WSOD opening Site Editor due to lack of trailing slash in http://domain/subdirectory?_wp-find-template=true

[bobbingwide]

Description

In a test environment created with Heart Internet, opening the site editor beta fails with a WSOD.
Analysis indicates this is due to the 404 returned for the following request

http://217.199.187.57/scalar-wp-dev32.co.uk?_wp-find-template=true

when there's a trailing slash after the subdirectory name the request works

http://217.199.187.57/scalar-wp-dev32.co.uk/?_wp-find-template=true

By default Heart Internet hosting doesn't support automatic 301 redirects for requests pointing to directories.
Visiting the "home" URL for this site also produces a 404 until the trailing slash is added.

http://217.199.187.57/scalar-wp-dev32.co.uk/

Note: This problem was analysed in bobbingwide/bobbingwide#49
There is some work in progress to better deal with 404's once they've happened.
I believe that changing the URL to include the trailing slash will be a better solution for this specific problem.

Step-by-step reproduction instructions

Pre-requisite: Create a hosting environment which doesn't support automatic 301 redirects when visiting a subdirectory.

1. Install WordPress 5.9
2. Activate Twenty Twenty-Two
3. Visit the site eg. http://217.199.187.57/scalar-wp-dev32.co.uk/
4. Click on the "home" link or visit http://217.199.187.57/scalar-wp-dev32.co.uk
5. If you get a 404 then continue. If not revisit the pre-requisite
6. Login and choose Edit site or Appearance > Editor beta
7. It should fail with a WSOD.
8. View the console log. See the 404

Screenshots, screen recording, code snippet

#

Environment info

• WordPress 5.9
• Theme: Twenty Twenty-Two
• Problem occurs with / wthout Gutenberg 12.5.4

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

[bobbingwide]

An update from the person who has the problem.

1. It occurs with Heart Internet
2. It occurs with "domain references"
3. He can't use FSE for staging, test or dev sites; only with live domains !

He has 140 sites on Heart Internet using domain references.

Heart Internet's position is...

"This case has been reviewed by my senior colleagues and there currently aren't any issues with the hosting package that'll cause that part of the admin area not to work. I could be an issue with the corefiles for scalar-wp-dev32.co.uk."

Followed up with..

I've consulted with my senior colleagues and in the current case it wouldn't be advisable to use temp URL.
An alternative is to edit the hosts file on your computer
( https://www.howtogeek.com/howto/27350/beginner-geek-how-to-edit-your-hosts-file/ )
In general, WordPress has issues with temp URLs due to the use of permalinks.

[youknowriad]

I think #48363 fixed this issue.