MediaWiki:Common.js

if(document.getElementById("toc")){
  var toc_height = document.getElementById("toc").offsetHeight;
  var toc_to_top = document.getElementById("bodyContent").offsetTop;
  var toc_scroll_top = toc_height + toc_to_top + 20;
  window.onscroll = function() {
    var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
    if(scrollTop > toc_scroll_top){
      document.getElementById('toc').style.position = 'fixed';
    } else {
      document.getElementById('toc').style.position = '';
    }
  }
}