scrollTransitionToSlide_(isMovingForward, nextSlide, callback) {
    this.el.style.overflow = 'hidden';

    if (!isMovingForward) {
      nextSlide.moveBeforeFirst();
      nextSlide.show();
      scrollTo(this.currentSlide_.el.offsetTop, 0);
    } else {
      nextSlide.show();
    }

    scrollTo(nextSlide.el.offsetTop, 500, () => {
      this.currentSlide_.hide();

      if (isMovingForward) {
        this.currentSlide_.moveAfterLast();
      }

      this.el.style.overflow = 'auto';
      setTimeout(() => {
        callback.call(this, nextSlide);
      }, 150);
    });
  }
Transitions to a slide, doing the scroll animation.
@param {boolean} isMovingForward Whether we're going forward or backwards.
@param {Slide} nextSlide Next slide.
@param {Function} callback Callback to be called upon finishing. This is an
async function so it'll happen once the scroll animation finishes.
@private
@see scrollTo