position-relative-to.js

function getPositionReference(node) {
  let pos = getComputedStyle(node).getPropertyValue("position");
  switch (pos) {
    case "relative":
      return node;
    case "absolute":
      let parentPosition = "static";
      let parent = node.parentNode;
      while (parentPosition == "static" && parent.parentNode) {
        parent = parent.parentNode;
      }
      return parent;
    case "fixed":
      return window; // corresponds to the viewport, not to any element...
    case "sticky":
      // this acts like position: relative in some cases and position: fixed in others
      // mainly depends on scroll position
      // TODO.
      return null;
  }
}