페이지가 처음 로딩 되었을 때 애니메이션 처리가 되지 않는 느낌입니다

페이지가 맨 처음 로딩 되었을 때에는 애니메이션이 작동하지 않다가... 페이지 섹션을 일정 부분 진행하고 다시 윗방향 스크롤을 걸었을때 애니메이션이 정상작동을 합니다.

그 후부터는 페이지의 시작 위치로 이동해서 시작부터 차근차근 내렸을때 정상작동을 합니다.

원인이 잘 생각이 나지 않네요

Main.js

(() => {

    let yOffset = 0; // window.pageYOffset 대신에 쓸 변수
    let prevScrollHeight = 0; // 현재 스크롤 위치(yOffset) 보다 이전에 위치한 스크롤 섹션들의 스크롤 높이값의 합
    let currentScene = 0; // 현재 활성화된 (눈 앞에 보고있는) 씬 (Scrollsection)
    let enterNewScene = false; // 새로운 Scene이 시작된 순간 true
    // sceneInfo는 각 scene에 대한 정보를 담고 있다.
    const sceneInfo = [
        {
            // 0
            type: 'sticky',
            heightNum: 5, // 브라우저 높이의 5배로 scrollHeight 세팅
            scrollHeight: 0,
            objs: {
                container: document.querySelector('#scroll-section-0'),
                messageA: document.querySelector('#scroll-section-0 .main-message.a'),
                messageB: document.querySelector('#scroll-section-0 .main-message.b'),
                messageC: document.querySelector('#scroll-section-0 .main-message.c'),
                messageD: document.querySelector('#scroll-section-0 .main-message.d')
            },
            values: {
                messageA_opacity_in: [0, 1, { start: 0.1, end: 0.2 }],
                messageB_opacity_in: [0, 1, { start: 0.3, end: 0.4 }],
                messageC_opacity_in: [0, 1, { start: 0.5, end: 0.6 }],
                messageD_opacity_in: [0, 1, { start: 0.7, end: 0.8 }],
                messageA_translateY_in: [20, 0, { start: 0.1, end: 0.2 }],
                messageB_translateY_in: [20, 0, { start: 0.3, end: 0.4 }],
                messageC_translateY_in: [20, 0, { start: 0.5, end: 0.6 }],
                messageD_translateY_in: [20, 0, { start: 0.7, end: 0.8 }],
                messageA_opacity_out: [1, 0, { start: 0.25, end: 0.3 }],
                messageB_opacity_out: [1, 0, { start: 0.45, end: 0.5 }],
                messageC_opacity_out: [1, 0, { start: 0.65, end: 0.7 }],
                messageD_opacity_out: [1, 0, { start: 0.85, end: 0.9 }],
                messageA_translateY_out: [0, -20, { start: 0.25, end: 0.3 }],
                messageB_translateY_out: [0, -20, { start: 0.45, end: 0.5 }],
                messageC_translateY_out: [0, -20, { start: 0.65, end: 0.7 }],
                messageD_translateY_out: [0, -20, { start: 0.85, end: 0.9 }]
            }
        },
        {
            // 1
            type: 'normal',
            // heightNum: 5, // type normal에서는 필요 없음
            scrollHeight: 0,
            objs: {
                container: document.querySelector('#scroll-section-1'),
                content: document.querySelector('#scroll-section-1 .description')
            }
        },
        {
            // 2
            type: 'sticky',
            heightNum: 5,
            scrollHeight: 0,
            objs: {
                container: document.querySelector('#scroll-section-2'),
                messageA: document.querySelector('#scroll-section-2 .a'),
                messageB: document.querySelector('#scroll-section-2 .b'),
                messageC: document.querySelector('#scroll-section-2 .c'),
                pinB: document.querySelector('#scroll-section-2 .b .pin'),
                pinC: document.querySelector('#scroll-section-2 .c .pin')
            },
            values: {
                messageA_translateY_in: [20, 0, { start: 0.15, end: 0.2 }],
                messageB_translateY_in: [30, 0, { start: 0.5, end: 0.55 }],
                messageC_translateY_in: [30, 0, { start: 0.72, end: 0.77 }],
                messageA_opacity_in: [0, 1, { start: 0.15, end: 0.2 }],
                messageB_opacity_in: [0, 1, { start: 0.5, end: 0.55 }],
                messageC_opacity_in: [0, 1, { start: 0.72, end: 0.77 }],
                messageA_translateY_out: [0, -20, { start: 0.3, end: 0.35 }],
                messageB_translateY_out: [0, -20, { start: 0.58, end: 0.63 }],
                messageC_translateY_out: [0, -20, { start: 0.85, end: 0.9 }],
                messageA_opacity_out: [1, 0, { start: 0.3, end: 0.35 }],
                messageB_opacity_out: [1, 0, { start: 0.58, end: 0.63 }],
                messageC_opacity_out: [1, 0, { start: 0.85, end: 0.9 }],
                pinB_scaleY: [0.5, 1, { start: 0.5, end: 0.55 }],
                pinC_scaleY: [0.5, 1, { start: 0.72, end: 0.77 }],
                pinB_opacity_in: [0, 1, { start: 0.5, end: 0.55 }],
                pinC_opacity_in: [0, 1, { start: 0.72, end: 0.77 }],
                pinB_opacity_out: [1, 0, { start: 0.58, end: 0.63 }],
                pinC_opacity_out: [1, 0, { start: 0.85, end: 0.9 }]
            }
        },
        {
            // 3
            type: 'sticky',
            heightNum: 5,
            scrollHeight: 0,
            objs: {
                container: document.querySelector('#scroll-section-3'),
                canvasCaption: document.querySelector('.canvas-caption')
            },
            values: {

