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HTML

              
                <body>
  <nav id="toc">
  </nav>
  <main>
    <h1>New Horizons</h1>
    <p>
      Live life at your own pace as you garden, fish, decorate, hunt for bugs and fossils, get to know the animal residents, and more. The time of day and seasons match real life, so something is happening on your island whether you’re there or not.
    </p>
    <p>Oh--just watch out for those shifty racoons.</p>
    <h2>Tools</h2>
    <p>New Horizons has all sorts of tools that you can use to your disposal!</p>
    <h3>Shovel</h3>
    <p>This tool's versatility knows no bounds! Hole digging, rock smacking, tree uprooting goodness!</p>
    <h3>Bug Net</h3>
    <p>Catch some bugs--if you are into that kind of thing.</p>
    <h3>Slingshot</h3>
    <p>Make presents fall from the sky! Just watch out for the river!</p>
    <h3>Fishing Rod</h3>
    <p>After all that shovel work its nice to take a load of, craft a nice coconut juice, and fish.</p>
    <h3>Ladder</h3>
    <p>For those annoying cliffs.</p>
    <h3>Axe</h3>
    <p>For choppin' trees--just watch out for them bees!</p>
    <h3>Vaulting Pole</h3>
    <p>For those like me, that are too poor to afford a bridge.</p>
    <h3>Watering Can</h3>
    <p>Those flowers won't crossbreed themselves.</p>
    <h2>Flower Crossbreeding</h2>
    <h3>Lilies</h3>
    <ul>
      <li>Red + Red = Black</li>
      <li>Yellow + Red = Orange</li>
      <li>Red + White = Pink</li>
    </ul>
    <h3>Roses</h3>
    <ul>
      <li>Red + Red = Black</li>
      <li>Red + White = Pink</li>
      <li>Yellow + Red = Orange</li>
      <li>White + White = Purple</li>
    </ul>
    <h3>Mums</h3>
    <ul>
      <li>White + Red = Pink</li>
      <li>White + White = Purple</li>
      <li>Purple + Purple = Green</li>
    </ul>
    <h3>Pansies</h3>
    <ul>
      <li>Red + Red = Purple</li>
      <li>White + White = Blue</li>
      <li>Yellow + Red = Orange</li>
      <li>Blue + Blue = Purple</li>
      <li>Red + Red = Purple</li>
    </ul>
    <h3>Windflower</h3>
    <ul>
      <li>Orange + Red = Pink</li>
      <li>Orange + White = Blue</li>
      <li>White + White = Blue</li>
      <li>Blue + Pink = Purple</li>
      <li>Blue + Blue = Purple</li>
    </ul>
    <h3>Hyacinths</h3>
    <ul>
      <li>Red + White = Pink</li>
      <li>White + White = Blue</li>
      <li>Red + Yellow = Orange</li>
      <li>Blue + Blue = Purple</li>
    </ul>
    <h3>Cosmos</h3>
    <ul>
      <li>Yellow + Red = Orange</li>
      <li>Red + White = Pink</li>
      <li>Red + Red = Black</li>
    </ul>
    <h3>Tulips</h3>
    <ul>
      <li>Red + Red = Black</li>
      <li>Red + Yellow = Orange</li>
      <li>Red + White = Pink</li>
      <li>Black + Black = Purple</li>
    </ul>
  </main>
</body>
              
            
!

CSS

              
                body {
  display: flex;
  flex-flow: space-between;
  height: 100vh;
  font-family: "Rubik";
  margin: 0;
  color: #333344;
  line-height: 1.53em;
  font-size: 1.2rem;
  background-color: #efeeff;
}

h1,
h2,
h3,
h4,
h5,
h6 {
  font-family: "Rubik Mono One";
  font-weight: 300;
  margin-top: 3rem;
  color: #484866;
}

main {
  flex: 3 3 100%;
  overflow: auto;
  padding: 2rem 3rem;
}

nav {
  flex: 1 1 300px;
  background-color: #333344;
  min-width: 240px;
  paddings: 1rem;
  overflow-y: auto;
}

h1,
h2,
p {
  max-width: 700px;
}

a {
  color: #eeeeff;
  text-decoration: none;
}

.active {
  color: #ff8888;
  font-weight: bold;
}

ul {
  list-style: none;
  padding-left: 1.25rem;
}

ul li::before {
  content: "\2022";
  color: #eeeeff;
  font-weight: bold;
  display: inline-block;
  width: 1em;
  margin-left: -1em;
}

              
            
!

