const currentSection = ref()
onMounted(() => {
  const observer = new IntersectionObserver((entries) => {
    entries.forEach((entry) => {
        if(entry.intersectionRatio > 0) {
          currentSection.value = entry.target.getAttribute('id')
        }
      })
     document.querySelectorAll('article h2').forEach((section) => {
        observer.observe(section)
     })
})

then add a class to the active currentSection.