<div role="tab" {{action "toggleActive"}} class="panel-heading {{if collapsed "collapsed"}}">
    <h4 class="panel-title">
        <a href="#">
            {{title}}
        </a>
    </h4>
</div>
{{#bs-collapse collapsed=collapsed class="panel-collapse"}}
    <div class="panel-body">
        {{yield}}
    </div>
{{/bs-collapse}}
<div role="tab" {{action "toggleActive"}} class="panel-heading {{if collapsed "collapsed" "expanded"}}">
    <h4 class="panel-title">
        <a href="#">
            {{title}}
        </a>
    </h4>
</div>
{{#bs-collapse collapsed=collapsed class="panel-collapse"}}
    <div class="panel-body">
        {{yield}}
    </div>
{{/bs-collapse}}

Add expanded class name for accordion item.
feat(AccordionItem): Add expanded class name for accordion item.

Thanks to @Charizard 