Very basic seamless iframes

gofer.js

window.addEventListener("message", e => {
  let d = e.data;

  if(d.sentinel == 'gofer' && d.type == 'resize') {
    let f = document.querySelector(`iframe[src="${d.location}"]`);
    if(f && f.style) {
      f.style.height = `${d.height}px`;
    }
  }
});

Code on the iframe should be something like the following:

window.addEventListener('load', function(e) {
  window.parent.postMessage({
    sentinel: 'gofer',
    type: 'resize',
    height: document.body.scrollHeight,
    location: window.location.href
  }, '*');
});