New Twiddle

controllers.application\.js

import Controller from '@ember/controller';
import EmberObject, { action, computed } from '@ember/object';

const PROPERTY_BAG = Symbol('PROPERTY_BAG');

function legacyTrackedVanilla(target, key, descriptor) {
  target[PROPERTY_BAG] = target[PROPERTY_BAG] ?? EmberObject.create();
  target[PROPERTY_BAG].set(key, descriptor.initializer?.());
  descriptor.get = function() {
    console.log(1); // NEVER CALLED
    return target[PROPERTY_BAG].get(key);
  };
  descriptor.set = function(value) {
    console.log(2); // NEVER CALLED
    this.notifyPropertyChange(key);
    return target[PROPERTY_BAG].set(key, value);
  };
  return descriptor;
}

function legacyTrackedComputed(target, key, descriptor) {
  target[PROPERTY_BAG] = target[PROPERTY_BAG] ?? EmberObject.create();
  target[PROPERTY_BAG].set(key, descriptor.initializer?.());
  descriptor.value = computed({
    get() {
      console.log(1); // NEVER CALLED
      return target[PROPERTY_BAG].get(key);
    },
    set(_, value) {
      console.log(2); // NEVER CALLED
      this.notifyPropertyChange(key);
      return target[PROPERTY_BAG].set(key, value);
    },
  });
  return descriptor;
}

export default class ApplicationController extends Controller {
  @legacyTrackedVanilla testVanillaDecorator = 1;
  @legacyTrackedComputed testComputedDecorator = 1;

  @action
  testIt() {
    this.testVanillaDecorator = this.testVanillaDecorator + 1;
    this.testComputedDecorator = this.testComputedDecorator + 1;
  }

}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
<button type="button" {{on "click" this.testIt}}>Click!</button>
<br>
<br>

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": "3.12.2",
    "ember-template-compiler": "3.12.2",
    "ember-testing": "3.12.2"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}