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HTML

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About the <head>

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Insecure Resource

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

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JavaScript

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JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <div id="smooth-content">
	<div class="section-container" id="panelcontainer">
		<section class="panel" id="panel_0"></section>
		<section class="panel" id="panel_1"></section>
		<section class="panel" id="panel_2"></section>
		<section class="panel" id="panel_3"></section>
	</div>
</div>
!

CSS 

              
                @charset "UTF-8";

html,
body {
	padding: 0;
	margin: 0;
}

body {
	width: 100%;
	height: 100vh;
	position: absolute;
	overflow-x: hidden;
	top: 0;
	left: 0;
	padding: 0;
	margin: 0;
}

#panelcontainer {
	height: 100%;
	position: relative;
}

.panel {
	position: absolute;
	width: 100%;
	height: 100vh;
	background-repeat: no-repeat;
	background-size: cover;
	background-position: center center;
	will-change: opacity, transform;
}

#panel_0 {
	background-image: url("https://via.placeholder.com/800x800/ffffff?text=one");
	background-color: olive;
}

#panel_1 {
	background-image: url("https://via.placeholder.com/800x800/000000?text=two");
	background-color: teal;
}

#panel_2 {
	background-image: url("https://via.placeholder.com/800x800/ffffff?text=three");
	background-color: steelBlue;
}

#panel_3 {
	background-image: url("https://via.placeholder.com/800x800/000000?text=four");
	background-color: sienna;
}
!

JS 

              
                // simplified down to remove the classes and function calls
// just show the bare necessities as simply as possible
gsap.registerPlugin(ScrollTrigger, ScrollSmoother);

// create the scrollSmoother before your scrollTriggers
ScrollSmoother.create({
	smooth: 1,
	smoothTouch: 0.1
});

let panels = gsap.utils.toArray(".panel");
let container = document.querySelector("#panelcontainer");
let winW = Math.min(window.innerWidth, document.documentElement.clientWidth);
let winH = Math.max(window.innerHeight, document.documentElement.clientHeight);

panels.forEach((panel, i) => {
	// simplified setupsizes
	gsap.set(panel, {
		height: Math.floor(winH) + "px", // make it a little taller than the screen to allow for bars
		width: Math.floor(winW) + "px"
	});

	if (i === 0) return;
	gsap.set(panel, { yPercent: 4, autoAlpha: 0, scale: 1.08 });
});

let tl = gsap.timeline();

tl.addLabel("panel0");

panels.forEach((panel, i) => {
	tl.to(panel, { yPercent: 0, autoAlpha: 1, scale: 1 });
	tl.addLabel("panel" + i);
});

gsap.set("#panelcontainer", {
	height: (panels.length * Math.floor(winH)).toString() + "px",
	width: winW + "px"
});

// typo
// console.log(panels.lenght, "🙈");

ScrollTrigger.create({
	trigger: container,
	start: "top top",
	end: `${(panels.length * Math.floor(winH)).toString()}px bottom`, // there was a typo here (panels.lenght) - returning undefined
	scrub: true,
	pin: true,
	pinSpacing: false,
	markers: true,
	// invalidateOnRefresh: true,
	snap: {
		snapTo: "labelsDirectional",
		duration: { min: 0.1, max: 0.3 },
		directional: true,
		inertia: false
	},
	animation: tl,
	id: "panelScrollTrigger",
	onLeave: (self) => {
		console.log("🧨 leave");
	},
	onToggle: (self) => {
		console.log("snap toggle");
	},
	onSnapComplete: (self) => {
		console.log("Snap complete");
	}
});
!
999px

Console