Accessible multiselect widget (based on https://www.24a11y.com/2019/select-your-poison-part-2/)

multiselect.css

.combo {
  display: block;
  margin-bottom: 1.5em;
  // max-width: 400px;
  position: relative;
}

.combo::after {
  border-bottom: 2px solid rgba(0,0,0,.5);
  border-right: 2px solid rgba(0,0,0,.5);
  content: '';
  display: block;
  height: 12px;
  pointer-events: none;
  position: absolute;
  right: 16px;
  top: 50%;
  transform: translate(0, -65%) rotate(45deg);
  width: 12px;
}

.input-wrapper {
  border-radius: 4px;
}

.combo-input {
  background-color: #f5f5f5;
  border: 2px solid rgba(0,0,0,.5);
  border-radius: 4px;
  display: block;
  font-size: 1em;
  min-height: calc(1.4em + 26px);
  padding: 12px 16px 14px;
  text-align: left;
  width: 100%;
}

select.combo-input {
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
}

.open .form-control {
  border-bottom-left-radius: 0;
  border-bottom-right-radius: 0;
}

.combo-input:focus {
  border-color: #0067b8;
  box-shadow: 0 0 4px 2px #0067b8;
  outline: 5px solid transparent;
}

.combo-label {
  display: block;
  font-size: 20px;
  font-weight: 100;
  margin-bottom: 0.25em;
}

.combo-menu {
  background-color: #f5f5f5;
  border: 1px solid rgba(0,0,0,.42);
  border-radius: 0 0 4px 4px;
  display: none;
  max-height: 300px;
  overflow-y:scroll;
  left: 0;
  position: absolute;
  top: 100%;
  width: 100%;
  z-index: 100;
}

.open .combo-menu {
  display: block;
}

.combo-option {
  padding: 10px 12px 12px;
}

.combo-option.option-current,
.combo-option:hover {
  background-color: rgb(198, 246, 213);
}

.combo-option.option-selected {
  padding-right: 30px;
  position: relative;
}

.combo-option.option-selected::after {
    content: '✓';
    position: absolute;
    right: 0.5rem;
    top: calc(50% - 1rem);
    font-size: 2rem;
    line-height: 2rem;
}

/* multiselect list of selected options */
.selected-options {
  list-style-type: none;
  margin: 0;
  // max-width: 400px;
  padding: 0;
}

.selected-options li {
  display: inline-block;
  margin-bottom: 5px;
}

.remove-option {
  @apply bg-primary-500;
  border-radius: 3px;
  color: #fff;
  font-size: 0.75em;
  font-weight: bold;
  margin-bottom: 6px;
  margin-right: 6px;
  padding: 0.25em 1.75em 0.25em 0.25em;
  position: relative;
}

.remove-option:focus {
  border-color: #baa1dd;
  box-shadow: 0 0 3px 1px #6200ee;
  outline: 3px solid transparent;
}

.remove-option::before,
.remove-option::after {
  border-right: 2px solid #fff;
  content: "";
  height: 1em;
  right: 0.75em;
  position: absolute;
  top: 50%;
  width: 0;
}

.remove-option::before {
  transform: translate(0, -50%) rotate(45deg);
}

.remove-option::after {
  transform: translate(0, -50%) rotate(-45deg);
}


.multiselect-inline {
  align-items: center;
  background-color: #f5f5f5;
  border: 2px solid rgba(0,0,0,.42);
  border-radius: 4px;
  display: flex;
  flex-wrap: wrap;
  min-height: calc(1.4em + 26px);
  padding: 12px 16px 14px;
}

.multiselect-inline .selected-options {
  flex: 0 1 auto;
}

.multiselect-inline .selected-options li {
  margin-bottom: 0;
}

.multiselect-inline .combo-input {
  border: none;
  flex: 1 1 35%;
  min-height: calc(1.4em - 2px);
  padding: 0;
}

.multiselect-inline .combo-input:focus {
  box-shadow: none;
  outline: none;
}

.multiselect-inline:focus-within {
  box-shadow: 0 0 3px 2px #0067b8;
  outline: 5px solid transparent;
}

multiselect.html.erb

<%= form_with(model: @review) do |form| %>
  <%= multiselect form, :recipients, current_account.users.pluck(:email) %>
<% end %>

multiselect_controller.js

import { Controller } from "stimulus"

