Updating from dependent key

controllers.application.js

import Ember from 'ember';
import { computed } from '@ember/object';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  newAppName: computed('appName', {
    get() {
    	return this.appName
    },
    set(key, value) {
    	return value;
    },
  })
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

NewAppName: {{newAppName}}<br>
<input value={{newAppName}} oninput={{action (mut newAppName) value="target.value"}}><br>

AppName: {{appName}}<br>
<input value={{appName}} oninput={{action (mut appName) value="target.value"}}>

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"
  }
}