Navigation: Improve how invalid non-link blocks appear in Navigation screen

[noisysocks]

If a user activates a theme that has add_theme_support( 'block-nav-menus' ), adds a non-link block to their Navigation, then switches away to a theme that doesn't support block-nav-menus, they will still see the block they added in the Navigation screen like so:

In this state, it is not possible to add additional non-link blocks. The previously added non-link block remains there so that no user data is lot as a result of changing theme.

Because the theme does not support block-nav-menus, this non-link block will not display in the site's frontend. The user must delete the block and potentially replace it with a regular link.

We should indicate to the user that the block is invalid and won't display in the site's frontend. The user should either change theme or delete/replace the block.

[shaunandrews]

Would it make sense to not show these blocks? Or to somehow collect them into an "Unsupported Blocks" section?

[draganescu]

Yes it would make sense as the blocks will not be rendered on the front end anyway. But maybe instead of hiding, mark them like #23573 suggests?

[Thelmachido]

Closing this issue due to the Navigation Editor project being moved to an inactive status on the feature projects page in coordination with the project leads. If this work is picked back up, issues can be revisited and reopened as needed.