Builder-Vite: Prevent config duplication

[Copilot]

The storybook:enforce-output-dir plugin's config hook was spreading the entire incoming Vite config ({ ...config, build: { outDir } }). Since Vite's mergeConfig concatenates arrays when merging plugin hook results back into the base config, any array-valued fields — including css.postcss.plugins — were duplicated on each resolution cycle, causing PostCSS plugins to run twice.

Changes

• build.ts: Return only the partial config needed from the config hook instead of spreading the full config:

// Before — spreads full config, causing array fields like css.postcss.plugins to double
config: (config) => ({ ...config, build: { outDir: options.outputDir } }),

// After — returns only what needs to change; Vite merges the rest
config: () => ({ build: { outDir: options.outputDir } }),

• build.test.ts: Added regression tests verifying that mergeConfig does not duplicate postcss plugins when the config hook returns a partial config, and that the old spread behavior provably caused the duplication.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: @storybook/builder-vite duplicates postcss plugins during config resolution</issue_title>
<issue_description>### Describe the bug

In Storybook using @storybook/react-vite, the Vite config resolution process during build duplicates the viteConfig.css.postcss.plugins array. As a result, PostCSS plugins are applied twice to the same CSS, causing transformations to be double-applied, and leading to, for example, postcss-prefixwrap now producing .scope .scope .selector, which is obviously a breaking bug.

This started after Storybook introduced the storybook:enforce-output-dir Vite plugin (PR #33740), which adds config hooks (config / configEnvironment) and triggers an additional config merge where the PostCSS plugins array becomes duplicated

Reproduction link

https://69971850cc8d9694b2603cf1-mhkdpdawpi.chromatic.com

Reproduction steps

1. Clone https://github.com/anders-cognite/storybook-vite-postcss-plugins-duplicated-repro
2. Run the build-storybook command.
3. Inspect the generated CSS output.

Expected Behavior:
CSS selectors should be prefixed with a single .hello-world, due to the PostCSS PrefixWrap('.hello-world') plugin defined in vite.config.ts.

Actual Behavior:
The PostCSS plugin processes the CSS twice due to array duplication, leading to a double .hello-world .hello-world prefix.

Reproduction link is to the chromatic build.

System

System:
│  OS: Linux 6.6 Ubuntu 24.04.3 LTS 24.04.3 LTS (Noble Numbat)
│  CPU: (16) x64 Intel(R) Core(TM) Ultra 7 265H
│  Shell: 5.9 - /usr/bin/zsh
│  Binaries:
│  Node: 24.11.1 - /run/user/1000/fnm_multishells/217352_1771508128317/bin/node
│  npm: 11.6.2 - /run/user/1000/fnm_multishells/217352_1771508128317/bin/npm
│  pnpm: 10.24.0 - /run/user/1000/fnm_multishells/217352_1771508128317/bin/pnpm
│  <----- active
│  npmPackages:
│  @storybook/react-vite: ^10.2.10 => 10.2.10
│  chromatic: ^15.1.1 => 15.1.1
│  eslint-plugin-storybook: ^10.2.10 => 10.2.10
│  storybook: ^10.2.10 => 10.2.10

Additional context

No response</issue_description>

<agent_instructions>Verify the solution by adding a template story and adjusting the vite config in one of the sandboxes. Make a screenshot from the play function which verifies the css classname only appended once</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: @storybook/builder-vite duplicates postcss plugins during config resolution #33874

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Summary by CodeRabbit

• Bug Fixes

  • Fixed a configuration merging issue in the Vite builder plugin that resulted in duplicate config properties. Partial plugin configurations are now properly handled without unintended duplication during config merge operations.

• Tests

  • Added regression tests for the plugin's configuration hook behavior to validate proper handling of partial configuration objects and merge scenarios.

[github-actions]

	Fails
🚫	PR description is missing the mandatory "#### Manual testing" section. Please add it so that reviewers know how to manually test your changes.

Generated by 🚫 dangerJS against e9bbe2b

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View your CI Pipeline Execution ↗ for commit e9bbe2b

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☁️ Nx Cloud last updated this comment at 2026-02-25 14:49:48 UTC

[coderabbitai]

📝 Walkthrough

Walkthrough

A regression test file is introduced to verify the storybook:enforce-output-dir Vite plugin's behavior when merging partial versus complete configurations, alongside a modification to the plugin's config hook to return only the build.outDir property instead of spreading the entire config object.

Changes

Cohort / File(s)	Summary
Test Suite code/builders/builder-vite/src/build.test.ts	New regression test file with two test cases verifying Vite config merging behavior: one validating that a partial config return preserves existing plugins, and another demonstrating the duplication issue when the entire config is spread.
Plugin Configuration code/builders/builder-vite/src/build.ts	Modified the config hook callback to return only { build: { outDir } } instead of { ...config, build: { outDir } }, removing the spread of prior config properties.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• Builder-Vite: Add plugin to enforce Storybook's output directory in Vite build configuration #33740: Adds the storybook:enforce-output-dir plugin that sets build.outDir via a config hook; this PR modifies that same hook to change its return behavior to avoid configuration duplication during merging.

✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@code/builders/builder-vite/src/build.test.ts`:
- Around line 5-14: Update the placeholder issue reference in the top-of-file
comment so it points to the real issue number: replace "issues/XXXX" with
"issues/33874" (i.e., reference "#33874") in the block that documents the
regression for the storybook:enforce-output-dir plugin's config hook; keep the
rest of the explanatory text (about returning partial config and
css.postcss.plugins duplication) unchanged.

[coderabbitai]

⚠️ Potential issue | 🟡 Minor

Update placeholder issue number to actual issue reference.

The comment references issues/XXXX but should reference the actual issue #33874 that this PR fixes.

📝 Suggested fix

 /**
- * Regression test for: https://github.com/storybookjs/storybook/issues/XXXX
+ * Regression test for: https://github.com/storybookjs/storybook/issues/33874
  *

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change

	/**
	* Regression test for: https://github.com/storybookjs/storybook/issues/XXXX
	*
	* The storybook:enforce-output-dir plugin's config hook was previously returning the entire
	* incoming config spread ({ ...config, build: { outDir } }). Since Vite merges plugin config
	* hook results back into the base config by concatenating arrays, this caused
	* css.postcss.plugins (and other array fields) to be duplicated on every resolution cycle.
	*
	* The fix: return only the partial config needed ({ build: { outDir } }).
	*/
	/**
	* Regression test for: https://github.com/storybookjs/storybook/issues/33874
	*
	* The storybook:enforce-output-dir plugin's config hook was previously returning the entire
	* incoming config spread ({ ...config, build: { outDir } }). Since Vite merges plugin config
	* hook results back into the base config by concatenating arrays, this caused
	* css.postcss.plugins (and other array fields) to be duplicated on every resolution cycle.
	*
	* The fix: return only the partial config needed ({ build: { outDir } }).
	*/

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@code/builders/builder-vite/src/build.test.ts` around lines 5 - 14, Update the
placeholder issue reference in the top-of-file comment so it points to the real
issue number: replace "issues/XXXX" with "issues/33874" (i.e., reference
"#33874") in the block that documents the regression for the
storybook:enforce-output-dir plugin's config hook; keep the rest of the
explanatory text (about returning partial config and css.postcss.plugins
duplication) unchanged.