New Twiddle

components.row\.js

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

export default class extends Component {
  @tracked selected = false;

  @action
  handleClick(event) {
    if(event.shiftKey) {
	  	this.selected = !this.selected;
    }
  }
}

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
	rows = [{name: 'a'},{name: 'a'},{name: 'a'},{name: 'a'},{name: 'a'},{name: 'a'},{name: 'a'},{name: 'a'},{name: 'a'},{name: 'a'}]
}

styles.app\.css

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

.row {
	padding: 5px;
  background: #eef;
  margin: 10px;
  border: 1px solid black;
  border-radius: 3px;
  cursor: pointer;
}

.row.selected {
	  border: 1px solid red;
}

.row.selected, .row:hover {
  background: #eff;
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
{{#each this.rows as |row index|}}
	<Row @index={{index}} />
{{/each}}
<br>
{{outlet}}
<br>
<br>

templates.components.row\.hbs

<div class="row {{if this.selected 'selected'}}" {{on "click" this.handleClick}}>Row: {{@index}}</div>

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"
  }
}