Trigger a callback when an element is resized
function observeResize (el, cb) {
  assert.ok(isDom(el), 'observe-resize: el should be a valid DOM element')
  assert.equal(typeof cb, 'function', 'observe-resize: cb should be type function')

  // Make this function a noop in non-browser environments
  if (typeof window !== 'object') return

  var called = false
  var frame = document.createElement('iframe')
  frame.setAttribute('class', '__observe-resize__')
  el.appendChild(frame)

  assert.ok(frame.contentWindow, 'observe-resize: no contentWindow detected - cannot start observing')
  frame.contentWindow.onresize = handleResize

  return function stopObserving () {
    if (frame.parentNode) frame.parentNode.removeChild(frame)
  }

  function handleResize () {
    if (called) return
    called = true
    window.requestAnimationFrame(function () {
      called = false
      cb(el)
    })
  }
}