Track user scrolling through page with GA - Copied from: https://pastebin.com/8wjY9eHa, see original article https://growthrocks.com/blog/scroll-tracking-google-analytics/

scroll.html

<script>
document.addEventListener('scroll', function(){
 
    var h =  document.documentElement,
        b =  document.body,
        st =  'scrollTop',
        sh =  'scrollHeight';
 
    var percent = parseInt ( (h[st]||b[st]) / ((h[sh]||b[sh]) - h.clientHeight) * 100);
 
  console.log(percent + "%");
if (percent == 25)
{
    ga('send', 'event', 'Scrolling', 'moreThan25%', '{{Page URL}}', { 'nonInteraction': 1 });
}
else if (percent == 50)
{
    ga('send', 'event', 'Scrolling', 'moreThan50%', '{{Page URL}}', { 'nonInteraction': 1 });
}
else if (percent == 75)
{
    ga('send', 'event', 'Scrolling', 'moreThan75%', '{{Page URL}}', { 'nonInteraction': 1 });
}
else if (percent == 90)
{
    ga('send', 'event', 'Scrolling', 'moreThan90%', '{{Page URL}}', { 'nonInteraction': 1 });
}
   
});
 
  </script>