mouseEnter / mouseLeave example

components.background-trigger.js

import Ember from 'ember';
import { htmlSafe } from '@ember/string';

export default Ember.Component.extend({
  classNames: ['background-trigger'],	
	classNameBindings: [
    'active:hovered', // when 'active' is true - bind the CSS class 'hovered' to this component
  ],
  thing: null,
	active: false,
 
  mouseEnter() {
		this.set('active', true);
	},
  
  mouseLeave() {
    this.set('active', false);
  },
  
  backgroundStyle: Ember.computed('thing', function() {
    const image = this.get('thing.image');
		return htmlSafe(`background-image: url('${image}');`);
  }),

});

components.welcome-area.js

import Ember from 'ember';

export default Ember.Component.extend({
	classNames: ['welcome-area'],

  items: [
    {
      name: "one",
      image: "https://placehold.it/1600/0066ff",
    },
    {
      name: "two",
      image: "https://placehold.it/1600/ff0066",
    },
    {
      name: "three",
      image: "https://placehold.it/1600/6600ff",
    },
  ],
  
	// set background image...

});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'sub-component changes parent class?',
});

styles.app.css

* {box-sizing: border-box};

html, body {
  height: 100%;
}

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

.welcome-area {
	/* */
}

.background-fill {
  width: 20px;
  height: 20px;
  border: 1px solid red;
  background-size: cover;
  background-position: center;
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  pointer-events: none;
  opacity: 0;
  transition: opacity 1s;
}

.background-trigger {
	cursor: pointer; 
}

.background-trigger.hovered .background-fill{
	z-index: -1;
  opacity: 1;
}

templates.application.hbs

<h1>{{appName}}</h1>

<WelcomeArea />

templates.components.background-trigger.hbs

<div class='background-fill'
	style={{backgroundStyle}}
>
</div>

<h3>Background trigger {{thing.name}}</h3>

templates.components.welcome-area.hbs

<h2>Welcome area</h2>

{{#each items as |item|}}
	<BackgroundTrigger @thing={{item}} />
{{/each}}

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"
  }
}