EXPERIMENT Make toPositionInFlatTree() to return null for DOM position not in flat tree

third_party/WebKit/Source/core/editing/Position.cpp

Index: third_party/WebKit/Source/core/editing/Position.cpp
diff --git a/third_party/WebKit/Source/core/editing/Position.cpp b/third_party/WebKit/Source/core/editing/Position.cpp
index c2d3e6d0e318a359e6c23725bc00cdfafb4acd47..f561a912bf9a474152ffc9134e0e917658d1da21 100644
--- a/third_party/WebKit/Source/core/editing/Position.cpp
+++ b/third_party/WebKit/Source/core/editing/Position.cpp
@@ -541,6 +541,12 @@ PositionInFlatTree toPositionInFlatTree(const Position& pos) {
 
   if (anchor->isShadowRoot())
     return PositionInFlatTree(anchor->ownerShadowHost(), pos.anchorType());
+  if (anchor->canParticipateInFlatTree() &&
+      !FlatTreeTraversal::parent(*anchor)) {
+    // Since this function is a primitive function, we do not adjust |pos|
+    // to somewhere in flat tree.
+    return PositionInFlatTree();
+  }
   // TODO(yosin): Once we have a test case for SLOT or active insertion point,
   // this function should handle it.
   return PositionInFlatTree(anchor, pos.anchorType());