Description

Fixes a bug when using Lightning CSS with Vite Server and html-proxy

Additional context

The issue was originally found when attempting to use Storybook with + Vitejs 4.4 with the lightningcss optioins enabled.

The shell app builds but when build the preview iframe it breaks with the error:

node:internal/process/promises:288
            triggerUncaughtException(err, true /* fromPromise */);
            ^

[Error: GenericFailure] {
  code: 'GenericFailure',
  plugin: 'vite:css',
  id: '\x00/iframe.html?html-proxy&direct&index=0.css',
  pluginCode:[css code]
}

I would guess the problem is here: https://github.com/vitejs/vite/blob/main/packages/vite/src/node/plugins/css.ts#L2176:L2181

When processing CSS from proxy the filePath is an empty string or a relative path without a filename so we fail the first condition and try to read the file from the file system.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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