How to make the main menu of the site on top of the modal window without fixing?

Question

css

Please tell me, the site has a main menu in the nav tag. When opening a modal window, this menu should remain on top of all windows, including the modal one. I tried to make a fixed menu and z-index - I get the desired effect. But when scrolling the modal, the nav stays fixed, which is not true. How to do it right?