Indeterminate demo

controllers.application\.js

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

export default class ApplicationController extends Controller {
  @tracked isChecked = false;
  @tracked isIndeterminate = false;

  cycle = () => {
    if (this.isChecked) {
      this.isChecked = false;
      this.isIndeterminate = false;
    } else if (this.isIndeterminate) {
      this.isChecked = true;
      this.isIndeterminate = false;
    } else {
      this.isChecked = false;
      this.isIndeterminate = true;
    }
  }
}

styles.app\.css

.demo-wrapper {
  display: flex;
  gap: 5px;
}

templates.application\.hbs

{{!-- https://discuss.emberjs.com/t/what-is-embers-magic-concerning-checkbox-indeterminate/16981 --}}

<div class="demo-wrapper">
  <button {{on "click" this.cycle}}>
    Click me to cycle states
  </button>

  <input
    type="checkbox"
    checked={{this.isChecked}}
    indeterminate={{this.isIndeterminate}}
  >
</div>

twiddle\.json

{
  "version": "0.17.1",
  "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/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}