Answers for "handle scroll css"

smooth scroll css

html {
  scroll-behavior: smooth;
}

/* No support in IE, or Safari
You can use this JS polyfill for those */
http://iamdustan.com/smoothscroll/

js onscroll event

// HTML:
<element onscroll="myScript">
// JavaScript:
object.addEventListener("scroll", myScript)
// or
object.onscroll = function() { /*...*/ }