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HTML

              
                <!DOCTYPE html>
<html lang="ja">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>floating-item/smooth-scroll</title>
  <link rel="stylesheet" href="style.css">
</head>

<body>
  <header class="l-header" id="js-header">
    <div class="l-inner">
      <p>リンクを押すと対象のセクションへスムーススクロールします</p>
      <nav class="p-header__nav">
        <ul class="p-header__list" id="js-global-nav-list">
          <li class="p-header__item"><a href="#section1">section1</a></li>
          <li class="p-header__item"><a href="#section2">section2</a></li>
          <li class="p-header__item"><a href="#section3">section3</a></li>
          <li class="p-header__item"><a href="#section4">section4</a></li>
          <li class="p-header__item"><a href="#section5">section5</a></li>
        </ul>
      </nav>
    </div>
  </header>

  <main id="observerArea">
    <section class="p-section1 l-section" id="section1">
      <div class="l-inner">
        <h2 class="p-section__title">セクション1</h2>
      </div>
    </section>

    <section class="p-section2 l-section" id="section2">
      <div class="l-inner">
        <h2 class="p-section__title">セクション2</h2>
      </div>
    </section>

    <section class="p-section3 l-section" id="section3">
      <div class="l-inner">
        <h2 class="p-section__title">セクション3</h2>
      </div>
    </section>

    <section class="p-section4 l-section" id="section4">
      <div class="l-inner">
        <h2 class="p-section__title">セクション4</h2>
      </div>
    </section>

    <section class="p-section5 l-section" id="section5">
      <div class="l-inner">
        <h2 class="p-section__title">セクション5</h2>
      </div>
    </section>

    <a href="#" class="p-topup" id="topup">TOPへ戻る</a>
    <script src="index.js"></script>

  </main>
</body>

</html>

              
            
!

CSS

              
                .l-header {
  padding: 20px;
  height: 100px;
  text-align: center;
  background-color: #333;
  color: #FFF;
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
}

.l-inner {
  max-width: 1000px;
  margin: 0 auto;
}

.p-header__nav {
  margin-top: 10px;
}

.p-header__list {
  display: flex;
  justify-content: center;
}


.p-header__item {
  font-size: 20px;
}

.p-section1 {
  margin-top: 100px;
}


.p-header__item:not(:last-child) {
  margin-right: 20px;
}


.p-header__item > a {
  display: block;
  padding: 10px;
  transition: .3s;
}


.p-header__item > a:hover {
  background-color: #FFF;
  color: #333;
  transition: .3s;
}


.l-section {
  padding: 200px 0;
  border-top: 1px solid #333;
  text-align: center;
}

.p-section__title {
  font-size: 20px;
}

.p-section2,
.p-section4 {
  background-color: #E0E0E0;
}

.p-section5 {
  border-bottom: 1px solid #333;

}

.p-topup {
  display: inline-block;
  background-color: #FFCCCC;
  padding: 30px;
  border-radius: 50px;
  position: fixed;
  bottom: 10px;
  right: 20px;
  opacity: 0;
  transition: opacity .3s;
}
.p-topup.is-show {
  transition: opacity .3s;
  opacity: 1;
}

/* reset.css */
/*! destyle.css v2.0.2 | MIT License | https://github.com/nicolas-cusan/destyle.css */

/* Reset box-model and set borders */
/* ============================================ */

*,
::before,
::after {
  box-sizing: border-box;
  border-style: solid;
  border-width: 0;
}

/* Document */
/* ============================================ */

/**
 * 1. Correct the line height in all browsers.
 * 2. Prevent adjustments of font size after orientation changes in iOS.
 * 3. Remove gray overlay on links for iOS.
 */

html {
  line-height: 1.15; /* 1 */
  -webkit-text-size-adjust: 100%; /* 2 */
  -webkit-tap-highlight-color: transparent; /* 3*/
}

/* Sections */
/* ============================================ */

/**
 * Remove the margin in all browsers.
 */

body {
  margin: 0;
}

/**
 * Render the `main` element consistently in IE.
 */

main {
  display: block;
}

/* Vertical rhythm */
/* ============================================ */

p,
table,
blockquote,
address,
pre,
iframe,
form,
figure,
dl {
  margin: 0;
}

/* Headings */
/* ============================================ */

h1,
h2,
h3,
h4,
h5,
h6 {
  font-size: inherit;
  line-height: inherit;
  font-weight: inherit;
  margin: 0;
}

