javascript - 如何检测 ""的 20% 是否可见？

Question

正如您在这里看到的，我有两个部分占据了整个页面。我想在滚动时检测，如果第二部分可见 20%，如果可见，则强制它平滑滚动并锁定该部分，以便占用整个空间。如果添加更多，与其他部分相同。

(我自己能做的顺利的部分)。

有人知道我怎么能做那样的事情吗？

* {
    margin: 0;
    padding: 0;
}

#s1 {
  height: 100vh;
  background: red;
}

#s2 {
  height: 100vh;
  background: blue;
}

<section id="s1"></section>
<section id="s2"></section>

Answer 1

var section = document.querySelector('section'),
    sectionHeight = section.offsetHeight,
    lastScrollTop = 0;
document.addEventListener("scroll", function() {
    var scrollY = window.pageYOffset || document.documentElement.scrollTop;

    if (scrollY > lastScrollTop) { //on scroll down
        if (scrollY >= sectionHeight * 0.2 && scrollY < sectionHeight) { //if 20% of section2 is visible
            window.scrollTo(0, sectionHeight * 1); //scroll to section2
        } else if (scrollY >= sectionHeight * 1.2 && scrollY < 2 * sectionHeight) { //if 20% of section3 is visible
            window.scrollTo(0, sectionHeight * 2); //scroll to section3
        } else if (scrollY >= sectionHeight * 2.2) { //if 20% of section4 is visible
            window.scrollTo(0, sectionHeight * 3); //scroll to section4
        }
    }
    lastScrollTop = scrollY <= 0 ? 0 : scrollY;
});

* {
    margin: 0;
    padding: 0;
}
html,body{
  scroll-behavior: smooth;
}
section{
  height: 100vh;
}
#s1 {
  background: red;
}
#s2 {
  background: blue;
}
#s3{
  background: yellow;
}
#s4{
  background:purple;
}

<section id="s1"></section>
<section id="s2"></section>
<section id="s3"></section>
<section id="s4"></section>