Detect CSS Sticky State Changes in JavaScript – Sticky Observer

Sticky Observer is a lightweight JavaScript utility that allows you to monitor CSS position: sticky elements within your document.

It uses the IntersectionObserver API to track the elements’ states and tells you exactly when these elements become stuck or unstuck during scrolling.

Features:

• Event Driven: Dispatches sticky-change events when elements transition between stuck and unstuck states.
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• Sentinel Based: Uses invisible sentinel elements and IntersectionObserver to track sticky behavior.
• Highly Customizable: Supports custom scroll containers, debug mode with visual indicators, and extended sticky behavior options.
• Automatic Parent Positioning: Detects and corrects statically positioned parent elements that would break sticky behavior.

Use Cases:

• Sticky header effects: Change a header’s appearance when it becomes stuck. Add shadows, modify logos, or update navigation styles.
• Scroll-based UI updates: Update progress indicators, table of contents, or breadcrumbs based on which section headers are currently sticky.
• Analytics tracking: Measure how long users interact with sticky elements. Log when important content remains in view during scrolling.
• Dynamic layout adjustments: Modify adjacent content layout when an element sticks. Prevent overlapping content or adjust spacing dynamically.

See It In Action:

How To Use Sticky Observer:

1. Install StickyObserver with NPM and import it into your JS.

# NPM
$ npm install @bramus/sticky-observer

import { StickyObserver } from '@bramus/sticky-observer';

2. Select a target element, and define what happens when the state changes.

// Create an observer for all h3 headings
const observer = StickyObserver.observe('h3');

// Listen for sticky state changes
observer.addEventListener('sticky-change', (e) => {
  const { target, stuck } = e.detail;
  // Apply visual changes based on sticky state
  if (stuck) {
    target.style.boxShadow = '0 2px 8px rgba(0,0,0,0.1)';
  } else {
    target.style.boxShadow = 'none';
  }
  console.log(`Element ${stuck ? 'stuck' : 'unstuck'}:`, target);
});

3. Your sticky elements need properly positioned parent containers:

/* Parent container must have positioning context */.container {
  position: relative; /* Required for sentinels to work */}

/* Your sticky element */h2 {
  position: sticky;
  top: 0;
  background: white;
  z-index: 10;
}

4. Advanced usages with configuration options and events.

// Observe elements within a specific scroll container
const scrollContainer = document.querySelector('.scrollable-area');
const observer = StickyObserver.observe('.sticky-header', {

  // Enable debug mode to visualize sentinels
  debug: true,

  // Specify a custom scroll container instead of viewport
  container: scrollContainer,
  
  // Keep reporting "stuck" even after scrolling past the sticky edge
  remainStickyBeyondStickyEdge: false

});

// The observer extends EventTarget, so you can remove listeners
const handler = (e) => {
  console.log('State changed:', e.detail);
};
observer.addEventListener('sticky-change', handler);

// Later: clean up
observer.removeEventListener('sticky-change', handler);
observer.disconnect(); // Stop all observations

How It Works:

When you observe an element, Sticky Observer injects two hidden div elements into the parent container. It places a Top Sentinel immediately before the target element and a Bottom Sentinel immediately after it.

The library then creates an IntersectionObserver. It watches these sentinels to see when they intersect with the viewport or the defined container. If the Top Sentinel scrolls out of view while the element remains visible, the library determines the element is “stuck.”

Conversely, interactions with the Bottom Sentinel determine when the element is pushed out of the sticky state by the bottom of its container. This logic allows the library to fire events without constantly polling the scroll position.

FAQs:

Q: Why does the library warn about parent positioning, and can I ignore it?
A: The warning appears when a sticky element’s parent has position: static (the default). Absolute positioning of sentinel elements requires a positioned parent.

Q: How do I handle multiple sticky elements that stack on top of each other?
A: Each sticky element needs its own observer instance or you can observe them with a single selector that matches all elements. The top CSS property on each sticky element determines their stacking order. Set different top values to create stacked sticky headers. The library tracks each element’s state independently and fires separate events for each one.

Q: The stuck state triggers too early or too late. How do I adjust the detection threshold?
A: The library uses fixed thresholds for the IntersectionObserver instances (0 for top sentinel, 1 for bottom sentinel). These thresholds match the sticky behavior boundaries. If the timing feels off, check that your sticky element’s top CSS value matches your expectations.

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