This is pulling out just the navigator improvements from #21727. This brings also in the movers work, but shows it all together. The illustration here is for the navigator within the navigation block, however, this could apply to any place eventually where it's used.

What this does:

• Ellipsis menu comes in by the navigation element. For now, I brought in 'delete' and 'duplicate'.
• Brings in movers from 5.4.1 Backports #21793

Note, I also brought in an active state that could be explored without.

Exploring moving the +

One potential could be to as @mtias noted in #21727 (comment):

The action to add a sub-menu should be available in the corresponding navigator item (either in the ellipsis menu or as its own button).

During this he also said:

The ellipsis menu in the navigator UI should replicate the block ellipsis menu (duplicate, remove, etc).

You can see the ellipsis being used above, however what the adding could evolve to is explored here:

A benefit of this could be removing completely the + and just having you click to add directly where you are. I really like this idea as it takes away the visual complexity.

Further exploring

A few other options could be:

Making the size wider

Removing icons

It's good to keep the spec for this a little narrow to bring in iterations, so whilst there could be more, for now these bring in what was being explored in a larger issue.