Event to detect when position:sticky is triggered

css, html, javascript, jquery, position

Solution

Demo with IntersectionObserver (use a trick):

// get the sticky element
const stickyElm = document.querySelector('header')

const observer = new IntersectionObserver( 
  ([e]) => e.target.classList.toggle('isSticky', e.intersectionRatio < 1),
  {threshold: [1]}
);

observer.observe(stickyElm)

body{ height: 200vh; font:20px Arial; }

section{
  background: lightblue;
  padding: 2em 1em;
}

header{
  position: sticky;
  top: -1px;                       /* ➜ the trick */

  padding: 1em;
  padding-top: calc(1em + 1px);    /* ➜ compensate for the trick */

  background: salmon;
  transition: .1s;
}

/* styles for when the header is in sticky mode */
header.isSticky{
  font-size: .8em;
  opacity: .5;
}

<section>Space</section>
<header>Sticky Header</header>

The `top` value needs to be `-1px` or the element will never intersect with the top of the browser window (thus never triggering the intersection observer).

To counter this `1px` of hidden content, an additional `1px` of space should be added to either the border or the padding of the sticky element.

Alternatively, if you wish to keep the CSS as is (`top:0`), then you can apply the "correction" at the intersection observer-level by adding the setting `rootMargin: '-1px 0px 0px 0px'` (as @mattrick showed in his answer)

Demo with old-fashioned `scroll` event listener:

- auto-detecting first scrollable parent

- Throttling the scroll event

- Functional composition for concerns-separation

- Event callback caching: `scrollCallback` (to be able to unbind if needed)

// get the sticky element
const stickyElm = document.querySelector('header');

// get the first parent element which is scrollable
const stickyElmScrollableParent = getScrollParent(stickyElm);

// save the original offsetTop. when this changes, it means stickiness has begun.
stickyElm._originalOffsetTop = stickyElm.offsetTop;


// compare previous scrollTop to current one
const detectStickiness = (elm, cb) => () => cb & cb(elm.offsetTop != elm._originalOffsetTop)

// Act if sticky or not
const onSticky = isSticky => {
   console.clear()
   console.log(isSticky)
   
   stickyElm.classList.toggle('isSticky', isSticky)
}

// bind a scroll event listener on the scrollable parent (whatever it is)
// in this exmaple I am throttling the "scroll" event for performance reasons.
// I also use functional composition to diffrentiate between the detection function and
// the function which acts uppon the detected information (stickiness)

const scrollCallback = throttle(detectStickiness(stickyElm, onSticky), 100)
stickyElmScrollableParent.addEventListener('scroll', scrollCallback)



// OPTIONAL CODE BELOW ///////////////////

// find-first-scrollable-parent
// Credit: https://stackoverflow.com/a/42543908/104380
function getScrollParent(element, includeHidden) {
    var style = getComputedStyle(element),
        excludeStaticParent = style.position === "absolute",
        overflowRegex = includeHidden ? /(auto|scroll|hidden)/ : /(auto|scroll)/;

    if (style.position !== "fixed") 
      for (var parent = element; (parent = parent.parentElement); ){
          style = getComputedStyle(parent);
          if (excludeStaticParent && style.position === "static") 
              continue;
          if (overflowRegex.test(style.overflow + style.overflowY + style.overflowX)) 
            return parent;
      }

    return window
}

// Throttle
// Credit: https://jsfiddle.net/jonathansampson/m7G64
function throttle (callback, limit) {
    var wait = false;                  // Initially, we're not waiting
    return function () {               // We return a throttled function
        if (!wait) {                   // If we're not waiting
            callback.call();           // Execute users function
            wait = true;               // Prevent future invocations
            setTimeout(function () {   // After a period of time
                wait = false;          // And allow future invocations
            }, limit);
        }
    }
}

header{
  position: sticky;
  top: 0;

  /* not important styles */
  background: salmon;
  padding: 1em;
  transition: .1s;
}

header.isSticky{
  /* styles for when the header is in sticky mode */
  font-size: .8em;
  opacity: .5;
}

/* not important styles*/

body{ height: 200vh; font:20px Arial; }

section{
  background: lightblue;
  padding: 2em 1em;
}

<section>Space</section>
<header>Sticky Header</header>

Here's a React component demo which uses the first technique

Problem

Original source

Related problems

• Targeting position:sticky elements that are currently in a 'stuck' state
• Simple throttle in JavaScript
• Find first scrollable parent