            }
        }
    ];
    
    function setLayout() {
        // 각 스크롤 섹션의 높이 세팅
        for (let i = 0; i < sceneInfo.length; i++) {
            if(sceneInfo[i].type === 'sticky') {
                sceneInfo[i].scrollHeight = sceneInfo[i].heightNum * window.innerHeight; // window.innerHeight는 브라우저 창의 높이
            } else if(sceneInfo[i].type === 'normal') {
                sceneInfo[i].scrollHeight = sceneInfo[i].objs.container.offsetHeight
            }
                sceneInfo[i].objs.container.style.height = `${sceneInfo[i].scrollHeight}px`
        }

        yOffset = window.scrollY;

        let totalScrollHeight = 0;
        for (let i = 0; i < sceneInfo.length; i++) {
            totalScrollHeight += sceneInfo[i].scrollHeight;
            if(totalScrollHeight >= yOffset) { // 이미지로 그려보면 이해가 쉬움
                currentScene = i;
                break;
            }
        }
        document.body.setAttribute('id', `show-scene=${currentScene}`)
    }

    function calcValues (values, currentYOffset) { // currentYOffset은 현재 씬에서 얼마나 스크롤 됐는지를 의미
        let rv;
        // 현재 씬(스크롤섹션)에서 스크롤된 범위를 비율로 구하기
        const scrollHeight = sceneInfo[currentScene].scrollHeight
        const scrollRatio = currentYOffset / sceneInfo[currentScene].scrollHeight;

        if (values.length === 3) {
            // start ~ end 사이의 애니메이션을 실행
            const partScrollStart = values[2].start * scrollHeight;
            const partScrollEnd = values[2].end * scrollHeight;
            const partScrollHeight = partScrollEnd - partScrollStart

            if (currentYOffset >= partScrollStart && currentYOffset <= partScrollEnd) {
                rv = (currentYOffset - partScrollStart) / partScrollStart * (values[1] - values[0]) + values[0];
            } else if (currentYOffset < partScrollStart) {
                rv = values[0];
            } else if (currentYOffset > partScrollEnd) {
                rv = values[1];
            }
        } else {
            rv = scrollRatio * (values[1] - values[0]) + values[0];
        }

        return rv;
    }

    function playAnimation() {
        const objs = sceneInfo[currentScene].objs;
        const values = sceneInfo[currentScene].values;
        const currentYOffset = yOffset - prevScrollHeight;
        const scrollHeight = sceneInfo[currentScene].scrollHeight;
        const scrollRatio = currentYOffset / scrollHeight;

