moebius#138: Optimize operations on root of deeply-nested frame tree

https://github.com/MoonchildProductions/moebius/pull/138

dom/base/nsINode.cpp

@@ -1907,6 +1907,10 @@ void
 nsINode::doRemoveChildAt(uint32_t aIndex, bool aNotify,
                          nsIContent* aKid, nsAttrAndChildArray& aChildArray)
 {
+  // NOTE: This function must not trigger any calls to
+  // nsIDocument::GetRootElement() calls until *after* it has removed aKid from
+  // aChildArray. Any calls before then could potentially restore a stale
+  // value for our cached root element, per note in nsDocument::RemoveChildAt().
   NS_PRECONDITION(aKid && aKid->GetParentNode() == this &&
                   aKid == GetChildAt(aIndex) &&
                   IndexOf(aKid) == (int32_t)aIndex, "Bogus aKid");