Indeterminate checkboxes

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  indeterminate: false,
  actions: {
    toggleIndeterminate() {
      this.toggleProperty('indeterminate');
    }
  }
});

templates.application.hbs

<button {{action 'toggleIndeterminate'}}>toggle indeterminate</button>
<br />
<div>
<input type="checkbox" indeterminate={{indeterminate}} /><label>Indeterminate? {{indeterminate}}</label>
</div>
<div>
{{input type="checkbox" indeterminate=indeterminate}}<label>Indeterminate? {{indeterminate}}</label>
</div>

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}