Ember runloop demonstration with a computed property and an observer

components.foo-bar.js

import Ember from 'ember';

export default Ember.Component.extend({
  sourceInternal: 'default',
  logs: [],
  
  source: Ember.computed('sourceInternal', {
    get: function() {
      this._log(`[source getter] sourceInternal=${this.get('sourceInternal')}`);
      return this.get('sourceInternal');
    },
    set: function(key, newSource) {
      this._log(`[source setter] sourceInternal=${newSource}`);
      return this.set('sourceInternal', newSource);
    }
  }),
  
  _observeSourceInternal: Ember.observer('sourceInternal', function() {
    this._logSource('[observer]');
    setTimeout(() => { this._logSource('[delayed]') }, 1000);
  }),
  
  _logSource: function(prefix) {
    this._log(`${prefix} source=${this.get('source')} sourceInternal=${this.get('sourceInternal')}`);
  },
  
  _log: function(log) {
    let logs = this.get('logs');
    logs.pushObject(log);
  },  

  actions: {
    clearLogs: function() {
      this.set('logs', []);
    }
  }
});

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.flex {
  display: flex;
}

.col {
  border: 1px solid gray;
  border-radius: 8px;
  padding: 16px;
  margin: 2px;
}

.console {
  background-color: #dedede;
  min-height: 200px;
  border-radius: 4px;
  padding: 10px;
  line-height: 1.5;
}

templates.application.hbs

{{foo-bar}}

templates.components.foo-bar.hbs

source:
{{input value=source}}
<br>
<br>

Change the source value to see how sourceInternal is updated. This demo uses an observer to simulate the behavior I'm seeing in Evergreen, and a delay to show how sourceInternal is updated after the run loop completes.

<br>
<br>

<button {{action "clearLogs"}}>Clear logs</button>
<br>
<br>

<div class="console">
  {{#each logs as |str|}}
    {{str}}
    <br>
  {{/each}}
</div>

twiddle.json

{
  "version": "0.10.4",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.18.2",
    "ember-template-compiler": "2.18.2"
  },
  "addons": {}
}