Previewing custom meta for unpublished nav menu items in the customizer does not work

[theenoahmason]

There is a caveat when working with WP Backstage Nav Menu Items in the WordPress Customizer. Since WordPress doesn't publish newly added menu items added in the Customizer until the changes are published, previewing of custom meta fields that may or may not be used in the actual output of the menu item will not be previewable. The Customizer assigns a negative post ID (placeholder), and does not save the menu item as a post in the DB until the customizer changes are published, meaning there is no way to get the post meta for this non-existing post. We will continue to investigate ways to get around this, but at this time there simply is none. In the mean time, previewing works great for custom meta on menu items that do already exist in the DB.

This ticket will be used to track findings on this issue.