I am trying to detect the child which is at the topmost position of a scrollable div. I am trying to use the Intersection Observer API.

code example https://codesandbox.io/s/interesting-dan-tqy6p?file=/src/components/Planner/Planner.js&resolutionWidth=320&resolutionHeight=810

In my case,

1. I want to change the current week when the user scrolls the scrollable div.
2. Figure out which week is at the top of the scrollable div and update the week in the header.