Use Document, rather than document element, for implicit root.

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

Index: third_party/WebKit/Source/core/dom/IntersectionObservation.cpp
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObservation.cpp b/third_party/WebKit/Source/core/dom/IntersectionObservation.cpp
index 3bb9aed1aa1db7e89140063a78126833707fe813..59b94832fb44446e1f10852c86db1a52632b7917 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObservation.cpp
+++ b/third_party/WebKit/Source/core/dom/IntersectionObservation.cpp
@@ -23,10 +23,15 @@ IntersectionObservation::IntersectionObservation(IntersectionObserver& observer,
 {
 }
 
+Element* IntersectionObservation::target() const
+{
+    return toElement(m_target.get());

[ojan, 2016/01/12 01:28:18]

How do you know this is an Element and not the Document?

Nit: I'd leave this in the header.

[szager1, 2016/01/12 21:19:51]

The target can only ever be an Element; only the root can be a Document.  The
IntersectionObservation constructor takes an Element argument, so this must be
an Element.

The only reason I didn't inline target() is because it would require adding
'#include "core/dom/Element.h"' to the header.  I don't feel strongly either
way, let me know what you prefer.

+}
+
 void IntersectionObservation::initializeGeometry(IntersectionGeometry& geometry)
 {
     ASSERT(m_target);
-    LayoutObject* targetLayoutObject = m_target->layoutObject();
+    LayoutObject* targetLayoutObject = target()->layoutObject();
     if (targetLayoutObject->isBoxModelObject())
         geometry.targetRect = toLayoutBoxModelObject(targetLayoutObject)->visualOverflowRect();
     else
@@ -40,7 +45,7 @@ void IntersectionObservation::clipToRoot(LayoutRect& rect)
     // TODO(szager): the writing mode flipping needs a test.
     ASSERT(m_target);
     LayoutObject* rootLayoutObject = m_observer->rootLayoutObject();
-    LayoutObject* targetLayoutObject = m_target->layoutObject();
+    LayoutObject* targetLayoutObject = target()->layoutObject();
     targetLayoutObject->mapToVisibleRectInAncestorSpace(toLayoutBoxModelObject(rootLayoutObject), rect, nullptr);
     if (rootLayoutObject->hasOverflowClip()) {
         LayoutBox* rootLayoutBox = toLayoutBox(rootLayoutObject);
@@ -54,10 +59,10 @@ void IntersectionObservation::clipToRoot(LayoutRect& rect)
 
 void IntersectionObservation::clipToFrameView(IntersectionGeometry& geometry)
 {
-    Element* rootElement = m_observer->root();
+    Node* rootNode = m_observer->root();
     LayoutObject* rootLayoutObject = m_observer->rootLayoutObject();
-    if (rootElement == rootElement->document().documentElement()) {
-        geometry.rootRect = LayoutRect(rootElement->document().view()->visibleContentRect());
+    if (rootLayoutObject->isLayoutView()) {
+        geometry.rootRect = LayoutRect(toLayoutView(rootLayoutObject)->frameView()->visibleContentRect());
         m_observer->applyRootMargin(geometry.rootRect);
         geometry.intersectionRect.intersect(geometry.rootRect);
     } else {
@@ -68,7 +73,7 @@ void IntersectionObservation::clipToFrameView(IntersectionGeometry& geometry)
         m_observer->applyRootMargin(geometry.rootRect);
     }
 
-    LayoutPoint scrollPosition(rootElement->document().view()->scrollPosition());
+    LayoutPoint scrollPosition(rootNode->document().view()->scrollPosition());
     geometry.targetRect.moveBy(-scrollPosition);
     geometry.intersectionRect.moveBy(-scrollPosition);
     geometry.rootRect.moveBy(-scrollPosition);
@@ -83,7 +88,7 @@ bool IntersectionObservation::computeGeometry(IntersectionGeometry& geometry)
 {
     ASSERT(m_target);
     LayoutObject* rootLayoutObject = m_observer->rootLayoutObject();
-    LayoutObject* targetLayoutObject = m_target->layoutObject();
+    LayoutObject* targetLayoutObject = target()->layoutObject();
     if (!rootLayoutObject->isBoxModelObject())
         return false;
     if (!targetLayoutObject->isBoxModelObject() && !targetLayoutObject->isText())
@@ -152,7 +157,7 @@ void IntersectionObservation::computeIntersectionObservations(double timestamp)
 void IntersectionObservation::disconnect()
 {
     if (m_target)
-        m_target->ensureIntersectionObserverData().removeObservation(this->observer());
+        target()->ensureIntersectionObserverData().removeObservation(this->observer());
     m_observer->removeObservation(*this);
     m_observer.clear();
 }