code: function repositionControls() {
      if (CTX.opt.responsive && nextBtn && prevBtn) {
        var btnTop = (getHeight() / 2) - (nextBtn.offsetHeight / 2);
        nextBtn.style.top = btnTop + 'px';
        prevBtn.style.top = btnTop + 'px';
      }
    }
docs: Moves controls to correct position