Twenty Nineteen: Mobile menu isn't scrollable

[kjellr]

As originally reported by @anevins in the Twenty Nineteen GitHub repo:

​https://github.com/WordPress/twentynineteen/issues/713

Twenty Nineteen's mobile menu uses a fixed position, and does not allow for vertical scrolling when menu items extend beyond the screen height. In this screenshot for instance, it's impossible to scroll down and see the additional menu items:

Normally, this would be solved by applying display: block (instead of table), and overflow-x: scroll to the submenu. However, in this case, that results in long menus visible under their children submenus:

[karmatosed]

I am still seeing this today so it is an issue I would recommend is resolved as if you have a long menu it does appear broken.