A React component example that makes it simple to position an element relative to a Slate node.

slate-node-portal.js

import React from 'react'
import Portal from 'react-portal'
import getOffsets from 'positions'
import { findDOMNode } from 'slate'

/**
 * No-op.
 *
 * @type {Function}
 */

function noop() {}

/**
 * A component that opens a portal relative to a Slate node.
 *
 * @type {Component}
 */

class SlateNodePortal extends React.Component {

  /**
   * Property types.
   *
   * @type {Object}
   */

  static propTypes = {
    children: React.PropTypes.any.isRequired,
    menuAnchor: React.PropTypes.string,
    node: React.PropTypes.object.isRequired,
    nodeAnchor: React.PropTypes.string,
    onClose: React.PropTypes.func,
    onOpen: React.PropTypes.func,
    onUpdate: React.PropTypes.func,
  };

  /**
   * Default properties.
   *
   * @type {Object}
   */

  static defaultProps = {
    menuAnchor: 'bottom middle',
    nodeAnchor: 'bottom middle',
    onClose: noop,
    onOpen: noop,
    onUpdate: noop,
  };

  /**
   * On portal open, store a reference to it and update the menu's position.
   *
   * @param {Element} portal
   */

  onOpen = (portal) => {
    this.portal = portal
    this.update()
    this.props.onOpen(portal)
  }

  /**
   * On portal update, update the menu's position.
   */

  onUpdate = () => {
    this.update()
    this.props.onUpdate()
  }

  /**
   * On portal close, remove the portal reference.
   */

  onClose = () => {
    this.portal = null
    this.props.onClose()
  }

  /**
   * Update the menu's position, relative to the current node.
   */

  update = () => {
    if (!this.portal) return
    const { node, nodeAnchor, menuAnchor } = this.props

    const menuEl = this.portal.firstChild
    menuEl.style.position = 'absolute'

    const nodeEl = findDOMNode(node)
    const offset = getOffsets(menuEl, menuAnchor, nodeEl, nodeAnchor)
    menuEl.style.top = `${offset.top}px`
    menuEl.style.left = `${offset.left}px`
  }

  /**
   * Render.
   *
   * @return {Element}
   */

  render = () => {
    const {
      children,
      menuAnchor,
      node,
      nodeAnchor,
      onClose,
      onOpen,
      onUpdate,
      ...props
    } = this.props

    return (
      <Portal
        {...props}
        onOpen={this.onOpen}
        onUpdate={this.onUpdate}
        onClose={this.onClose}
      >
        {children}
      </Portal>
    )
  }

}

/**
 * Export.
 *
 * @type {Component}
 */

export default SlateNodePortal

Sidenote for folks finding this: the <Portal /> needs an isOpened prop (isOpen if you're on react-portal v4) set, otherwise it doesn't show up.