/*
 * Helper constants and functions
 */

// make it easier for ourselves by putting some values in objects
// in TypeScript, these would be enums
const Keys = {
  Backspace: 'Backspace',
  Clear: 'Clear',
  Down: 'ArrowDown',
  End: 'End',
  Enter: 'Enter',
  Escape: 'Escape',
  Home: 'Home',
  Left: 'ArrowLeft',
  PageDown: 'PageDown',
  PageUp: 'PageUp',
  Right: 'ArrowRight',
  Space: ' ',
  Tab: 'Tab',
  Up: 'ArrowUp'
}

const MenuActions = {
  Close: 0,
  CloseSelect: 1,
  First: 2,
  Last: 3,
  Next: 4,
  Open: 5,
  Previous: 6,
  Select: 7,
  Space: 8,
  Type: 9
}

// filter an array of options against an input string
// returns an array of options that begin with the filter string, case-independent
function filterOptions(options = [], filter, exclude = []) {
  return options.filter((option) => {
    const matches = option.toLowerCase().indexOf(filter.toLowerCase()) === 0;
    return matches && exclude.indexOf(option) < 0;
  });
}

// return an array of exact option name matches from a comma-separated string
function findMatches(options, search) {
  const names = search.split(',');
  return names.map((name) => {
    const match = options.filter((option) => name.trim().toLowerCase() === option.toLowerCase());
    return match.length > 0 ? match[0] : null;
  })
  .filter((option) => option !== null);
}

// return combobox action from key press
function getActionFromKey(key, menuOpen) {
  // handle opening when closed
  if (!menuOpen && key === Keys.Down) {
    return MenuActions.Open;
  }

  // handle keys when open
  if (key === Keys.Down) {
    return MenuActions.Next;
  }
  else if (key === Keys.Up) {
    return MenuActions.Previous;
  }
  else if (key === Keys.Home) {
    return MenuActions.First;
  }
  else if (key === Keys.End) {
    return MenuActions.Last;
  }
  else if (key === Keys.Escape) {
    return MenuActions.Close;
  }
  else if (key === Keys.Enter) {
    return MenuActions.CloseSelect;
  }
  else if (key === Keys.Backspace || key === Keys.Clear || key.length === 1) {
    return MenuActions.Type;
  }
}

// get index of option that matches a string
function getIndexByLetter(options, filter) {
  const firstMatch = filterOptions(options, filter)[0];
  return firstMatch ? options.indexOf(firstMatch) : -1;
}

// get updated option index
function getUpdatedIndex(current, max, action) {
  switch(action) {
    case MenuActions.First:
      return 0;
    case MenuActions.Last:
      return max;
    case MenuActions.Previous:
      return Math.max(0, current - 1);
    case MenuActions.Next:
      return Math.min(max, current + 1);
    default:
      return current;
  }
}

// check if an element is currently scrollable
function isScrollable(element) {
  return element && element.clientHeight < element.scrollHeight;
}

// ensure given child element is within the parent's visible scroll area
function maintainScrollVisibility(activeElement, scrollParent) {
  const { offsetHeight, offsetTop } = activeElement;
  const { offsetHeight: parentOffsetHeight, scrollTop } = scrollParent;

  const isAbove = offsetTop < scrollTop;
  const isBelow = (offsetTop + offsetHeight) > (scrollTop + parentOffsetHeight);

  if (isAbove) {
    scrollParent.scrollTo(0, offsetTop);
  }
  else if (isBelow) {
    scrollParent.scrollTo(0, offsetTop - parentOffsetHeight + offsetHeight);
  }
}

/*
 * Multiselect Combobox w/ Buttons code
 */
const MultiselectButtons = function(el) {
  // element refs
  this.el = el;
  this.comboEl = el.querySelector('[role=combobox]');
  this.inputEl = el.querySelector('input[aria-autocomplete="list"]');
  this.listboxEl = el.querySelector('[role=listbox]');

  this.idBase = this.inputEl.id;
  this.selectedEl = document.getElementById(`${this.idBase}__selected`);
  this.selectedOptionTemplate = document.getElementById(`${this.idBase}__selected-option-template`);
  this.preselectedOptionButtons = Array.from(this.selectedEl.querySelectorAll('li button'));

  // data
  this.optionEls = Array.from(this.listboxEl.children);
  this.options = this.optionEls.map(option => option.innerText.trim())
  this.filteredOptions = this.options;

