Previewing a post with navigation block after importing a classic menu renders the fallback #59684

Description

When using the navigation block and importing a classic menu, the menu cannot be previewed until the user publishes the menu. When previewing a draft of a post, one expects to see the menu they chose in the editor, not a fallback menu. Seeing the fallback menu is a confusing UX for most users, who do not understand how, why, or when the fallback menu would be rendered.

Ideally, previewing a post with a draft menu will also preview the draft menu. Another, less desirable, option would be to make it more obvious that a menu is in a draft state. This might include a visual indicator on the menu itself, and/or alerting the user prior to previewing that the draft menu needs to be saved before it can be previewed.

Step-by-step reproduction instructions

1. Create a new post
2. Add a navigation block
3. Import a classic menu
4. Observe that the menu renders in the editor as expected
5. Preview the post in a new tab
6. On the frontend preview, the navigation block renders the fallback navigation menu, not the same menu as we see in the editor

Screenshots, screen recording, code snippet

Environment info

Tested on 6.4.3 as well as 6.5 RC1

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes