Table of Content for SharePoint Pages - https://fneve.blogspot.com/2017/10/table-of-content-for-sharepoint-pages.html

sp.tableOfContentsSticky.js

// Author: Fabian Neve / http://fneve.blogspot.ch/
// Date: 18.09.2017
// With input from https://blog.sharepointexperience.com/2014/10/sticky-stuff-in-sharepoint/
// 
// Top alignment of ToC has to be defined in sp.tableOfContents.css under #s4-workspace.scroll .right-wp-zone-col

var wrap = $("#s4-workspace");

wrap.on("scroll", function(e) {

if (this.scrollTop > 116 ) {
 wrap.addClass("scroll");
 } else {
 wrap.removeClass("scroll");
 } 
});