   //     - 1 means intersecting.
   //     No other threshold crossings are possible.
   //   - Otherwise:
@@ skipping 40 matching lines @@
     target()->ensureIntersectionObserverData().removeObservation(observer());
   m_observer.clear();
 }
 
 DEFINE_TRACE(IntersectionObservation) {
   visitor->trace(m_observer);
   visitor->trace(m_target);
 }
 
 }  // namespace blink