local-copy-ish

components.demo-local-copy-ish\.hbs

<div style='border: 1px solid blue'>
<h2>child</h2>
<p><code>this.foo</code> is {{this.foo}}</p>
<label>
	update child <input value={{this.foo}} {{on "input" this.updateFoo}}>
</label>
</div>

components.demo-local-copy-ish\.js

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

export default class extends Component {
  _prevArgFoo;
  @tracked _foo;

  get foo() {
    if (this._prevArgFoo !== this.args.foo) {
      this._prevArgFoo = this.args.foo;
      this._foo = this.args.foo;
    }

    return this._foo;
  }

  updateFoo = ({ target: { value } }) => (this._foo = value);
}

controllers.application\.js

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

export default class ApplicationController extends Controller {
  @tracked foo = 'neato';

  updateFoo = ({ target: { value } }) => (this.foo = value);
}

templates.application\.hbs

<div style='border: 1px solid green'>
<h1>parent</h1>
<p><code>this.foo</code> is {{this.foo}}</p>
<DemoLocalCopyIsh @foo={{this.foo}} />
<label>
	update parent <input value={{this.foo}} {{on "input" this.updateFoo}}>
</label>
</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"
  }
}