While Foundations's off-canvas menu allows us to forget about writing
CSS, it also leads to complicated HTML.
Ideally Foundation would provide an easy way to simplify what we're
doing, but I haven't found anything in the documentation.
We could simplify the HTML a bit more if we used a CSS grid layout
instead of a flex one, but old browsers have better support for the
latter.
Note we're using `breakpoint(medium)` so we can group the CSS for small
screens and follow SCSS-Lint rules at the same time.
Also note behavior of the main area when the menu appears on small
screens is slightly different: it doesn't move the main content to the
right. I've done it this way so we don't have any overflow issues,
unlike the previous version.
There's a small issue using a label and a checkbox to enable/disable the
menu: sighted keyboard users with a small screen might not be able to
enable the menu. So we're adding the `:focus-within` pseudoclass so the
menu can be normally navigated using the keyboard. Even if old browsers
don't support this pseudoclass, we believe the probability of a sighted
user using a small screen, navigating with the keyboard and using an old
browser is really low, particularly in the admin area.
We're also adding the `aria-hidden` attribute on the label, since the
menu is never hidden for screen readers and so having a control to show
it could be confusing. Since the label is not focusable, we're complying
with the fourth ARIA rule:
> Do not use role="presentation" or aria-hidden="true" on a focusable
> element .
>
> Using either of these on a focusable element will result in some users
> focusing on 'nothing'.
