Function _getOrigin

src/js/utils/bounds.js:185–216

(body)

}

function _getOrigin(body) {
  // getBoundingClientRect is unfortunately too accurate.  It introduces a pixel or two of
  // jitter as the user scrolls that messes with our ability to detect if two positions
  // are equivilant or not.  We place an element at the top left of the page that will
  // get the same jitter, so we can cancel the two out.
  let node = zeroElement;
  if (!node || !body.contains(node)) {
    node = document.createElement('div');
    node.setAttribute('data-tether-id', uniqueId());
    extend(node.style, {
      top: 0,
      left: 0,
      position: 'absolute'
    });

    body.appendChild(node);

    zeroElement = node;
  }

  const id = node.getAttribute('data-tether-id');
  if (isUndefined(zeroPosCache[id])) {
    zeroPosCache[id] = _getActualBoundingClientRect(node);

    // Clear the cache when this position call is done
    defer(() => {
      delete zeroPosCache[id];
    });
  }

  return zeroPosCache[id];
}