did insert test

controllers.application\.js

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


export default class ApplicationController extends Controller {
  @tracked appName = 'Ember Twiddle';
	@tracked items = [];


  @action
  update() {
    this.items.pushObject(1);
  }

}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>  
{{#if this.items.length}}
    test
   {{/if}}
  <div {{did-insert this.update}}>   
  </div>
{{outlet}}
<br>
<br>

twiddle\.json

{
  "version": "0.17.0",
  "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/3.3.1/jquery.js",
    "ember": "3.17.0",
    "@ember/render-modifiers": "1.0.2",
    "ember-template-compiler": "3.17.0",
    "ember-testing": "3.17.0"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "@ember/render-modifiers": "1.0.2"
  }
}