Function checkVisibleSections

src/components/SectionProvider.tsx:70–109

()

  useEffect(() => {
    function checkVisibleSections() {
      let { innerHeight, scrollY } = window
      let newVisibleSections = []

      for (
        let sectionIndex = 0;
        sectionIndex < sections.length;
        sectionIndex++
      ) {
        let { id, headingRef, offsetRem = 0 } = sections[sectionIndex]

        if (!headingRef?.current) {
          continue
        }

        let offset = remToPx(offsetRem)
        let top = headingRef.current.getBoundingClientRect().top + scrollY

        if (sectionIndex === 0 && top - offset > scrollY) {
          newVisibleSections.push('_top')
        }

        let nextSection = sections[sectionIndex + 1]
        let bottom =
          (nextSection?.headingRef?.current?.getBoundingClientRect().top ??
            Infinity) +
          scrollY -
          remToPx(nextSection?.offsetRem ?? 0)

        if (
          (top > scrollY && top < scrollY + innerHeight) ||
          (bottom > scrollY && bottom < scrollY + innerHeight) ||
          (top <= scrollY && bottom >= scrollY + innerHeight)
        ) {
          newVisibleSections.push(id)
        }
      }

      setVisibleSections(newVisibleSections)
    }

    let raf = window.requestAnimationFrame(() => checkVisibleSections())
    window.addEventListener('scroll', checkVisibleSections, { passive: true })