New Twiddle

components.select-list\.js

import Component from "@glimmer/component";
import { tracked } from "@glimmer/tracking";
import { action } from "@ember/object";

class Cell {
  @tracked value;
  constructor(key, value) {
    this.key = key;
    this.value = value;
  }
}

function wrap(index, length) {
  let wrapped = Math.abs(index) % length;
  return index < 0 ? length - wrapped : wrapped;
}

function clamp(index, length) {
  return Math.max(0, Math.min(index, length - 1));
}

export default class SelectList extends Component {
  selectedIndexCell = new Cell(0, -1);

  get selectedIndex() {
    let selectedIndexCell = this.selectedIndexCell;
    let length = this.items.length;
    if (selectedIndexCell.key !== length) {
      let clampedIndex = clamp(selectedIndexCell.value, length);
      selectedIndexCell = this.selectedIndexCell = new Cell(
        length,
        clampedIndex
      );
    }
    return selectedIndexCell.value;
  }

  set selectedIndex(value) {
    this.selectedIndexCell.value = wrap(value, this.items.length);
  }

  get items() {
    return this.args.items;
  }

  @action _didInsertElement() {
    document.addEventListener("keydown", this.onKeydown);
  }

  @action _willDestroyElement() {
    document.removeEventListener("keydown", this.onKeydown);
  }

  @action onKeydown(e) {
    switch (e.key) {
      case "ArrowUp":
        return this.selectPrev();
      case "ArrowDown":
        return this.selectNext();
      // case 'Enter':
      //   return this.didCommit();
      // case 'Escape':
      //.  return this.didCancel();
    }
  }

  @action selectPrev() {
    this.selectedIndex--;
  }

  @action selectNext() {
    this.selectedIndex++;
  }
}

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';
import { tracked } from '@glimmer/tracking';

const ITEMS = ['one', 'two', 'three', 'four'];

export default class ApplicationController extends Controller {
  @tracked items = [...ITEMS];
  
  @action updateItems(count) {
    this.items = ITEMS.slice(0, count);
  }
}

styles.app\.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

ul {
  border: 1px solid black;
}

li.selected {
  background: #efefef;
}

templates.application\.hbs

<SelectList @items={{this.items}} />

<p>
  <button {{on "click" (fn this.updateItems 1)}}>One Items</button>
  <button {{on "click" (fn this.updateItems 2)}}>Two Items</button>
  <button {{on "click" (fn this.updateItems 3)}}>Three Items</button>
  <button {{on "click" (fn this.updateItems 4)}}>Four Items</button>
</p>

<p>
  Test case:
  
  <ol>
    <li>Press down arrow twice to get to "three".</li>
    <li>Press "Three Items".</li>
    <li>Selection should remain at "Three Items".</li>
    <li>Press "Two Items".</li>
    <li>Selection should move to "Two Items".</li>
    <li>Press "Three Items".</li>
    <li>Selection should remain at "Two Items".</li>
  </ol>
</p>

templates.components.select-list\.hbs

<ul
	{{did-insert this._didInsertElement}}
  {{will-destroy this._willDestroyElement}}
>
  {{#each @items as |item index|}}
    <li class={{if (eq index this.selectedIndex) "selected"}}>
      {{item}}
    </li>
  {{/each}}
</ul>

<p>selectedIndex: {{this.selectedIndex}}</p>

<p>Use up and down arrow keys to move selection</p>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "@ember/render-modifiers": "1.0.2",
    "ember-truth-helpers": "2.1.0"
  }
}