New Twiddle

components.box\.js

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

export default class extends Component {
}

components.parent\.js

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

export default class extends Component {
  data = [{ id: 1 }, {id: 2},{id: 3}];
  @tracked expanded = [];

	@action toggleOne(id) {
    if(this.expanded.includes(id)) {
      this.expanded = this.expanded.filter(x => x !== id);
    } else {
      this.expanded = [...this.expanded, id];
    }
  }

	@action toggleAll() {
    if(this.expanded.length === 0) {
      this.expanded = this.data.map(e => e.id);
    } else {
      this.expanded = [];
    }
  }
}

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
}

helpers.contains\.js

import { helper } from '@ember/component/helper';

export default helper(function contains([elem, arr]/*, hash*/) {
  return arr.includes(elem);
});

templates.application\.hbs

<Parent />

templates.components.box\.hbs

<div>
	<button {{on "click" @toggle}}>Toggle from Box</button>
  {{#if @expanded}}
    if expanded
  {{else}}
    is hidden
  {{/if}}
</div>

templates.components.parent\.hbs

Debug expanded: {{this.expanded}}
<button {{on "click" this.toggleAll}}>Toggle all</button>
{{#each this.data as |item|}}
	<Box @expanded={{contains item.id this.expanded}} @toggle={{fn this.toggleOne item.id}} />
{{/each}}

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"
  }
}