Wrong template displayed for hybrid themes in the post template editor

[jffng]

Description

The post template editor shows the wrong template for hybrid themes that contain a mix of block and php templates.

Step-by-step reproduction instructions

1. Activate a classic theme that has a single.php or page.php (e.g. twentytwentyone)
2. Add a templates/index.html file.
3. Go to add / edit a post or page.
4. Verify the template editor renders the index.html, instead of single.php.
5. Verify the front-end renders the single.php.

Screenshots, screen recording, code snippet

Kapture.2024-08-07.at.11.11.50.mp4

Environment info

• WP 6.6.1 with Twenty Twenty-One

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

[carlomanf]

I think these may be related and/or duplicate issues: #31633 #63124 #64303

[youknowriad]

I think we should disable template previews and template editing for hybrid themes unless we're certain that the associated template is a block template one.

[carolinan]

I tried looking into this today but was not able to get any closer to solving it.

[youknowriad]

@Mamaduka This issue is going to become more important to solve, now that "show template" is more prominent.

[Mamaduka]

I think an extra check (hasTemplate) added by @jorgefilipecosta in #68110, partially solves that issue.