Button Block: Add back compat for WP6.4 regarding HTML Tag Processor

[t-hamano]

Fixes #62986

What?

This PR fixes a fatal error in the Button block, caused by the WP_HTML_Tag_Processor class in WP6.4 not yet having the next_token() method.

Fatal error: Uncaught Error: Call to undefined method WP_HTML_Tag_Processor::next_token() in /var/www/html/wp-content/plugins/gutenberg/build/block-library/blocks/button.php:44

How?

A class with the next_token method exists in the Gutenberg plugin as Gutenberg_HTML_Tag_Processor_6_5. I set it up so that this class is used if the IS_GUTENBERG_PLUGIN variable is true. This prevents a critical error that occurs when the Gutenberg plugin is used in WP6.4.

In my opinion, the following flow would allow us to maintain full backward compatibility:

1. July 3rd (today): Gutenberg 18.7 was released.
2. Merge this PR.
3. July 16th: WordPress 6.6 will be officially released.
4. July 17th: Gutenberg 18.8 will be shipped with this PR included, which will fix the issue with WP6.4.
5. The Gutenberg plugin will drop support for WP6.4.
6. (TODO) Remove the fallback approach made in this PR.

Testing Instructions

• Deactivate the Gutenberg plugin and downgrade to WP 6.4.
• Insert a Button block into a post and publish it.
• Activate the Gutenberg plugin and visit the post.
  • trunk: You will get a critical error.
  • This PR: You should not get a critical error and the Button block should render.

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: dmsnell <dmsnell@git.wordpress.org>
Co-authored-by: anton-vlasenko <antonvlasenko@git.wordpress.org>
Co-authored-by: talldan <talldanwp@git.wordpress.org>
Co-authored-by: ndiego <ndiego@git.wordpress.org>
Co-authored-by: beckej13820 <beckej@git.wordpress.org>

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[github-actions]

Flaky tests detected in 7835c9f.
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/9778816524
📝 Reported issues:

• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js

[anton-vlasenko]

@t-hamano I'd be grateful if you could address my comment below. Thank you.

[talldan]

Thanks for addressing this. I realize it was my mistake that's lead to this situation.

July 16th: WordPress 6.6 will be officially released.
July 17th: Gutenberg 18.8 will be shipped with this PR included, which will fix the issue with WP6.4.

Given 18.8 is released after WordPress 6.6, I think support for WordPress 6.4 can be dropped in 18.8.

Perhaps this PR can be shipped in a Gutenberg 18.7.1 release (I'm happy to handle the release process).

[t-hamano]

Thank you everyone for your feedback. I think I've made the appropriate changes to the code, but I'd appreciate it if you could also check that this comment is clear.

Perhaps this PR can be shipped in a Gutenberg 18.7.1 release (I'm happy to handle the release process).

That is very much appreciated 🙇‍♂️

[talldan]

I'd appreciate it if you could also check that this comment is clear.

Yep, I think it's very clear 👍

[talldan]

I've cherry picked this to the 18.7 release branch - b6310be