Describe the bug

If a JS file is specified as the target in Lib Mode, CSS / SCSS can also be imported in the JS file. This CSS file is taken during the first creation and generates a style.css. But if you add a --watch parameter, new files are generated, even if there are changes in the CSS files, but no new CSS file.

Reproduction

export default {
  mode: 'development',
  build: {
    emptyOutDir: false, // workaround to prevent a missing style.css after the regeneration with --watch
    lib: {
      entry: './src/lib.js',
      name: 'Lib'
    },
  }
}

System Info

Output of npx envinfo --system --npmPackages vite,@vitejs/plugin-vue --binaries --browsers:

npx: Installierte 1 in 1.072s

  System:
    OS: macOS 11.2.3
    CPU: (12) x64 Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
    Memory: 2.79 GB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.16.1 - /usr/local/bin/node
    Yarn: 1.22.4 - ~/.yarn/bin/yarn
    npm: 6.14.12 - /usr/local/bin/npm
  Browsers:
    Chrome: 90.0.4430.212
    Edge: 89.0.774.45
    Firefox: 86.0.1
    Safari: 14.0.3
  npmPackages:
    vite: ^2.3.4 => 2.3.4 

Used package manager: npm

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