New Twiddle

components.child\.js

import Component from '@glimmer/component';

export default class extends Component {
  get innerNumber() {
    return this.args.number;
  }
}

components.parent\.js

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

export default class extends Component {
  @tracked number = 1;
	
	@action changeNumber() {
    this.number++;
  }
}

controllers.application\.js

import Controller from '@ember/controller';

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

templates.application\.hbs

<Parent/>

templates.components.child\.hbs

<div>Arg {{@number}}</div>
<div>Getter {{this.innerNumber}}</div>

templates.components.parent\.hbs

<button {{on 'click' this.changeNumber}}>Increment</button>

<Child @number={{this.number}}/>

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"
  }
}