New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
   actions: {
    itChanged: function() {
      console.log('it changed!');
    }
  }
});

templates.application.hbs

Good: {{input type="checkbox" checked=cbState1}} checkbox w/o action: 
{{#if cbState1}}
checked 
{{else}}
unchecked
{{/if}}

<br>

Broken: {{input type="checkbox" checked=cbState2 change=(action "itChanged")}} checkbox with change action: 
{{#if cbState2}}
checked
{{else}}
unchecked
{{/if}}

<br>

Good: {{input type="checkbox" checked=cbState3 click=(action "itChanged")}} checkbox with click action: 
{{#if cbState3}}
checked
{{else}}
unchecked
{{/if}}

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"
  }
}