/* Lists (enumeration) */
/* ============================================ */

ul,
ol {
  margin: 0;
  padding: 0;
  list-style: none;
}

/* Lists (definition) */
/* ============================================ */

dt {
  font-weight: bold;
}

dd {
  margin-left: 0;
}

/* Grouping content */
/* ============================================ */

/**
 * 1. Add the correct box sizing in Firefox.
 * 2. Show the overflow in Edge and IE.
 */

hr {
  box-sizing: content-box; /* 1 */
  height: 0; /* 1 */
  overflow: visible; /* 2 */
  border-top-width: 1px;
  margin: 0;
  clear: both;
  color: inherit;
}

/**
 * 1. Correct the inheritance and scaling of font size in all browsers.
 * 2. Correct the odd `em` font sizing in all browsers.
 */

pre {
  font-family: monospace, monospace; /* 1 */
  font-size: inherit; /* 2 */
}

address {
  font-style: inherit;
}

/* Text-level semantics */
/* ============================================ */

/**
 * Remove the gray background on active links in IE 10.
 */

a {
  background-color: transparent;
  text-decoration: none;
  color: inherit;
}

/**
 * 1. Remove the bottom border in Chrome 57-
 * 2. Add the correct text decoration in Chrome, Edge, IE, Opera, and Safari.
 */

abbr[title] {
  text-decoration: underline; /* 2 */
  text-decoration: underline dotted; /* 2 */
}

/**
 * Add the correct font weight in Chrome, Edge, and Safari.
 */

b,
strong {
  font-weight: bolder;
}

/**
 * 1. Correct the inheritance and scaling of font size in all browsers.
 * 2. Correct the odd `em` font sizing in all browsers.
 */

code,
kbd,
samp {
  font-family: monospace, monospace; /* 1 */
  font-size: inherit; /* 2 */
}

/**
 * Add the correct font size in all browsers.
 */

small {
  font-size: 80%;
}

/**
 * Prevent `sub` and `sup` elements from affecting the line height in
 * all browsers.
 */

sub,
sup {
  font-size: 75%;
  line-height: 0;
  position: relative;
  vertical-align: baseline;
}

sub {
  bottom: -0.25em;
}

sup {
  top: -0.5em;
}

/* Embedded content */
/* ============================================ */

/**
 * Prevent vertical alignment issues.
 */

img,
embed,
object,
iframe {
  vertical-align: bottom;
}

/* Forms */
/* ============================================ */

/**
 * Reset form fields to make them styleable
 */

button,
input,
optgroup,
select,
textarea {
  -webkit-appearance: none;
  appearance: none;
  vertical-align: middle;
  color: inherit;
  font: inherit;
  background: transparent;
  padding: 0;
  margin: 0;
  outline: 0;
  border-radius: 0;
  text-align: inherit;
}

/**
 * Reset radio and checkbox appearance to preserve their look in iOS.
 */

[type="checkbox"] {
  -webkit-appearance: checkbox;
  appearance: checkbox;
}

[type="radio"] {
  -webkit-appearance: radio;
  appearance: radio;
}

/**
 * Show the overflow in IE.
 * 1. Show the overflow in Edge.
 */

button,
input {
  /* 1 */
  overflow: visible;
}

/**
 * Remove the inheritance of text transform in Edge, Firefox, and IE.
 * 1. Remove the inheritance of text transform in Firefox.
 */

button,
select {
  /* 1 */
  text-transform: none;
}

/**
 * Correct the inability to style clickable types in iOS and Safari.
 */

button,
[type="button"],
[type="reset"],
[type="submit"] {
  cursor: pointer;
  -webkit-appearance: none;
  appearance: none;
}

button[disabled],
[type="button"][disabled],
[type="reset"][disabled],
[type="submit"][disabled] {
  cursor: default;
}

/**
 * Remove the inner border and padding in Firefox.
 */

button::-moz-focus-inner,
[type="button"]::-moz-focus-inner,
[type="reset"]::-moz-focus-inner,
[type="submit"]::-moz-focus-inner {
  border-style: none;
  padding: 0;
}

