active-element.js

/**
 * @param {!Element=}
 * @return {!Array<!Element>} containing focused element, deepest first
 */
function getActive(t = document.activeElement || document.body) {
  if (t.activeElement) {
    t = t.activeElement;
  }
  if (t.shadowRoot) {
    return [...getActive(t.shadowRoot), t];
  }
  return [t];
}