        switch (currentScene) {
            case 0:
                // console.log('0 play');
                if (scrollRatio <= 0.22) {
                    // in
                    objs.messageA.style.opacity = calcValues(values.messageA_opacity_in, currentYOffset);
                    objs.messageA.style.transform = `translate3d(0, ${calcValues(values.messageA_translateY_in, currentYOffset)}%, 0)`;
                } else {
                    // out
                    objs.messageA.style.opacity = calcValues(values.messageA_opacity_out, currentYOffset);
                    objs.messageA.style.transform = `translate3d(0, ${calcValues(values.messageA_translateY_out, currentYOffset)}%, 0)`;
                }

                if (scrollRatio <= 0.42) {
                    // in
                    objs.messageB.style.opacity = calcValues(values.messageB_opacity_in, currentYOffset);
                    objs.messageB.style.transform = `translate3d(0, ${calcValues(values.messageB_translateY_in, currentYOffset)}%, 0)`;
                } else {
                    // out
                    objs.messageB.style.opacity = calcValues(values.messageB_opacity_out, currentYOffset);
                    objs.messageB.style.transform = `translate3d(0, ${calcValues(values.messageB_translateY_out, currentYOffset)}%, 0)`;
                }

                if (scrollRatio <= 0.62) {
                    // in
                    objs.messageC.style.opacity = calcValues(values.messageC_opacity_in, currentYOffset);
                    objs.messageC.style.transform = `translate3d(0, ${calcValues(values.messageC_translateY_in, currentYOffset)}%, 0)`;
                } else {
                    // out
                    objs.messageC.style.opacity = calcValues(values.messageC_opacity_out, currentYOffset);
                    objs.messageC.style.transform = `translate3d(0, ${calcValues(values.messageC_translateY_out, currentYOffset)}%, 0)`;
                }

                if (scrollRatio <= 0.82) {
                    // in
                    objs.messageD.style.opacity = calcValues(values.messageD_opacity_in, currentYOffset);
                    objs.messageD.style.transform = `translate3d(0, ${calcValues(values.messageD_translateY_in, currentYOffset)}%, 0)`;
                } else {
                    // out
                    objs.messageD.style.opacity = calcValues(values.messageD_opacity_out, currentYOffset);
                    objs.messageD.style.transform = `translate3d(0, ${calcValues(values.messageD_translateY_out, currentYOffset)}%, 0)`;
                }

                break;

            case 2:
                // console.log('2 play');
                if (scrollRatio <= 0.32) {
                    // in
                    objs.messageA.style.opacity = calcValues(values.messageA_opacity_in, currentYOffset);
                    objs.messageA.style.transform = `translate3d(0, ${calcValues(values.messageA_translateY_in, currentYOffset)}%, 0)`;
                } else {
                    // out
                    objs.messageA.style.opacity = calcValues(values.messageA_opacity_out, currentYOffset);
                    objs.messageA.style.transform = `translate3d(0, ${calcValues(values.messageA_translateY_out, currentYOffset)}%, 0)`;
                }

                if (scrollRatio <= 0.67) {
                    // in
                    objs.messageB.style.transform = `translate3d(0, ${calcValues(values.messageB_translateY_in, currentYOffset)}%, 0)`;
                    objs.messageB.style.opacity = calcValues(values.messageB_opacity_in, currentYOffset);
                    objs.pinB.style.transform = `scaleY(${calcValues(values.pinB_scaleY, currentYOffset)})`;
                } else {
                    // out
                    objs.messageB.style.transform = `translate3d(0, ${calcValues(values.messageB_translateY_out, currentYOffset)}%, 0)`;
                    objs.messageB.style.opacity = calcValues(values.messageB_opacity_out, currentYOffset);
                    objs.pinB.style.transform = `scaleY(${calcValues(values.pinB_scaleY, currentYOffset)})`;
                }