JS

              
                let tocId = "toc";

let headings;
let headingIds = [];
let headingIntersectionData = {};
let headerObserver;

function setLinkActive(link) {
  const links = document.querySelectorAll(`#${tocId} a`);
  links.forEach((link) => link.classList.remove("active"));
  if (link) {
    link.classList.add("active");
  }
}

function getProperListSection(heading, previousHeading, currentListElement) {
  let listSection = currentListElement;
  if (previousHeading) {
    if (heading.tagName.slice(-1) > previousHeading.tagName.slice(-1)) {
      let nextSection = document.createElement("ul");
      listSection.appendChild(nextSection);
      return nextSection;
    } else if (heading.tagName.slice(-1) < previousHeading.tagName.slice(-1)) {
      let indentationDiff =
        parseInt(previousHeading.tagName.slice(-1)) -
        parseInt(heading.tagName.slice(-1));
      while (indentationDiff > 0) {
        listSection = listSection.parentElement;
        indentationDiff--;
      }
    }
  }
  return listSection;
}

function setIdFromContent(element, appendedId) {
  if (!element.id) {
    element.id = `${element.innerHTML
      .replace(/:/g, "")
      .trim()
      .toLowerCase()
      .split(" ")
      .join("-")}-${appendedId}`;
  }
}

function addNavigationLinkForHeading(heading, currentSectionList) {
  let listItem = document.createElement("li");
  let anchor = document.createElement("a");
  anchor.innerHTML = heading.innerHTML;
  anchor.id = `${heading.id}-link`;
  anchor.href = `#${heading.id}`;
  anchor.onclick = (e) => {
    setTimeout(() => {
      setLinkActive(anchor);
    });
  };
  listItem.appendChild(anchor);
  currentSectionList.appendChild(listItem);
}

function buildTableOfContentsFromHeadings() {
  const tocElement = document.querySelector(`#${tocId}`);
  const main = document.querySelector("main");
  if (!main) {
    throw Error("A `main` tag section is required to query headings from.");
  }
  headings = main.querySelectorAll("h1, h2, h3, h4, h5, h6");
  let previousHeading;
  let currentSectionList = document.createElement("ul");
  tocElement.appendChild(currentSectionList);

  headings.forEach((heading, index) => {
    currentSectionList = getProperListSection(
      heading,
      previousHeading,
      currentSectionList
    );
    setIdFromContent(heading, index);
    addNavigationLinkForHeading(heading, currentSectionList);

    headingIds.push(heading.id);
    headingIntersectionData[heading.id] = {
      y: 0
    };
    previousHeading = heading;
  });
}

function updateActiveHeadingOnIntersection(entry) {
  const previousY = headingIntersectionData[entry.target.id].y;
  const currentY = entry.boundingClientRect.y;
  const id = `#${entry.target.id}`;
  const link = document.querySelector(id + "-link");
  const index = headingIds.indexOf(entry.target.id);

  if (entry.isIntersecting) {
    if (currentY > previousY && index !== 0) {
      console.log(id + ":1 enter top");
    } else {
      console.log(id + ":2 enter bottom");
      setLinkActive(link);
    }
  } else {
    if (currentY > previousY) {
      console.log(id + ":3 leave bottom");
      const lastLink = document.querySelector(`#${headingIds[index - 1]}-link`);
      setLinkActive(lastLink);
    } else {
      console.log(id + ":4 leave top");
    }
  }

  headingIntersectionData[entry.target.id].y = currentY;
}

function observeHeadings() {
  let options = {
    root: document.querySelector("main"),
    threshold: 0.1
  };
  headerObserver = new IntersectionObserver(
    (entries) => entries.forEach(updateActiveHeadingOnIntersection),
    options
  );
  Array.from(headings)
    .reverse()
    .forEach((heading) => headerObserver.observe(heading));
}

window.addEventListener("load", (event) => {
  buildTableOfContentsFromHeadings();
  if ("IntersectionObserver" in window) {
    observeHeadings();
  }
});

window.addEventListener("unload", (event) => {
  headerObserver.disconnect();
});

              
            
!
999px

Console