Site Editor: Set IFRAME_REQUEST constant for classic theme site preview

[t-hamano]

What?

Follow-up #69514
Related core PR: WordPress/wordpress-develop#8475

Note

Don't add Backport to WP 6.8 Beta/RC label to this PR. This is because the auto cherry-pick has failed in #69514. I plan to submit another PR to the wp/6.8 branch that includes this PR and #69514.

Why?

In #69514, We added a get parameter to hide admin bar in the classic theme site preview. However, we found that some plugins load code into the preview.

How?

To solve this problem, it seems best to define IFRAME_REQUEST instead of filtering show_admin_bar hook.

Testing Instructions

• Activate Twenty Twenty-One.
• Install and activate https://ja.wordpress.org/plugins/query-monitor/ plugin.
• Access Appearance > Design.
• Confirm that the admin bar isn't displayed within the site preview.
• Confirm that the Query Monitor plugin doesn't load anything.
• Reload your browser a few times and check again.

Before	After

[Mamaduka]

Great catch, @t-hamano! Today, I also learned about IFRAME_REQUEST 😄

[github-actions]

Flaky tests detected in 2859a73.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/13829249888
📝 Reported issues:

• [Flaky Test] image inserted via link should appear in the frontend published post content #52030 in /test/e2e/specs/editor/blocks/image.spec.js

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: t-hamano <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.