Fix flex child fixed width and introduce max width option

[tellthemachines]

Trac ticket: https://core.trac.wordpress.org/ticket/65462

Use of AI Tools

Changes copied from WordPress/gutenberg#79073 with the help of copilot /gpt 5.3 codex. Human reviewed by myself.

[github-actions]

Hi there! 👋

Thank you for your contribution to WordPress! 💖

It looks like this is your first pull request to wordpress-develop. Here are a few things to be aware of that may help you out!

No one monitors this repository for new pull requests. Pull requests must be attached to a Trac ticket to be considered for inclusion in WordPress Core. To attach a pull request to a Trac ticket, please include the ticket's full URL in your pull request description.

Pull requests are never merged on GitHub. The WordPress codebase continues to be managed through the SVN repository that this GitHub repository mirrors. Please feel free to open pull requests to work on any contribution you are making.

More information about how GitHub pull requests can be used to contribute to WordPress can be found in the Core Handbook.

Please include automated tests. Including tests in your pull request is one way to help your patch be considered faster. To learn about WordPress' test suites, visit the Automated Testing page in the handbook.

If you have not had a chance, please review the Contribute with Code page in the WordPress Core Handbook.

The Developer Hub also documents the various coding standards that are followed:

• PHP Coding Standards
• CSS Coding Standards
• HTML Coding Standards
• JavaScript Coding Standards
• Accessibility Coding Standards
• Inline Documentation Standards

Thank you,
The WordPress Project

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[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.

Core Committers: Use this line as a base for the props when committing in SVN:

Props isabel_brison, andrewserong.

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

[andrewserong]

This is testing well for me on the site frontend and it's a faithful backport of WordPress/gutenberg#79073 (that approach is really nice IMO as it's backwards compatible while adding either a new "Max" option or a new "real fixed" option, depending on how you look at it 😄)

I used the following markup to test (first paragraph is preserved in its output from prior to this PR (the old "fixed" now known as "max), the new "fixed" paragraph only works with this PR applied):

<!-- wp:group {"layout":{"type":"flex","flexWrap":"nowrap"}} -->
<div class="wp-block-group"><!-- wp:paragraph {"style":{"layout":{"selfStretch":"fixed","flexSize":"500px"}},"backgroundColor":"accent-2"} -->
<p class="has-accent-2-background-color has-background">Previous fixed width (now max)</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph {"style":{"layout":{"selfStretch":"fixed"}}} -->
<p>Another paragraph</p>
<!-- /wp:paragraph --></div>
<!-- /wp:group -->

<!-- wp:group {"layout":{"type":"flex","flexWrap":"nowrap"}} -->
<div class="wp-block-group"><!-- wp:paragraph {"style":{"layout":{"selfStretch":"fixedNoShrink","flexSize":"500px"}},"backgroundColor":"accent-2"} -->
<p class="has-accent-2-background-color has-background">New fixed width (called fixed)</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph {"style":{"layout":{"selfStretch":"fixed"}}} -->
<p>Another paragraph</p>
<!-- /wp:paragraph --></div>
<!-- /wp:group -->

<!-- wp:paragraph -->
<p></p>
<!-- /wp:paragraph -->

Here's how it renders on the site frontend:

LGTM!

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62505
GitHub commit: f7baebc

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.