storeScrollPosition() {

		clearTimeout( this.storeScrollPositionTimeout );

		this.storeScrollPositionTimeout = setTimeout( () => {
			sessionStorage.setItem( 'reveal-scroll-top', this.viewportElement.scrollTop );
			sessionStorage.setItem( 'reveal-scroll-origin', location.origin + location.pathname );

			this.storeScrollPositionTimeout = null;
		}, 50 );

	}
Persists the current scroll position to session storage
so that it can be restored.