Menus: make tabs panels more accessible for keyboard users

[audrasjb]

In #48655, the discussion moved from a better handling of Add menu item action in Menus screen to further accessibility enhancements (like using aria labels, regions, and so on).

As @azaozz pointed out in the WP 5.3 post-mortem, we should rather use a dedicated ticket as the proposed enhancement doesn't cover the bug intially raised by the ticket reporter.

Let's improve navigation for keyboard users in Menus screen in this specific ticket.

To summarize, we need to improve the keyboard navigation in Add menu item sections tab panels.

@afercia proposed to follow the recommendations available on this post: ​https://developer.paciellogroup.com/blog/2016/02/short-note-on-improving-usability-of-scrollable-regions/

[audrasjb]

Related: #48655

[audrasjb]

Menus: make tabs panels more accessible for keyboard users

[audrasjb]

Add box-shadow on panel container only on focus, for better keyboard navigation

[audrasjb]

Add box-shadow on panel container only on focus, for better keyboard navigation

[afercia]

This ticket was discussed during today's accessibility bug-scrub. Ideally, this UI should be an ARIA tabbed interface so that the role=region would be a more appropriate role=tabpanel. This is a separate issue though that can be iterated later.

Marking for commit for final review.

[SergeyBiryukov]

I think it would make sense to use existing post type and taxonomy labels here:

• Most Recent → stays as is, doesn't have a direct match.
• Search Results → $post_type->labels->search_items
• All Items → $post_type->labels->all_items

• Most Used → $taxonomy->labels->most_used
• All Items → $taxonomy->labels->all_items
• Search Results → $taxonomy->labels->search_items

Looks good to me otherwise :)

[SergeyBiryukov]

In 47209:

Accessibility: Menus: Make tab panels in Add menu items section more accessible for keyboard users.

Props audrasjb, afercia.
Fixes #49211.