How to move the scrollbar down, or how to cover it with a fixed element?

Question

css

Fixed hat. There is no way to hide under it. Or just simply slide down.
Any advice will help
Thanks for the advice, but there is one new ans

When the menu opens, scroll crawls on it

Answer 1

The solution turned out to be simple. One had only to read the documentation
-ms-overflow-style: scrollbar;
The scrollbar becomes fixed
Thank you all!

Answer 2

The maximum you can do is something like this . Or replace the fixed width of the header with 100%, then it will not crawl under the scrollbar and there will be no need to fence what I wrote earlier.