Storybook: Scope Docs theme providers

[ciampo]

What?

Fixes a dev-mode Storybook warning on Docs pages:

ThemeProvider: More than one root provider (isRoot) is mounted on the same document.

Storybook now keeps the Design System theme decorator as a root provider in Canvas mode, but scopes it in Docs mode.

Why?

The Design System theme decorator applied isRoot to every matching story. Canvas pages render one story in the preview iframe, so there was only one root provider. Docs pages also run inside the preview iframe, but their Primary and Stories blocks render multiple stories into that same iframe document rather than separate iframe documents. Each decorated story therefore mounted a competing root provider against the same document.documentElement.

How?

Sets isRoot only when the decorated story is not rendered in Docs mode.

Testing Instructions

1. Run npm run lint:js -- storybook/decorators/with-design-system-theme.tsx.
2. Start Storybook in dev mode.
3. Open Components/Button Docs and confirm the browser console does not show the duplicate root-provider ThemeProvider warning.
4. Open the individual Components/Button Default story and confirm the warning is still absent.

Testing Instructions for Keyboard

Not applicable; this changes Storybook theme-provider wiring and does not change keyboard interaction.

Screenshots or screencast

Not applicable.

Use of AI Tools

This PR was investigated and drafted with Codex.

[github-actions]

Size Change: 0 B

Total Size: 7.5 MB

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[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: mirka <0mirka00@git.wordpress.org>

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[github-actions]

Flaky tests detected in f60863f.
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/28241948094
📝 Reported issues:

• [Flaky Test] Should save the changes #78689 in /test/e2e/specs/editor/plugins/wp-editor-meta-box.spec.js

[mirka]

I like the refactor, but do I understand correctly that it's unrelated to the fix in question?

[mirka]

Not sure we need the tests, it's a good amount of overhead for a noncritical one-liner fix, and it's quite easy to notice any regressions in our daily exposure to the Storybook.

[ciampo]

@mirka less is more. PR is down to a single line diff :)