JSFiddle - React, Tailwind, and code Playground

https://jsfiddle.net/levchenko_d/djy4h7u0/3/#update

by levchenko_d

HTML

<script src="https://beat.today/app/js/app.min.js"></script>
<link rel="stylesheet" href="https://beat.today/app/css/style.min.css">
<div id="log"></div>
<div class="calendar-wrapper"></div>

CSS

.calendar-wrapper {
    position: relative;
    height: 300px;
    overflow: hidden;
    box-shadow: 0 1px 5px 1px rgba(0,0,0,0.3);
    margin: 10px;
    border-radius: 5px;
    margin: 20px auto;
    width: 400px;
}
.calendar-today{
  background: red;
}
.calendar-topbar{
  height: 50px;
  width: 100%;
  position: relactive;
  background: #fff;
  z-index: 9;
  box-shadow: 0 1px 3px rgba(0,0,0,0.1);
}

.calendar-view {
  position: relative;
  height: 250px;
  overflow-y: scroll;
 -webkit-overflow-scrolling: touch;
}

.calendar-year:first-of-type{
   
}

.calendar-month{
  position: relative;
  opacity: 0.5;
}

.year-title{
  display: none;
}
#log{
  display: none;
  position: fixed;
  width: 100px;
  max-height: 200px;
  overflow-y: scroll;
  background: #fff;
  z-index: 99999;
  box-shadow: 0 2px 3px silver;
 
}
.month-title{
  position: absolute;
  height: 36px;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  text-align: center;
  margin: auto;
  opacity: 0.1;
  font-size: 36px;
  z-index: 0;
}

.calendar-week{
  position: relative;
}

.calendar-week:after{
  display: table;
  content: '';
  clear: both;
}

.calendar-day{
  float: left;
  width: 14.285714285714286%;
  padding: 5px 10px;
  height: 50px;
  font-family: 'Muli','Lato','Open Sans','Helvetica Neue',Helvetica,sans-serif;
  font-size: 11px;
}

.calendar-day:hover{
  background: rgba(0,0,0,0.07);
}

.calendar-topbar-date {
    line-height: 50px;
    padding: 0 20px;
    font-family: 'Muli','Lato','Open Sans','Helvetica Neue',Helvetica,sans-serif;
    font-size: 14px;
    color: #565656;
}

.calendar-week[data-calendar-skip-days='1'] .calendar-day:first-of-type{
  margin-left: 14.285714285714286%;
}
.calendar-week[data-calendar-skip-days='2'] .calendar-day:first-of-type{
  margin-left: 28.571428571428573%;
}
.calendar-week[data-calendar-skip-days='3'] .calendar-day:first-of-type{
  margin-left: 42.85714285714286%;
}
.calendar-week[data-calendar-skip-days='4'] .calendar-day:first-of-type{
  margin-left:...

JavaScript

function parseIntSafe(n){
  return (parseInt(n, 10)||0);
}

App.Follow.prototype.getBounds = function(){
    var t=this;

    function getElementBounds(element, index){
      var topBound = element.offsetTop,
          bottomBound = topBound + element.size().y;

      element.followData = {
        bounds : {
          top: topBound,
          bottom: bottomBound
        }
      };

      element.addClass(t.o.followingClass);
    }

    t.elementDOMs.loop(getElementBounds);
  };

App.Follow.prototype.attachEvents = function(){
    var t=this;

    t.onScroll = function(){
	    t.scrollTop = App.getScrollTop(App.q('.calendar-view'));


      t.findMatchingBounds(function(element){
        if(!element){
          App.isFn(t.o.onNoMatch)(t.previousElement);
          if(t.previousElement){
            t.previousElement.removeClass(t.o.matchClass);
            App.isFn(t.o.onLeave)(t.previousElement);
          }
          t.previousElement = null;
        } else {
          element.addClass(t.o.matchClass);

          if(element != t.previousElement){
            if(t.previousElement){
              t.previousElement.removeClass(t.o.matchClass);
              App.isFn(t.o.onLeave)(t.previousElement);
            }

            App.isFn(t.o.onEnter)(element);
            t.previousElement = element;
          }
        }

      });
    };

    App.q('.calendar-view').addEventListener('scroll', t.onScroll);
  };

App.run = function(fn, times, callback){
	for(var i=0; i<times+1; i++){
  	if(i===times){
    	App.isFn(callback)();
    } else {
    	App.isFn(fn)(i);
    }
  }
};







var Calendar = function(options){
	var t = this;
  t.o = options;
  t.element = App.q(t.o.element);
  t.instances = {};
  t.curretDate = new Date();
  t.shownDate = new Date(); //the one is shown
  t.allowMonthDetection = true; //inertia scroll bug
  t.currentDayDate = 1; //outer iterator for createDay();
  t.isInitialMonth = true;// see Folllow
  t.rowHeight = t.o.rowHeight || 50;
 ...