/**
 * Restore the focus styles unset by the previous rule.
 */

button:-moz-focusring,
[type="button"]:-moz-focusring,
[type="reset"]:-moz-focusring,
[type="submit"]:-moz-focusring {
  outline: 1px dotted ButtonText;
}

/**
 * Remove arrow in IE10 & IE11
 */

select::-ms-expand {
  display: none;
}

/**
 * Remove padding
 */

option {
  padding: 0;
}

/**
 * Reset to invisible
 */

fieldset {
  margin: 0;
  padding: 0;
  min-width: 0;
}

/**
 * 1. Correct the text wrapping in Edge and IE.
 * 2. Correct the color inheritance from `fieldset` elements in IE.
 * 3. Remove the padding so developers are not caught out when they zero out
 *    `fieldset` elements in all browsers.
 */

legend {
  color: inherit; /* 2 */
  display: table; /* 1 */
  max-width: 100%; /* 1 */
  padding: 0; /* 3 */
  white-space: normal; /* 1 */
}

/**
 * Add the correct vertical alignment in Chrome, Firefox, and Opera.
 */

progress {
  vertical-align: baseline;
}

/**
 * Remove the default vertical scrollbar in IE 10+.
 */

textarea {
  overflow: auto;
}

/**
 * Correct the cursor style of increment and decrement buttons in Chrome.
 */

[type="number"]::-webkit-inner-spin-button,
[type="number"]::-webkit-outer-spin-button {
  height: auto;
}

/**
 * 1. Correct the outline style in Safari.
 */

[type="search"] {
  outline-offset: -2px; /* 1 */
}

/**
 * Remove the inner padding in Chrome and Safari on macOS.
 */

[type="search"]::-webkit-search-decoration {
  -webkit-appearance: none;
}

/**
 * 1. Correct the inability to style clickable types in iOS and Safari.
 * 2. Change font properties to `inherit` in Safari.
 */

::-webkit-file-upload-button {
  -webkit-appearance: button; /* 1 */
  font: inherit; /* 2 */
}

/**
 * Clickable labels
 */

label[for] {
  cursor: pointer;
}

/* Interactive */
/* ============================================ */

/*
 * Add the correct display in Edge, IE 10+, and Firefox.
 */

details {
  display: block;
}

/*
 * Add the correct display in all browsers.
 */

summary {
  display: list-item;
}

/*
 * Remove outline for editable content.
 */

[contenteditable] {
  outline: none;
}

/* Table */
/* ============================================ */

table {
  border-collapse: collapse;
  border-spacing: 0;
}

caption {
  text-align: left;
}

td,
th {
  vertical-align: top;
  padding: 0;
}

th {
  text-align: left;
  font-weight: bold;
}

/* Misc */
/* ============================================ */

/**
 * Add the correct display in IE 10+.
 */

template {
  display: none;
}

/**
 * Add the correct display in IE 10.
 */

[hidden] {
  display: none;
}

              
            
!

JS

              
                const globalNav = document.getElementById("js-global-nav-list");
globalNav.addEventListener("click", function (e) {
  e.preventDefault();
  const globalNavItem = e.target;
  const headerHeight = document.getElementById("js-header").clientHeight;
  const href = globalNavItem.getAttribute("href");
  const sectionName = href.replace("#", "");
  const sectionId = document.getElementById(sectionName);
  const rect = sectionId.getBoundingClientRect().top;
  const offset = window.pageYOffset;
  const target = rect + offset - headerHeight;
  window.scrollTo({
    top: target,
    behavior: "smooth"
  });
});

const topUp = document.getElementById("topup");
topUp.addEventListener("click", function (e) {
  e.preventDefault();
  window.scrollTo({
    top: 0,
    behavior: "smooth"
  });
});

const callback = (entries) => {
  entries.forEach((entry) => {
    if (entry.boundingClientRect.top < 0) {
      topUp.classList.remove("is-show");
      if (entry.isIntersecting) {
        topUp.classList.remove("is-show");
      } else {
        topUp.classList.add("is-show");
      }
    }
  });
};
const observer = new IntersectionObserver(callback);
observer.observe(document.getElementById("section2"));

              
            
!
999px

Console