findCommonOffsetParent.ts

/** Get the offset ancestors of a node. */
function getOffsetAncestors(node: HTMLElement): Set<HTMLElement> {
  let ancestors = new Set<HTMLElement>()
  let current: Element | null = node.offsetParent
  while (current) {
    if (current instanceof HTMLElement) {
      ancestors.add(current)
      current = current.offsetParent
    } else {
      break
    }
  }
  return ancestors
}

/** Find the common offset parent of a set of nodes. */
function findCommonOffsetParent(nodes: HTMLElement[] = []): HTMLElement {
  let commonAncestors = getOffsetAncestors(nodes[0])

  for (let index = 1; index < nodes.length; index++) {
    const ancestors = getOffsetAncestors(nodes[index])

    for (let ancestor of commonAncestors) {
      if (!ancestors.has(ancestor)) {
        commonAncestors.delete(ancestor)
      }
    }

    if (commonAncestors.size === 0) {
      return document.body
    }
  }

  return commonAncestors.size > 0
    ? commonAncestors.values().next().value
    : document.body
}