packages/material-ui/src/Modal/ModalManager.ts/getHiddenSiblings
function getHiddenSiblings(container: Element) {
  const hiddenSiblings: Element[] = [];
  [].forEach.call(container.children, (element: Element) => {
    if (element.getAttribute('aria-hidden') === 'true') {
      hiddenSiblings.push(element);
    }
  });
  return hiddenSiblings;
}