issue (solved)

controllers.application.js

import Ember from 'ember';
const {computed} = Ember;

let aCount = 0;
let bCount = 0;

let stableComputedKey = 0;

function stableComputed(...dependentKeys) {
  let func = dependentKeys.pop();
  let installed = false;
  let oldValues = [];
  let uniqueKey = `stableComputed-${stableComputedKey}`;
  if (dependentKeys.any(k => k.includes('@') || k.includes('[]'))) {
    throw new Error("Does not support array dependent keys");
  }
  stableComputedKey += 1;
  return computed(uniqueKey, function(...compArgs) {
    if (!installed) {
      // Should be done for every dependent key
      this.addObserver(dependentKeys[0], this, function() {
    
       let newValues = dependentKeys.map(k => this.get(k));
       if (!newValues.every((value, i) => oldValues[i] === value)) {
         this.notifyPropertyChange(uniqueKey);
       }
       oldValues = newValues;
      });
      installed = true;
      oldValues = dependentKeys.map(k => this.get(k));
    }
    return func.apply(this, compArgs);
  });
}

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  value: 'initial',
  constant: 'always the same',
  neverChangingProperty: computed('value', 'constant', function() {
    return this.get('constant')
  }),
 
  a: stableComputed('neverChangingProperty', function() {
    this.get('neverChangingProperty');
    aCount += 1;
    return aCount;
  }),

  b: computed('value', function() {
    bCount += 1;
    return bCount;
  }),
  
  actions: {
    changeValue() {
      this.set('constant', Math.random())
    },
    updateValue() {
      this.set('value', Math.random())
    },
    updateNeverChanging() {
      this.set('neverChangingProperty', Math.random())
    }
  }
  
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

a: {{a}}
<br>
b: {{b}}
<br>
value: {{value}}

<button {{action 'updateValue'}}>Update value</button>

<button {{action 'changeValue'}}>Change value</button>


{{neverChangingProperty}}

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}