Implement a tabs menu on mobile for the primary navigation in addition to the burger menu.

Requirements

• The tabs should lead to each parent/category page of the site.
• They should have a selected state on the frontend regardless the level of hierarchy (example: Parent page > sub page).
• To view all items users would be able to swipe horizontally.
• The tabs menu should be sticky at the top and disappear when users start scrolling down the page.

Feature Flag

• Information Architecture > Enable mobile tabs menu.
  Since this is the first IA feature flag, lets's create a new WP option (eg. planet4_ia) and make sure these are not localized by WPML.

To be refined

• Investigate: exclude on Posts and special pages (Search, 404, etc). Create follow up ticket.
• Look also into the native navigation block introduced in WP 5.9

Designs (Sizes M and S)