accessibility of slide-out menu (when activated by itself)

Hi,

I want to ensure keyboard-only accessibility for the site I'm building. I want to use only the slide-out menu for both desktop and mobile. However, keyboard tabbing loses focus once the menu opens - it tabs through the underlying page and I can't access the menu or close it (with keyboard only). However, the mobile version of the regular nav is indeed keyboard accessible. Any advice for keyboard-only access to slide-out navigation?