How to auto scroll on top of page in Lightning Component in Salesforce using CSS?

To make the scrollTop = 0 to work the container div has to have an internal scroll. Which we can get by either giving a height via px or via vh.
Try that out it should work. I was facing a similar issue while adding LWC inside Flexi Page. The Flexi page has its own scroller but for that, the scrollTop doesn’t work. But if we create a separate scroller at the top-level div then the scrollTop starts to work.
Check this out. I have created this and it works perfectly fine for me: https://webcomponents.dev/edit/3vpWJ46hxykfPSACfuNN