                if (scrollRatio <= 0.93) {
                    // in
                    objs.messageC.style.transform = `translate3d(0, ${calcValues(values.messageC_translateY_in, currentYOffset)}%, 0)`;
                    objs.messageC.style.opacity = calcValues(values.messageC_opacity_in, currentYOffset);
                    objs.pinC.style.transform = `scaleY(${calcValues(values.pinC_scaleY, currentYOffset)})`;
                } else {
                    // out
                    objs.messageC.style.transform = `translate3d(0, ${calcValues(values.messageC_translateY_out, currentYOffset)}%, 0)`;
                    objs.messageC.style.opacity = calcValues(values.messageC_opacity_out, currentYOffset);
                    objs.pinC.style.transform = `scaleY(${calcValues(values.pinC_scaleY, currentYOffset)})`;
                }

                break;

            case 3:
                // console.log('3 play');
                break;
        }
    }

    function scrollLoop() {
        enterNewScene = false;
        prevScrollHeight = 0; // PrevScrollHeight의 초기화가 함수 내에 있어야 맥북 기준 4 scene의 총합 11540이 찍히고 다음 scrollLoop() 이 실행될 때 다시 초기화 된다. (값이 누적되지 않는다.)
        for (let i = 0; i < currentScene; i++) {
            prevScrollHeight += sceneInfo[i].scrollHeight;
        }

        if(yOffset > prevScrollHeight + sceneInfo[currentScene].scrollHeight) {
            enterNewScene = true;
            currentScene++;
            document.body.setAttribute('id', `show-scene-${currentScene}`);
        }

        if(yOffset < prevScrollHeight) {
            enterNewScene = true;
            if (currentScene === 0) return; // scene 0에서 바운싱 스크롤이 일어났을 때 콘솔에 -값을 찍히지 않게 방지
            currentScene--;
            document.body.setAttribute('id', `show-scene-${currentScene}`);
        }

        if (enterNewScene) return;

        playAnimation();
    }

    window.addEventListener('scroll', () => {
        yOffset = window.scrollY;  // window.pageYOffset은 deprecated 되었다.
        scrollLoop()
    });
    // window.addEventListener('DOMContentLoaded', setLayout)
    window.addEventListener('load', setLayout)
    window.addEventListener('resize', setLayout) // 화면 크기가 바뀌었을때를 기준으로 setLayout을 정의하기

})();

main.css

@charset "utf-8";
html {
    font-family: 'Noto Sans KR' , sans-serif;
    font-size: 14px;
}
body {
    overflow-x: hidden;
    color: rgb(29, 29, 31);
    letter-spacing: -0.05em;
    background: white;
}
p {
    line-height: 1.6;
}
a {
    color: rgb(29, 29, 31);
    text-decoration: none;
}

.global-nav {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 44px;
    padding: 0 1rem;
}
.local-nav {
    position: absolute;
    top: 45px;
    left: 0;
    width: 100%;
    height: 52px;
    padding: 0 1rem;
    border-bottom: 1px solid #ddd;
}
.global-nav-links,
.local-nav-links {
    display: flex;
    align-items: center;
    max-width: 1000px;
    height: 100%;
    margin: 0 auto;
    /* background-color: gold; */
}
.global-nav-links {
    justify-content: space-between;
}
.local-nav-links .product-name {
    margin-right: auto;
    font-size: 1.4rem;
    font-weight: bold;
}
.local-nav-links a {
    font-size: 0.8rem;
}
.local-nav-links a:not(.product-name) {
    margin-left: 2em;
}
.scroll-section {
    border: 3px solid red;
    padding-top: 50vh;
}
#scroll-section-0 h1 {
    font-size: 4rem;
    text-align: center;
}
.main-message {
    display: flex;
    align-items: center;
    justify-content: center;
    top: 35vh;
    margin: 5px 0;
    height: 3em;
    font-size: 2.5rem;
    opacity: 0;
}
.main-message p {
    font-weight: bold;
    text-align: center;
    line-height: 1.2;
}
.main-message small {
    display: block;
    margin-bottom: 0.5em;
    font-size: 1.2rem;
}
#scroll-section-2 .main-message {
    font-size: 3.5rem;
}
.description {
    max-width: 1000px;
    margin: 0 auto;
    padding: 0 1rem;
    font-size: 1.2rem;
    color: #888
}
.description strong {
    float: left;
    margin-right: 0.2em;
    /* font-size: 3rem; */
    color: rgb(29, 29, 31);
}
.desc-message {
    width: 50%;
    font-weight: bold;
}
.pin {
    width: 1px;
    height: 100px;
    background: rgb(29,29,31);
}
#scroll-section-2 .b {
    top: 10%;
    left: 40%;
}
#scroll-section-2 .c {
    top: 15%;
    left: 45%;
}
.mid-message {
    max-width: 1000px;
    margin: 0 auto;
    padding: 0 1rem;
    font-size: 2rem;
    color: #888;
}
.mid-message strong {
    color: rgb(29,29,31);
}
.canvas-caption {
    max-width: 1000px;
    margin: 0 auto;
    padding: 0 1rem;
    font-size: 1.2rem;
    color: #888;
}
.footer {
    display: flex;
    align-items: center;
    justify-content: center;
    height: 7rem;
    color: white;
    background: darkorange;
}
.sticky-elem {
    display: none;
    position: fixed;
    left: 0;
    width: 100%;
}
body#show-scene-0 #scroll-section-0 .sticky-elem,
body#show-scene-1 #scroll-section-1 .sticky-elem,
body#show-scene-2 #scroll-section-2 .sticky-elem,
body#show-scene-3 #scroll-section-3 .sticky-elem {
    display: block;
}

