Summary

Fixes #93583.

On Windows (native, no WSL), sass-loader with the modern Dart Sass API fails to resolve relative @import statements inside node_modules packages. For example, @primer/css/color-modes/index.scss does @import './themes/light.scss', and the build fails with:

Error: Can't find stylesheet to import.
  @import './themes/light.scss'
  node_modules\@primer\css\color-modes\index.scss

Root cause

When the sass-loader webpack importer (webpackImporter: true, the default) can't resolve the path on Windows due to backslash/forward-slash path separator differences, it returns null. Dart Sass then falls back to its built-in resolver, but that resolver only searches the file's own directory and user-specified loadPaths. Since node_modules is not in loadPaths by default, the relative import inside the package fails.

Fix

In packages/next/src/build/webpack/config/blocks/css/index.ts, prepend <rootDirectory>/node_modules to the loadPaths option passed to sass-loader. This ensures Dart Sass can always resolve package imports natively, regardless of whether the webpack importer succeeds.

User-specified sassOptions.loadPaths values are preserved and appended after the default, so there is no breaking change.

- sassOptions,
+ sassOptions: {
+   ...restSassOptions,
+   loadPaths: [
+     path.join(ctx.rootDirectory, 'node_modules'),
+     ...(userLoadPaths ?? []),
+   ],
+ },

Why this is safe

• Adds a fallback resolution path; does not change how existing working imports resolve
• Dart Sass's own documentation recommends loadPaths for resolving packages from node_modules
• User-provided loadPaths values take precedence (appended after the default) and continue to work unchanged
• No effect on Turbopack (Turbopack has its own Sass pipeline)

Test plan

• On Windows 10/11 (native Node.js, no WSL): clone github/docs, run npm install && npm run dev — build should succeed without SCSS import errors
• On Linux/macOS: verify existing SCSS imports still work correctly (no regression)
• Projects with custom sassOptions.loadPaths values should still work

🤖 Generated with Claude Code