Description

We use a hybrid theme (use Block Editor for content, has a couple of Template Parts, but doesn't use FSE.)

When testing out 6.3-RC-1, I keep running into "The theme you are currently using is not compatible with the Site Editor".

Step-by-step reproduction instructions

Just one of may ways:

1. Appearance -> Template Parts.
2. Edit a template part.
3. Use "<" to go 'back' to Patterns, and select Manage all of my Patterns.
4. Hit my browser's back button, and voila.

Screenshots, screen recording, code snippet

Environment info

WordPress 6.3-RC-1. Windows 10, Chrome. No Gutenberg plugin.

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