@media(min-width: 1024px) {
    #scroll-section-0 h1 {
        font-size: 9vw;
    }
    .main-message {
        font-size: 4vw;
    }
    .description {
        font-size: 2rem;
    }
    .description strong {
        font-size: 6rem;
    }
    #scroll-section-2 .main-message {
        font-size: 6vw;
    }
    .main-message small {
        font-size: 1.5vw;
    }
    .desc-message {
        width: 20%;
    }
    #scroll-section-2 .b {
        top: 20%;
        left: 53%;
    }
    #scroll-section-2 .c {
        left: 55%;
    }
    .mid-message {
        font-size: 4vw;
    }
    .canvas-caption {
        font-size: 2rem;
    }
}

blank.html
<!doctype html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0"> <meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>AirMug Pro</title> <link href="https://fonts.googleapis.com/css2?family=Noto+Sans+KR:wght@400;900&display=swap" rel="stylesheet"> <link rel="stylesheet" href="css/default.css"> <link rel="stylesheet" href="css/main.css"> </head> <body> <div class="container"> <nav class="global-nav"> <div class="global-nav-links"> <a href="" class="global-nav-item">Rooms</a> <a href="" class="global-nav-item">Ideas</a> <a href="" class="global-nav-item">Stores</a> <a href="" class="global-nav-item">Contact</a> </div> </nav> <nav class="local-nav"> <div class="local-nav-links"> <a href="#" class="product-name">AirMug Pro</a> <a href="#">개요</a> <a href="#">제품사양</a> <a href="#">구입하기</a> </div> </nav> <section class="scroll-section" id="scroll-section-0"> <h1>AirMug Pro</h1> <div class="sticky-elem main-message a"> <p>온전히 빠져들게 하는<br>최고급 세라믹</p> </div> <div class="sticky-elem main-message b"> <p>주변 맛을 느끼게 해주는<br>주변 맛 허용 모드</p> </div> <div class="sticky-elem main-message c"> <p>온전히 편안한<br>맞춤형 손잡이</p> </div> <div class="sticky-elem main-message d"> <p>새롭게 입가를<br>찾아온 매혹</p> </div> </section> <section class="scroll-section" id="scroll-section-1"> <p class="description"> <strong>보통 스크롤 영역</strong> Lorem, ipsum dolor sit amet consectetur adipisicing elit. Dolore explicabo commodi facere nisi expedita eaque quasi doloremque eligendi natus architecto, facilis velit nobis voluptatum suscipit autem soluta debitis neque illum quibusdam similique odio maiores. Ab optio laborum eum sit praesentium, aperiam voluptatem nihil modi tempore a magni necessitatibus hic exercitationem temporibus doloremque error omnis. Sequi, repudiandae dolor eaque non soluta praesentium doloremque, doloribus, ea consequuntur sit perferendis ex! Repellendus natus harum soluta dolorem voluptas alias qui, ipsa vero! Alias voluptate libero, facilis molestias nam aspernatur, amet accusantium consequatur a nostrum temporibus. Qui obcaecati optio quod iusto totam, aut iste dolor ea. Explicabo dolorem ducimus natus, officiis aut minima, ad sapiente voluptatem repellat aliquid suscipit at. Architecto nostrum perferendis, fugiat velit perspiciatis illum dignissimos, vitae delectus magnam