1. disable the Off Canvas Panel in Customizer > Layout > Off Canvas.
2. in Customizer > Layout > Primary Navigation, set the Mobile Menu Breakpoint to 0
Once thats done, you will be able to see how the items occupy that space.
And if need be we can provide some CSS to assist with adjusting its layout.
Easiest solution is to change menu mobile breakpoint in customizer to zero, that way it will never shift to mobile menu. I did this last week for a landing page I didnt want a menu on.