[ic-side-navigation] only one navigation item is viewable at 400% page zoom

[GCHQ-Developer-530]

Summary of the bug

When a page is zoomed to 400%, the side nav goes to top bar behaviour. When you open the menu of the top bar, only one navigation is viewable and any attempt to scroll or tab won't let you see any more navigation items.

Tabbing does tab through the items and then eventually you see the navigation item shown getting focus. But scrolling just seems to scroll the content behind the side nav and not the navigation items within.

🪜 How to reproduce

Tell us the steps to reproduce the problem:

1. Go to any side nav example
2. Zoom to 400%
3. Open menu
4. See the error

🧐 Expected behaviour

Maybe need some design input and investigation, but should the navigation items be displayed in a completely different way when it's this zoomed in? Could just show a row of icons under the top bar?