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HTML 

              
                <div class="container">
	<h1>🐛 Firefox Bug: CSS Scroll Snap "Jump"</h1>
	<p>In Firefox 77, trying to smooth scroll an element with <code>.scroll({ behavior: 'smooth' })</code> that also has CSS Scroll Snap applied to it can cause an unwanted "jump" and fail to scroll to the correct destination.</p>
	<p>For example, in the demo below you should be able to click on the edges of the slides peeking in from the side of the page.</p>
	<p>In all other major browsers, this works as expected (even in Safari, which requires a polyfill for smooth scrolling but supports CSS Scroll Snap natively).</p>

	<h2>Demo</h2>
</div>

<ul class="demo demo--center">
	<li class="demo__slide" tabindex="-1">Slide 1</li>
	<li class="demo__slide" tabindex="-1">Slide 2</li>
	<li class="demo__slide" tabindex="-1">Slide 3</li>
</ul>

<div class="container">
	<h2>Browser Support <small>(as of 2020-06-03)</small></h2>
	<small class="note">All links open in new tabs.</small>
	<table class="support-table">
		<thead>
			<tr>
				<th>🖥 Browser</th>
				<th>📋 Behavior</th>
				<th>🎬 Screencast</th>
			</tr>
		</thead>
		<tbody>
			<tr>
				<td>Chrome 83+</td>
				<td>✅</td>
				<td><a href="https://assets.codepen.io/146527/2020-06-03+CSS+Scroll+Snap+Bug+-+Chrome+83.mp4" target="_blank" rel="noopener" title="Watch screencast of Chrome 83">Watch screencast</a></td>
			</tr>
			<tr>
				<td>Chrome for Android 8+</td>
				<td>✅</td>
				<td><a href="https://assets.codepen.io/146527/2020-06-03+CSS+Scroll+Snap+Bug+-+Chrome+for+Android+8.mp4" target="_blank" rel="noopener" title="Watch screencast of Chrome for Android 8">Watch screencast</a></td>
			</tr>
			<tr>
				<td>Edge 83+</td>
				<td>✅</td>
				<td><a href="https://assets.codepen.io/146527/2020-06-03+CSS+Scroll+Snap+Bug+-+Edge+83.mp4" target="_blank" rel="noopener" title="Watch screencast of Edge 83">Watch screencast</a></td>
			</tr>
			<tr class="error">
				<td>Firefox 77 </td>
				<td>❌</td>
				<td><a href="https://assets.codepen.io/146527/2020-06-03+CSS+Scroll+Snap+Bug+-+Firefox+77.mp4" target="_blank" rel="noopener" title="Watch screencast of Firefox 77">Watch screencast</a></td>
			</tr>
			<tr>
				<td>Firefox for Android 8+</td>
				<td>✅</td>
				<td><a href="https://assets.codepen.io/146527/2020-06-03+CSS+Scroll+Snap+Bug+-+Firefox+for+Android+8.mp4" target="_blank" rel="noopener" title="Watch screencast of Firefox for Android 8">Watch screencast</a></td>
			</tr>
			<tr>
				<td>Opera 68+</td>
				<td>✅</td>
				<td><a href="https://assets.codepen.io/146527/2020-06-03+CSS+Scroll+Snap+Bug+-+Opera+68.mp4" target="_blank" rel="noopener" title="Watch screencast of Opera 68">Watch screencast</a></td>
			</tr>
			<tr>
				<td>Safari 13+</td>
				<td>✅</td>
				<td><a href="https://assets.codepen.io/146527/2020-06-03+CSS+Scroll+Snap+Bug+-+Safari+13.mp4" target="_blank" rel="noopener" title="Watch screencast of Safari 13">Watch screencast</a></td>
			</tr>
			<tr>
				<td>Safari for iOS 13+</td>
				<td>✅</td>
				<td><a href="https://assets.codepen.io/146527/2020-06-03+CSS+Scroll+Snap+Bug+-+Safari+for+iOS+13.mp4" target="_blank" rel="noopener" title="Watch screencast of Safari for iOS 13">Watch screencast</a></td>
			</tr>
		</tbody>
	</table>

	<h2>References</h2>
	<ul>
		<li><a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1643217" target="_blank" rel="noopener">Bugzilla: CSS Scroll Snap conflicts with JS scroll behavior smooth</a></li>
		<li><a href="https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Scroll_Snap" target="_blank" rel="noopener">MDN: CSS Scroll Snap</a></li>
		<li><a href="https://developer.mozilla.org/en-US/docs/Web/API/Element/scroll" target="_blank" rel="noopener">MDN: Element.scroll()</a></li>
	</ul>
</div>
!

CSS 

              
                /* ------------------------------ */
/* Base                           */
/* ------------------------------ */

html {
	line-height: 1.5;
	tab-size: 4;
	box-sizing: border-box;
}

*,
::after,
::before {
	box-sizing: inherit;
}

body {
	margin: 0;
	padding-bottom: 10rem;
}

.container {
	max-width: 40rem;
	margin: auto;
	padding-right: 1rem;
	padding-left: 1rem;
}



/* ------------------------------ */
/* Demo                           */
/* ------------------------------ */

.demo {
	--peek: 5rem;
	--gutter: 1.5rem;
	display: block;
	position: relative;
	margin: 0 auto;
	padding: 0;
	overflow: scroll;
	font-size: 0;
	list-style: none;
	white-space: nowrap;
	scroll-snap-type: x mandatory; /* ⚠️ */
}

.demo__slide {
	display: inline-block;
	width: calc(100% - var(--peek) * 2);
	height: 10rem;
	margin: 0 calc(var(--gutter) / 2);
	padding: 1rem;
	border: 2px solid black;
	font-size: 1rem;
	white-space: normal;
	scroll-snap-align: center; /* ⚠️ */
}

.demo__slide:first-child {
	margin-left: var(--peek);
}

.demo__slide:last-child {
	margin-right: var(--peek);
}

.demo__slide:nth-child(even) {
	background: #fff;
}

.demo__slide:nth-child(odd) {
	background: #eee;
}



/* ------------------------------ */
/* Browser Support                */
/* ------------------------------ */

.support-table {
	border-collapse: collapse;
	border: 1px solid #ccc;
	text-align: left;
}

.support-table thead {
	background: #eee;
}

.support-table th,
.support-table td {
	padding: 0.5rem 1rem;
	border-bottom: 1px solid #ccc;
}

.support-table .error {
	background: #fee;
}

.note {
	display: block;
	margin-bottom: 0.5rem;
	font-style: italic;
}
!

JS 

              
                // Includes smooth scroll polyfill for Safari
// https://github.com/iamdustan/smoothscroll#readme

const slider = document.querySelector('.demo');

// When someone focuses on a slide, smooth scroll to it.
slider.addEventListener('focusin', function (event) {
	const slide = event.target.closest('.demo__slide');

	if (!slide) {
		return;
	}

	// Calculate the center position for the slide relative to its container.
	const top = slide.offsetTop + slide.offsetHeight / 2 - slider.offsetHeight / 2;
	const left = slide.offsetLeft + slide.offsetWidth / 2 - slider.offsetWidth / 2;

	// Smooth scroll to the slide's center position.
	slider.scroll({ top, left, behavior: 'smooth' });
});
!
999px

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