  // state
  this.activeIndex = 0;
  this.open = false;
}

MultiselectButtons.prototype.init = function() {
  this.inputEl.addEventListener('input', this.onInput.bind(this));
  this.inputEl.addEventListener('blur', this.onInputBlur.bind(this));
  this.inputEl.addEventListener('click', () => this.updateMenuState(true));
  this.inputEl.addEventListener('keydown', this.onInputKeyDown.bind(this));

  this.optionEls.map((optionEl, index) => {
    optionEl.addEventListener('click', () => { this.onOptionClick(index); });
    optionEl.addEventListener('mousedown', this.onOptionMouseDown.bind(this));
  });

  console.log(this.preselectedOptionButtons)
  this.preselectedOptionButtons.map((buttonEl, index) => {
    var optionValue = buttonEl.querySelector('.combobox-remove-text').innerText.trim();
    var optionIndex = this.options.indexOf(optionValue);
    console.log(buttonEl, index, optionValue, optionIndex)
    buttonEl.addEventListener('click', () => { this.removeOption(index); });
  });
}

MultiselectButtons.prototype.filterOptions = function(value) {
  this.filteredOptions = filterOptions(this.options, value);

  // hide/show options based on filtering
  const options = this.el.querySelectorAll('[role=option]');
  [...options].forEach((optionEl) => {
    const value = optionEl.innerText;
    if (this.filteredOptions.indexOf(value) > -1) {
      optionEl.style.display = 'block';
    }
    else {
      optionEl.style.display = 'none';
    }
  });
}

MultiselectButtons.prototype.onInput = function() {
  const curValue = this.inputEl.value;
  this.filterOptions(curValue);

  // if active option is not in filtered options, set it to first filtered option
  if (this.filteredOptions.indexOf(this.options[this.activeIndex]) < 0) {
    const firstFilteredIndex = this.options.indexOf(this.filteredOptions[0]);
    this.onOptionChange(firstFilteredIndex);
  }

  const menuState = this.filteredOptions.length > 0;
  if (this.open !== menuState) {
    this.updateMenuState(menuState, false);
  }
}

MultiselectButtons.prototype.onInputKeyDown = function(event) {
  const { key } = event;

  const max = this.filteredOptions.length - 1;
  const activeFilteredIndex = this.filteredOptions.indexOf(this.options[this.activeIndex]);

  const action = getActionFromKey(key, this.open);

  switch(action) {
    case MenuActions.Next:
    case MenuActions.Last:
    case MenuActions.First:
    case MenuActions.Previous:
      event.preventDefault();
      const nextFilteredIndex = getUpdatedIndex(activeFilteredIndex, max, action);
      const nextRealIndex = this.options.indexOf(this.filteredOptions[nextFilteredIndex]);
      return this.onOptionChange(nextRealIndex);
    case MenuActions.CloseSelect:
      event.preventDefault();
      return this.updateOption(this.activeIndex);
    case MenuActions.Close:
      event.preventDefault();
      return this.updateMenuState(false);
    case MenuActions.Open:
      return this.updateMenuState(true);
  }
}

MultiselectButtons.prototype.onInputBlur = function() {
  if (this.ignoreBlur) {
    this.ignoreBlur = false;
    return;
  }

  if (this.open) {
    this.updateMenuState(false, false);
  }
}

MultiselectButtons.prototype.onOptionChange = function(index) {
  this.activeIndex = index;
  this.inputEl.setAttribute('aria-activedescendant', `${this.idBase}-${index}`);

  // update active style
  const options = this.el.querySelectorAll('[role=option]');
  [...options].forEach((optionEl) => {
    optionEl.classList.remove('option-current');
  });
  options[index].classList.add('option-current');

  if (this.open && isScrollable(this.listboxEl)) {
    maintainScrollVisibility(options[index], this.listboxEl);
  }
}

MultiselectButtons.prototype.onOptionClick = function(index) {
  this.onOptionChange(index);
  this.updateOption(index);
  this.inputEl.focus();
}

MultiselectButtons.prototype.onOptionMouseDown = function() {
  this.ignoreBlur = true;
}

MultiselectButtons.prototype.removeOption = function(index) {
  const option = this.options[index];

  // update aria-selected
  const options = this.el.querySelectorAll('[role=option]');
  options[index].setAttribute('aria-selected', 'false');
  options[index].classList.remove('option-selected');

  // remove button
  const buttonEl = document.getElementById(`${this.idBase}-remove-${index}`);
  this.selectedEl.removeChild(buttonEl.parentElement);
}

MultiselectButtons.prototype.selectOption = function(index) {
  const selected = this.options[index];
  this.activeIndex = index;

  // update aria-selected
  const options = this.el.querySelectorAll('[role=option]');
  options[index].setAttribute('aria-selected', 'true');
  options[index].classList.add('option-selected');

