TOC Active Highlight

<!-- Nav dots light up as their matching section scrolls into view -->
<!-- Uses view-timeline-name on each section + timeline-scope on the
     shared ancestor so a nav dot outside the section can reference it -->
<nav class="nuda-sd2-toc__nav">
  <span class="nuda-sd2-toc__item">Intro<span class="nuda-sd2-toc__dot nuda-sd2-toc__dot--1"></span></span>
  <span class="nuda-sd2-toc__item">Specs<span class="nuda-sd2-toc__dot nuda-sd2-toc__dot--2"></span></span>
  <span class="nuda-sd2-toc__item">Notes<span class="nuda-sd2-toc__dot nuda-sd2-toc__dot--3"></span></span>
</nav>
<section class="nuda-sd2-toc__section nuda-sd2-toc__section--1">…</section>
<section class="nuda-sd2-toc__section nuda-sd2-toc__section--2">…</section>
<section class="nuda-sd2-toc__section nuda-sd2-toc__section--3">…</section>

.nuda-sd2-toc {
  height: 160px;
  border: 1px solid rgba(255,255,255,.08);
  border-radius: 10px;
  background: #09090b;
  overflow: hidden;
}

.nuda-sd2-toc__scroller {
  height: 100%;
  overflow: auto;
  timeline-scope: --sd2sec1,--sd2sec2,--sd2sec3;
}

.nuda-sd2-toc__nav {
  position: sticky;
  top: 0;
  z-index: 2;
  display: flex;
  gap: .3rem;
  padding: .5rem .4rem;
  background: #09090b;
  border-bottom: 1px solid rgba(255,255,255,.08);
}

.nuda-sd2-toc__item {
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: .3rem;
  color: #a1a1aa;
  font-size: .62rem;
  text-transform: uppercase;
  letter-spacing: .04em;
}

.nuda-sd2-toc__dot {
  width: 16px;
  height: 3px;
  border-radius: 2px;
  background: #e4ff54;
  opacity: 0;
  transform: scaleX(.3);
  animation-timing-function: linear;
  animation-fill-mode: both;
}

.nuda-sd2-toc__dot--1 {
  animation-name: nuda-sd2-toc-dot;
  animation-timeline: --sd2sec1;
}

.nuda-sd2-toc__dot--2 {
  animation-name: nuda-sd2-toc-dot;
  animation-timeline: --sd2sec2;
}

.nuda-sd2-toc__dot--3 {
  animation-name: nuda-sd2-toc-dot;
  animation-timeline: --sd2sec3;
}

.nuda-sd2-toc__section {
  padding: 1.6rem 1rem;
  color: #a1a1aa;
  font-size: .75rem;
  line-height: 1.6;
}

.nuda-sd2-toc__section p {
  margin: 0;
}

.nuda-sd2-toc__section--1 {
  view-timeline-name: --sd2sec1;
}

.nuda-sd2-toc__section--2 {
  view-timeline-name: --sd2sec2;
}

.nuda-sd2-toc__section--3 {
  view-timeline-name: --sd2sec3;
}

@keyframes nuda-sd2-toc-dot {
  0%,100% {
    opacity: 0;
    transform: scaleX(.3);
  }
  40%,60% {
    opacity: 1;
    transform: scaleX(1);
  }
}

@supports not (animation-timeline:view()) {
  .nuda-sd2-toc__dot--1 {
    opacity: 1;
    transform: scaleX(1);
  }
}

@media (prefers-reduced-motion:reduce) {
  .nuda-sd2-toc__dot {
    animation: none;
    animation-timeline: none;
    opacity: 0;
    transform: scaleX(.3);
  }
  .nuda-sd2-toc__dot--1 {
    opacity: 1;
    transform: scaleX(1);
  }
}