New Twiddle

components.collapsible-panel-content.js

import Ember from 'ember';

export default Ember.Component.extend({
});

components.collapsible-panel-header.js

import Ember from 'ember';

export default Ember.Component.extend({
  click() {
  	this.get('onClick')();
  }
});

components.collapsible-panel.js

import Ember from 'ember';

export default Ember.Component.extend({
  isOpen: false,
  actions: {
    togglePanel() {
      this.toggleProperty('isOpen');
    }
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({

});

templates.application.hbs

{{#collapsible-panel as |panel|}}
  {{#panel.header}}
    Panel header (click me)
  {{/panel.header}}
  {{#panel.content}}
    Panel content
  {{/panel.content}}
{{/collapsible-panel}}

templates.components.collapsible-panel-content.hbs

{{#if isOpen}}
	{{yield}}
{{/if}}

templates.components.collapsible-panel-header.hbs

{{yield}}

templates.components.collapsible-panel.hbs

{{yield (hash
  header=(component "collapsible-panel-header" onClick=(action "togglePanel"))
  content=(component "collapsible-panel-content" isOpen=isOpen)
)}}

twiddle.json

{
  "version": "0.13.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.16.2",
    "ember-template-compiler": "2.16.2",
    "ember-testing": "2.16.2"
  },
  "addons": {
    "ember-data": "2.16.3"
  }
}