client/lib/perfmon/index.js
function stopMutationObserver() {
	if ( mutationObserver && mutationObserverActive ) {
		activePlaceholders = [];
		activePlaceholderEverDetected = false;
		mutationObserver.disconnect();
		mutationObserverActive = false;
	}
}
function stopMutationObserver() {
	if ( mutationObserver && mutationObserverActive ) {
		activePlaceholders = [];
		activePlaceholderEverDetected = false;
		mutationObserver.disconnect();
		mutationObserverActive = false;
	}
}
Stop observing the DOM changes and clean up all data structures