Because of the way replaceChild is defined (https://dom.spec.whatwg.org/#dom-node-replacechild), it should add two mutation records when the inserted node was originally a sibling of the reference child. e.g. node.replaceChild(node.lastChild, node.firstChild) when node has two children. This bug was found by the newly added test: LayoutTests/http/tests/w3c/dom/nodes/MutationObserver-childList.html
<rdar://problem/22571887>
*** This bug has been marked as a duplicate of bug 149546 ***