Scrollspy Vanilla Vertical Menu Overflow-Y:scroll menu

Question

I'm doing something with Scrollspy Vertical Menu. But I ran into a problem and couldn't find a solution. when I scroll, I want it to follow the nav on my sidebar slider. Because I can't see what part he's in when there's a lot of sections. how can I solve this? I tried bootstrap's scrollspy component, but it didn't work there, I wonder if anyone has a better solution?

https://codepen.io/lonelylonely/pen/mdxZBRM

const sections = document.querySelectorAll("section");
const navLinks = document.querySelectorAll("nav a");
const nav = document.querySelector("nav");
const lastElHeight =
  sections[sections.length - 1].getBoundingClientRect().height + 300;

window.onscroll = (el) => {
  sections.forEach((section) => {
    let top = window.scrollY;
    let navHeight = nav.offsetHeight;
    let offset = section.offsetTop;
    let height = section.offsetHeight;
    let id = section.getAttribute("id");

    if (top >= offset && top < offset + height) {
      navLinks.forEach((links) => {
        links.classList.remove("active");
        document.querySelector(`nav a[href*="${id}"]`).classList.add("active");
      });
    }

    if (
      sections[sections.length - 1].getBoundingClientRect().top < lastElHeight
    ) {
      navLinks.forEach((links) => {
        links.classList.remove("active");
        document.querySelector(`nav a[href*="${id}"]`).classList.add("active");
      });
    }
  });
};

<nav style="height: 200px; overflow-x: scroll;">
  <a href="#section-1" class="active">Section 1</a>
  <a href="#section-2">Section 2</a>
  <a href="#section-3">Section 3</a>
  <a href="#section-4">Section 4</a>
  <a href="#section-5">Section 5</a>
  <a href="#section-6">Section 6</a>
  <a href="#section-7">Section 7</a>
  <a href="#section-8">Section 8</a>
  <a href="#section-9">Section 9</a>
  <a href="#section-10">Section 10</a>
</nav>

<div class="section-container" tabindex="0">
  <section id="section-1">Section 1 <p>(200px height)</p>
  </section>
  <section id="section-2">Section 2 <p>(800px height)</p>
  </section>
  <section id="section-3">Section 3 <p>(400px height)</p>
  </section>
  <section id="section-4">Section 4 <p>(1200px height)</p>
  </section>
  <section id="section-5">Section 5 <p>(600px height)</p>
  </section>
  <section id="section-6">Section 6 <p>(600px height)</p>
  </section>
  <section id="section-7">Section 7 <p>(600px height)</p>
  </section>
  <section id="section-8">Section 8 <p>(600px height)</p>
  </section>
  <section id="section-9">Section 9 <p>(600px height)</p>
  </section>
  <section id="section-10">Section 10 <p>(600px height)</p>
  </section>
</div>

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  display: flex;
  position: relative;
}

html {
  scroll-behavior: smooth;
  overflow-y: scroll;
  height: 100vh;
}

nav {
  display: flex;
  position: fixed;
  flex-direction: column;
  top: 0;
  width: 20%;
}

nav a {
  padding: 10px 25px;
  color: black;
  font-size: 20px;
  text-transform: capitalize;
  background-color: lightgray;
  font-family: sans-serif;
  text-align: center;
  text-decoration: none;
}

nav a.active,
nav a:hover {
  background-color: darkgray;
}

section {
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 800;
  text-transform: uppercase;
  font-size: 7rem;
  font-family: sans-serif;
  border: 1px solid gray;
  flex-direction: column;
}

section p {
  font-size: 1rem;
  font-weight: 300;
}

section:nth-child(odd) {
  background-color: #eee;
}

.section-container {
  width: 80%;
  margin-left: auto;
}

#section-1 {
  height: 200px;
}

#section-2 {
  height: 800px;
}

#section-3 {
  height: 400px;
}

#section-4 {
  height: 1200px;
}

#section-5 {
  height: 600px;
}

#section-6 {
  height: 600px;
}

#section-7 {
  height: 600px;
}

#section-8 {
  height: 600px;
}

#section-9 {
  height: 600px;
}

#section-10 {
  height: 600px;
}