Disable loading of Custom CSS for atomic sites in jetpack-mu-wpcom

[monsieur-z]

Proposed changes:

#37794 caused the wpcomsh plugin to throw the exception: PHP Fatal error: Cannot redeclare jetpack_register_css_preprocessors().

This PR prevents the exception from being thrown by temporarily disabling the loading of the Custom CSS for atomic sites in the jetpack-mu-wpcom package (the Custom CSS will still be available in the Jetpack plugin for now). See conversation linked below for more details.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1719591775246119-slack-C05PV073SG3

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Spin up a WoA developer site
• Install and activate a classic theme, such as Twenty-Eleven
• From /wp-admin/admin.php?page=jetpack_modules, activate the Custom CSS module
• Install the Jetpack Beta Tester plugin
• In WPadmin, go to Jetpack > Beta Tester
• Select this branch for the WordPress.com Features

• In the admin, visit Appearance > Additional CSS
• Make sure you see the options above and that you can write additional CSS

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
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• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

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Once your PR is ready for review, check one last time that all required checks appearing at the bottom of this PR are passing or skipped.
Then, add the "[Status] Needs Team Review" label and ask someone from your team review the code. Once reviewed, it can then be merged.
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Mu Wpcom plugin:

• Next scheduled release: July 2, 2024.
• Scheduled code freeze: June 24, 2024.

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

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Wpcomsh plugin:

• Next scheduled release: July 2, 2024.
• Scheduled code freeze: June 24, 2024.

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

[anomiex]

See conversation linked below for more details.

TL;DR or for any non-Automatticians who wind up here: Jetpack gets updated weekly on WoA sites, while wpcomsh generally gets updated more often. With a version of wpcomsh containing #37794 and a version of Jetpack from earlier this week not containing #37794, wpcomsh loads first, defines the function, and there isn't yet the guard in Jetpack against redefining it. 💥

This PR removes the call in wpcomsh that loads all the code added in #37794, so wpcomsh doesn't define the functions. We'll revert this next week after Jetpack gets updated on WoA sites.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix/use-jetpack-plugin-custom-css-for-atomic branch.

  • For jetpack-mu-wpcom changes, also add define( 'JETPACK_MU_WPCOM_LOAD_VIA_BETA_PLUGIN', true ); to your wp-config.php file.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/use-jetpack-plugin-custom-css-for-atomic
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2