New Twiddle

controllers.a\.js

import Controller from '@ember/controller';

export default Controller.extend({
  
  get routeHeader () {
    return document.getElementById('route-header')
  }
  
});

controllers.application\.js

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

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';

	@tracked isNavHidden = false

	@action toggleNav () {
    this.isNavHidden = !this.isNavHidden
  }
}

controllers.b\.js

import Controller from '@ember/controller';

export default Controller.extend({
  
  get routeHeader () {
    return document.getElementById('route-header')
  }
  
});

router\.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route('a')
  this.route('b')
});

export default Router;

routes.a\.js

import Route from '@ember/routing/route';

export default Route.extend({
});

routes.b\.js

import Route from '@ember/routing/route';

export default Route.extend({
});

routes.index\.js

import Route from '@ember/routing/route';

export default Route.extend({
  redirect () {
    this.transitionTo('a')
  }
});

styles.app\.css

body {
  margin: 0;
}

.application {
  height: 100vh;
  display: flex;
  flex-direction: column;
  background-color: green;
}

.header {
  height: 4rem;
  display: flex;
  flex-direction: row;
}

.logo {
  width: 4rem;
  background-color: purple;
  cursor: pointer;
}

.route-header {
  flex: 1;
  background-color: blue;
}

.menu {
  width: 10rem;
  background: pink;
}

.main {
  flex: 1;
  display: flex;
  flex-direction: row;
  background-color: red;
}

.nav {
  width: 4rem;
  background-color: green;
  transition: width 0.2s;
}

.nav--hidden {
  width: 0rem;
}

.content {
  flex: 1;
  display: flex;
  flex-direction: column;
}

.route-content {
  flex: 1;
  display: flex;
  flex-direction: column;
 	background-color: orange; 
}

.route-content--center {
  flex: 1;
  align-self: center;
  width: 10rem;
  background-color: yellow;
  display: flex;
  flex-direction: column;
}

templates.a\.hbs

{{#-in-element this.routeHeader}}
  Stuff for route A header
{{/-in-element}}

<div class='route-content'>
	Stuff for route A content
</div>

templates.application\.hbs

<div class='application'>
	<div class='header'>
    <div class='logo' {{on 'click' this.toggleNav}}>
    </div>
    <div id='route-header' class='route-header'>
    </div>
    <div class='menu'>
    </div>
  </div>
  
  <div class='main'>
    <div class='nav {{if isNavHidden 'nav--hidden'}}'>
      <LinkTo @route='a'>a</LinkTo>
      <LinkTo @route='b'>b</LinkTo>
    </div>
    <div class='content'>
  		{{outlet}}
    </div>
  </div>
</div>

templates.b\.hbs

{{#-in-element this.routeHeader}}
  Stuff for route B header
{{/-in-element}}

<div class='route-content'>
	<div class='route-content--center'>
		Stuff for route B content
   </div>
</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"
  }
}