update-class-names-timer

components.my-component-nested.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: ['red'],
  didInsertElement() {
    window.nested = this;
  }
});

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  shelfClassNames: null,
  
  didInsertElement() {
    this._super(...arguments);
    
    setTimeout(() => {
      Ember.set(this, 'shelfClassNames', 'blue')
    }, 100);
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

styles.app.css

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

templates.application.hbs

<MyComponent />

templates.components.my-component-nested.hbs

yo yo

templates.components.my-component.hbs

<MyComponentNested @classNames={{this.shelfClassNames}} />

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}