Run loop coalesce

controllers.application.js

import Ember from 'ember';
import { later, debounce } from '@ember/runloop';

export default Ember.Controller.extend({
  logs: undefined,
  
  init() {
    this._super(...arguments);
    this.logs = [];
  },
  
  actions: {
  	case1() {
      this.log('---');
    	debounce(this, this.sync, "debounce", 1000);
      later(this, this.sync, "later", 2000);
    },
    
  	case2() {
      this.log('---');
      later(this, this.sync, "later", 2000);
    	debounce(this, this.sync, "debounce", 1000);
    },
  },
  
  sync(schedulerType) {
    this.log(`syncing via: ${schedulerType}`);
  },
  
  log(message) {
    this.logs.insertAt(0, message);
  },
});

templates.application.hbs

<h1>Debounce + later coelessing</h1>

<p style="color: blue">
	Since Ember 3.4, debounce and later seem to coalesce in a way that they didn't in 3.3 (see <a href="https://github.com/BackburnerJS/backburner.js/pull/319">this PR</a>)
</p>

Try these:

<button {{action 'case1'}}>debounce@1000, later@2000</button>
<button {{action 'case2'}}>later@2000, debounce@2000</button>

<hr>

<ul>
{{#each this.logs as |log|}}
  <li>{{log}}</li>
{{/each}}
</ul>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}