function initialize(nativeEventTarget) {
    root = nativeEventTarget;
    startText = getText();
    return true;
  }
These variables store information about text content of a target node,
allowing comparison of content before and after a given event.

Identify the node where selection currently begins, then observe
both its text content and its current position in the DOM. Since the
browser may natively replace the target node during composition, we can
use its position to find its replacement.