Vertical scrolling of the browser scrolls a single block on the page. How is it done?

Question

css

There is a page with content - on the right side of the browser there is a standard vertical scrolling of the page.
How to make it so that when a block appears on this page (for example, a div with content), vertical scrolling affects only this block (i.e. scrolls only this div)?

For example, this is implemented by Vkontakte ( https://vk.com/team?w=page-22822305_51878110 )

Answer 1

for body do overflow: hidden when opening modal

Answer 2

First you need to monitor activity using the tools at www.brendangregg.com/linuxperf.html + https://habrahabr.ru/company/oleg-bunin/blog/319020/
Measure and then, if there are specific questions, ask.

Answer 3

Switch to nginx?

Answer 4

Now your database can't cope, most likely due to slow disks or lack of memory.
Try to optimize it for the new number of connections from Apache,