You can attach an onscroll listener to detect when the user scrolls. Inside the listener, you need to do some math to calculate which element is currently viewed. The variables you can use for the calculation are scrollTop (on the container, used to determine the current scroll position) and the clientHeight of each element.
let nameDisplayDiv = document.getElementById('display')
let containerDiv = document.getElementById('container')
function refresh() {
let scrollTop = containerDiv.scrollTop + containerDiv.clientHeight / 2
let height = 0
for (let child of containerDiv.children) {
let top = height
let bottom = height += child.clientHeight
if (top < scrollTop && bottom > scrollTop) {
// Found the element that's currently viewed!
nameDisplayDiv.innerHTML = child.style.backgroundColor
break
}
}
}
containerDiv.onscroll = refresh
Here's a working fiddle:
https://jsfiddle.net/uxh91Leq/2/