.my-custom-scrollbar {

  position: relative;

  width: 800px;

  height: 400px;

  overflow: auto;

}

var myCustomScrollbar = document.querySelector('.my-custom-scrollbar');

Ps.initialize(myCustomScrollbar);

​

var scrollbarY = myCustomScrollbar.querySelector('.ps.ps--active-y>.ps__scrollbar-y-rail');

​

myCustomScrollbar.onscroll = function() {

    scrollbarY.style.cssText = `top: ${this.scrollTop}px!important; height: 400px; right: ${-this.scrollLeft}px`;  

}