blanditiis omnis ratione nihil minus deserunt repudiandae necessitatibus et, tempore cupiditate sapiente? Debitis obcaecati repellendus corrupti odit incidunt deleniti exercitationem dolorum ipsum consequatur rerum maxime, corporis placeat deserunt veniam cum qui dolorem nulla fugit eius ipsam quibusdam saepe! Aut culpa excepturi perferendis, sint animi tempora. Officia necessitatibus libero impedit alias commodi excepturi accusamus molestiae modi laudantium voluptates totam earum autem eos numquam at consequuntur ipsam repellat, perspiciatis veniam? Expedita nobis ducimus quisquam accusantium! Velit! </p> </section> <section class="scroll-section" id="scroll-section-2"> <div class="sticky-elem main-message a"> <p> <small>편안한 촉감</small> 입과 하나 되다 </p> </div> <div class="sticky-elem desc-message b"> <p> 편안한 목넘김을 완성하는 디테일한 여러 구성 요소들, 우리는 이를 하나하나 새롭게 살피고 재구성하는 과정을 거쳐 새로운 수준의 머그, AirMug Pro를 만들었습니다. 입에 뭔가 댔다는 감각은 어느새 사라지고 오롯이 당신과 음료만 남게 되죠. </p> <div class="pin"></div> </div> <div class="sticky-elem desc-message c"> <p> 디자인 앤 퀄리티 오브 스웨덴,<br>메이드 인 차이나 </p> <div class="pin"></div> </div> </section> <section class="scroll-section" id="scroll-section-3"> <p class="mid-message"> <strong>Retina 머그</strong> 아이디어를 광할하게 펼칠<br> 아름답고 부드러운 음료 공간. </p> <p class="canvas-caption"> Lorem ipsum dolor sit amet consectetur adipisicing elit. Expedita nulla nesciunt tempora asperiores culpa, illo et assumenda vel odio voluptatem? Saepe rerum qui accusamus maiores ex libero natus obcaecati accusantium aperiam eligendi ut et facere quis molestias quos repudiandae, omnis facilis deserunt cum! Expedita ipsam aliquid beatae doloremque, officia explicabo non neque reprehenderit qui quo, recusandae incidunt saepe totam! Adipisci dolorem inventore soluta. Natus pariatur sit quisquam dicta placeat! Molestias voluptas eos asperiores maxime, dolorem corporis eum quos iste dolores eveniet, cumque officiis beatae adipisci! Corporis quibusdam voluptates explicabo officiis quis cupiditate qui officia expedita. Praesentium placeat debitis recusandae ipsa similique optio accusamus, omnis vero a ducimus blanditiis fugit asperiores maiores mollitia? Pariatur exercitationem culpa, fugiat accusantium soluta incidunt beatae debitis laborum, harum neque aliquam, asperiores quod! Error numquam voluptate aut corrupti nam! Iusto illum accusamus sapiente asperiores quo eum perspiciatis aliquam blanditiis dolorem natus, modi magnam molestiae accusantium porro quisquam iure tempora laborum deserunt recusandae omnis dolore eos pariatur eveniet. Exercitationem mollitia inventore at quasi aperiam blanditiis minima accusantium, delectus, possimus quidem labore sapiente eius sint molestias doloribus est autem cupiditate iure veniam optio! Placeat dolor et, eos, saepe nemo tempore aliquid alias, facere pariatur optio sint nam ea. </p> </section> <footer class="footer"> 2023-08-01 Jun </footer> </div> <script src="js/main.js"></script> </body> </html>