The resizeIframe() function has been updated to use ES6 syntax and includes optimization with requestAnimationFrame() to reduce layout thrashing.

resizeIframe.js

const buffer = 20; // scroll bar buffer
const iframe = document.getElementById('ifm');

const pageY = elem => elem.offsetParent ? (elem.offsetTop + pageY(elem.offsetParent)) : elem.offsetTop;

const resizeIframe = () => {
  window.requestAnimationFrame(() => {
    const height = document.documentElement.clientHeight;
    const iframeTop = pageY(document.getElementById('ifm'));
    const heightDiff = height - iframeTop + buffer;
    const newHeight = (heightDiff < 0) ? 0 : heightDiff;
    document.getElementById('ifm').style.height = `${newHeight}px`;
  });
};

if (iframe.attachEvent) {
  iframe.attachEvent("onload", resizeIframe);
} else {
  iframe.onload = resizeIframe;
}

window.onresize = resizeIframe;