Floating menu with scrolling, like in VK?

Question

css

In groups in vk , the side menu (on the right) is fixed, but is screwed to its end and back if it is longer than the height of the window.
How to implement this? I rummaged through the pages of the page, did not find it.

Answer 1

https://abouolia.github.io/sticky-sidebar/examples...

Answer 2

There, JS switches the position: fixed class and, depending on the scrolling, margin-top (indent from above) changes, such as if the edge of the block has reached the edge of the viewport -> fixed, if both the top edge of the block and the bottom are not touching the edge of the viewport, then it is set margin-top of the current scroll (window.scrollY)
From this we
dance about clientHeight, offsetY and scrollTop