mobile menu is not working

"don't go down" ? what 's that ?
the menu is planned to be loaded at the top of the page or in the place of the module, you have choosen the top of the page and it is there. It works

look at this web-site www.stilnyeokna.ru/

top menu has property position: fixed; and go down with scrolling

Mobile menu should have the same behaviour I think.

this is the main menu and has no dropdown, another user has tested but if you fix it at the top and the menu is longer than the screen you can no more go on the items that are not shown in the screen area
this is not allowable for me and would break the navigation

But its possible to make scroll inside mobile menu module. I saw such menu

actually if you are at the top of the page and you scroll, you are scrolling the menu too

no. You can check it on www.steklo-group.ru
If you scroll down the page mobile menu go up and not visible any more.