Ghost-Admin/app/components/power-select-vertical-collection-options.hbs

<ul role="listbox" aria-controls="ember-power-select-trigger-{{@select.uniqueId}}" {{did-insert this.addHandlers}} ...attributes>
    {{#if @select.loading}}
        {{#if @loadingMessage}}
            <li class="ember-power-select-option ember-power-select-option--loading-message" role="option">{{@loadingMessage}}</li>
        {{/if}}
    {{/if}}

    {{#vertical-collection @options minHeight=30 estimateHeight=6 bufferSize=10 as |opt index|}}
        <li class="ember-power-select-option"
            aria-selected="{{ember-power-select-is-selected opt @select.selected}}"
            aria-disabled={{if opt.disabled "true"}}
            aria-current="{{eq opt @select.highlighted}}"
            data-option-index="{{@groupIndex}}{{index}}"
            role="option">
            {{yield opt @select}}
        </li>
    {{/vertical-collection}}
</ul>