Navigation: Avoid layout shifts between placeholder and loaded menu

[jasmussen]

The new Navigation placeholder state is simpler, but is always 48px tall, which means when you load a menu that has a different size, there's a small layout shift.

We can optimize that so the container is the same height as an eventual menu will be. That should ensure a uniform height between placeholder, the spinner that might briefly get shown, and the loaded menu.

It likely won't always be perfect, for example if a menu is set to collapse to an overlay, or if a menu has more than 1 row of items, but that would be acceptable.

GIF showing 4 distinct heights of navigation:

• Resting state (48px hard coded)
• Loading state (same height as spinner)
• Empty (24px inserter button height)
• Menu (same height as an menu item)

One potential fix is to output a hidden menu item with zero width, and use that item to size out both the setup state, loading, and the empty states.

[jasmussen]

Moving towards a behavior where the placeholder is never shown with a fallback always in place, this is no longer relevant.