Possible benchmarks:

1. Click "Create 2000 DOM nodes" - runs without any listeners
2. Click "set up DOMNodeInserted listener" (only once), then "Create 2000 DOM nodes" - runs same benchmark with a listener
3. Click "set up MutationObserver on container" (only once), then "Create 2000 DOM nodes" - since the MutationObserver is only listening for changes to container's children, but the DOM nodes are created in pane1, there's no callback
4. Click "set up MutationObserver on pane1" (only once), then "Create 2000 DOM nodes" - will get 2001 notifications, for the 2000 created nodes plus the BR after "pane1"