JSFiddle - React, Tailwind, and code Playground

CSS

.postit { 
    font:16px/1.4 Arial, sans-serif;
    padding:10px;
    cursor:default;
    position:fixed;
    background:yellow;
    border:1px solid #666;
    box-shadow:0 0 10px #ccc;
    width:150px; 
    height:160px;
}

JavaScript

window.onclick = function ( e ) {
    if ( e.target.className === 'postit' ) { return; }
    var div = document.createElement( 'div' );
    div.contentEditable = true;
    div.className = 'postit';
    div.style.left = e.clientX + 'px';
    div.style.top = e.clientY + 'px';
    document.body.appendChild( div );
};