adds class 'stuck' to elements that are stuck. Uses Bootstraps' `position-sticky` class as a selector.

hnl.stickyStuck

const stickies = document.body.querySelectorAll('.position-sticky');

window.addEventListener('scroll', function(){
  stickies.forEach(function(el){
    let stuck = false;
    const rect = el.getBoundingClientRect();
    if (window.getComputedStyle(el).bottom == 'auto') {
      stuck = rect.top === parseInt(window.getComputedStyle(el).top, 10);
    } else if (window.getComputedStyle(el).top == 'auto') {
      stuck = rect.height + rect.top < window.innerHeight;
    }
    el.classList.toggle('stuck',stuck);
    
  });
});