  // add remove option button
  const listItem = this.selectedOptionTemplate.content.cloneNode(true);
  const buttonEl = listItem.querySelector('button');
  const spanEl = buttonEl.querySelector('span');
  const inputEl = listItem.querySelector('input[type="hidden"]')

  buttonEl.id = `${this.idBase}-remove-${index}`;
  buttonEl.addEventListener('click', () => { this.removeOption(index); });
  spanEl.innerHTML = selected;
  inputEl.value = selected;
  
  this.selectedEl.appendChild(listItem);
}

MultiselectButtons.prototype.updateOption = function(index) {
  const option = this.options[index];
  const optionEls = this.el.querySelectorAll('[role=option]');
  const optionEl = optionEls[index];
  const isSelected = optionEl.getAttribute('aria-selected') === 'true';

  if (isSelected) {
    this.removeOption(index);
  }

  else {
    this.selectOption(index);
  }

  this.inputEl.value = '';
  this.filterOptions('');
}

MultiselectButtons.prototype.updateMenuState = function(open, callFocus = true) {
  this.open = open;

  this.comboEl.setAttribute('aria-expanded', `${open}`);
  open ? this.el.classList.add('open') : this.el.classList.remove('open');
  callFocus && this.inputEl.focus();
}



export default class extends Controller {
  connect() {
    const multiButtonComponent = new MultiselectButtons(this.element, []);
    multiButtonComponent.init();
  }
}

multiselect_helper.rb

module MultiselectHelper
  def multiselect(form_builder, attribute, options)
    id_base = "combobox-#{attribute}"
    preselected_options = form_builder.object.public_send(attribute)

    content_tag(:div, data: {controller: "multiselect"}) do
      safe_join([
        form_builder.label(attribute, id: "#{id_base}__label", class: "funky-label"),
        # used as descriptive text for option buttons; if used within the button text itself, it ends up being read with the input name
        content_tag(:span, "remove", id: "#{id_base}__remove", class: "hidden"),
        content_tag(:ul, id: "#{id_base}__selected", class: "selected-options space-x-2") do
          safe_join([
            content_tag(:template, id: "#{id_base}__selected-option-template") do
              content_tag(:li) do
                safe_join([
                  form_builder.hidden_field(attribute, multiple: true, value: nil),
                  content_tag(:button, class: "btn-rounded space-x-1", aria: {describedby: "#{id_base}__remove"}) do
                    safe_join([
                      content_tag(:span, nil, class: "combobox-remove-text"),
                      content_tag(:span, "×", class: "combobox-remove-icon")
                    ])
                  end
                ])
              end
            end,
            (
              safe_join(
                preselected_options.map.with_index do |selected_option, i|
                  content_tag(:li) do
                    safe_join([
                      form_builder.hidden_field(attribute, multiple: true, value: selected_option),
                      content_tag(:button, type: :button, id: "#{id_base}-remove-#{i}", class: "btn-rounded space-x-1", aria: {describedby: "#{id_base}__remove"}) do
                        safe_join([
                          content_tag(:span, selected_option, class: "combobox-remove-text"),
                          content_tag(:span, "×", class: "combobox-remove-icon")
                        ])
                      end
                    ])
                  end
                end
              )
            )
          ])
        end,
        content_tag(:div, class: "combo js-multi-buttons mb-0") do
          safe_join([
            content_tag(:div, class: "input-wrapper", role: "combobox", aria: {haspopup: "listbox", expanded: false, owns: "listbox"}) do
              form_builder.text_field(attribute, value: nil, name: nil, id: "#{id_base}", class: "form-control", placeholder: "Start typing...", aria: {activedescendant: nil, autocomplete: "list", labelledby: "#{id_base}__label #{id_base}__selected"})
            end,
            content_tag(:div, id: "#{id_base}__listbox", class: "combo-menu", role: "listbox", aria: {multiselectable: true}) do
              safe_join(options.map.with_index do |option, i|
                content_tag(:div, option, id: "#{id_base}__combobox-#{i}", class: ["combo-option", ('option-current' if i.zero?), ('option-selected' if preselected_options.include?(option))], role: "option", aria: {selected: preselected_options.include?(option)})
              end)
            end
          ])
        end
      ])
    end
  end
end

This is awesome. I tried it out in Firefox and got errors here: https://gist.github.com/fractaledmind/92c6fdddb0d8e6c132183801ebb1485c#file-multiselect_controller-js-L182

Changed to

Array.from(options).forEach((optionEl) => {

worked. I guess the NodeList returned by querySelectorAll can't be used with the ... spread in Firefox.