Navigation menu block: tabbing from an active submenu list item sends focus to the sidebar

[enriquesanchez]

This issue comes from the the accessibility test performed by the Accessibility team (#20369).

One of the issues raised was the unpredictability when attempting to tab from one child block to another in the same list, or if it's the last, to the next top-level item.

Tabbing from an active submenu list item sent focus to the sidebar, when I had expected to move to the next top level menu item.

[enriquesanchez]

I think that if we were to fix this issue of changing modes in sub links this would no longer be a problem.

The recent addition of the new tabbing behavior in 'Edit' mode allows for easier access to the editor sidebar when in 'Edit' mode, and this is totally fine.

The problem here is that the modes are changing without request from the user. Tabbing between child links moves from 'Select' to 'Edit' unexpectedly.