Swap window scroll from main content to modal window, and vise versa.

SwapScroll.js

// Needs to be clean up.
var mainContent = document.getElementById('main');

var open = function (target) {
  var closeButton = target.querySelector('p.button button');
  mainContent.style.top = -1 * document.body.scrollTop + 'px';
  mainContent.style.position = 'fixed';
  target.setAttribute('aria-hidden', 'false');
  document.body.scrollTop = 0;
  closeButton.addEventListener('click', close);
};

var close = function (event) {
  var element = event.currentTarget;
  var target = element.parentNode.parentNode;
  var scrollTop = parseInt(mainContent.style.top.replace('px', '')) * -1;
  mainContent.style.position = 'static';
  mainContent.style.top = 0;
  target.setAttribute('aria-hidden', 'true');
  document.body.scrollTop = scrollTop;
  element.removeEventListener('click', close);
};