Global Styles: Put the inheritance UI behind a Gutenberg experiment

[ramonjd]

What?

Follow up to #80555. See #80649.

Replaces the IS_GUTENBERG_PLUGIN gate on the Global Styles inheritance UI with a Gutenberg experiment, gutenberg-global-styles-inheritance-ui, under Blocks on Settings > Experiments.

Why?

See: #80649 (comment) We're punting it to 7.2.

The treatment was on for every Gutenberg plugin user. Behind an experiment it is opt-in.

How?

• lib/experimental/experiments/load.php registers the experiment.
• lib/experimental/editor-settings.php sets window.__experimentalGlobalStylesInheritanceUI when it is on.
• The six block-supports panels, ColorGradientDropdownItem and BackgroundImagePanel all read that flag.

No server-side gating was added. The Global Styles payload the resolver reads is already sent to the editor for other features.

Testing Instructions

1. Go to Settings > Experiments. Confirm "Global Styles inheritance in the block inspector" appears under Blocks and is unchecked.
2. Open a post, add a Heading and select it. Set a text colour on the block.
3. Confirm no control label has a dotted underline and no blue reset dot appears next to any control.
4. Tick the experiment and save. Reload the post.
5. Confirm labels for inherited controls now have a dotted underline, and the blue reset dot appears next to the text colour.
6. Untick the experiment, reload, and confirm step 3 again.

[github-actions]

Size Change: +31 B (0%)

Total Size: 7.76 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	428 kB	+31 B (+0.01%)

_{compressed-size-action}

[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: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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

[github-actions]

Flaky tests detected in 9e66390.
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/30418912400
📝 Reported issues:

• [Flaky Test] should allow user to add and remove multiple local font files #78760 in /test/e2e/specs/admin/font-library.spec.js

[andrewserong]

Thanks for the updates and back and forth, this is all testing nicely for me!

LGTM 🚀

[github-actions]

There was a conflict while trying to cherry-pick the commit to the wp/7.1 branch. Please resolve the conflict manually and create a PR to the wp/7.1 branch.

PRs to wp/7.1 are similar to PRs to trunk, but you should base your PR on the wp/7.1 branch instead of trunk.

# Checkout the wp/7.1 branch instead of trunk.
git checkout wp/7.1

# Create a new branch for your PR.
git checkout -b my-branch

# Cherry-pick the commit.
git cherry-pick 01a197a247115610d8ab84572bbb1042d849f5d1

# Check which files have conflicts.
git status

# Resolve the conflict...
# Add the resolved files to the staging area.
git status
git add .
git cherry-pick --continue

# Push the branch to the repository
git push origin my-branch

# Create a PR and set the base to the wp/7.1 branch.
# See https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-base-branch-of-a-pull-request.

[t-hamano]

This PR was backported by #80818.