Watch mode generates corrupt CSS on second build

[Fd929c2CE5fA]

Describe the bug

When using vite api build with watch mode, even if saved without changes, the generated second or later automatic build will produce an incorrect CSS file, it contains irrelevant svelte (script/html) code.

After testing, the "@sveltejs/vite-plugin-svelte": "5.1.1" version works correctly and the "@sveltejs/vite-plugin-svelte": "6.0.0" and "@sveltejs/vite-plugin-svelte": "6.1.0" versions can reproduce the error.

That is, the issue exist in plugin version > 6.

Reproduction URL

https://stackblitz.com/edit/vitejs-vite-q6tkjnjy?file=src%2Flib%2FCounter.svelte&terminal=dev

Reproduction

1. The first automatic build (node watch.js) after opening the stackblitz link generates the correct CSS:

dist/assets/index-8HoovIC0.css

:root {
  font-family: system-ui, Avenir, Helvetica, Arial, sans-serif;
  line-height: 1.5;
  font-weight: 400;

  color-scheme: light dark;
  color: rgba(255, 255, 255, 0.87);
  background-color: #242424;

  font-synthesis: none;
  text-rendering: optimizeLegibility;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

a {
  font-weight: 500;
  color: #646cff;
  text-decoration: inherit;
}
a:hover {
  color: #535bf2;
}

body {
  margin: 0;
  display: flex;
  place-items: center;
  min-width: 320px;
  min-height: 100vh;
}

h1 {
  font-size: 3.2em;
  line-height: 1.1;
}

.card {
  padding: 2em;
}

#app {
  max-width: 1280px;
  margin: 0 auto;
  padding: 2rem;
  text-align: center;
}

button {
  border-radius: 8px;
  border: 1px solid transparent;
  padding: 0.6em 1.2em;
  font-size: 1em;
  font-weight: 500;
  font-family: inherit;
  background-color: #1a1a1a;
  cursor: pointer;
  transition: border-color 0.25s;
}
button:hover {
  border-color: #646cff;
}
button:focus,
button:focus-visible {
  outline: 4px auto -webkit-focus-ring-color;
}

@media (prefers-color-scheme: light) {
  :root {
    color: #213547;
    background-color: #ffffff;
  }
  a:hover {
    color: #747bff;
  }
  button {
    background-color: #f9f9f9;
  }
}

  .logo.svelte-11cv5lq {
    height: 6em;
    padding: 1.5em;
    will-change: filter;
    transition: filter 300ms;
  }
  .logo.svelte-11cv5lq:hover {
    filter: drop-shadow(0 0 2em #646cffaa);
  }
  .logo.svelte.svelte-11cv5lq:hover {
    filter: drop-shadow(0 0 2em #ff3e00aa);
  }
  .read-the-docs.svelte-11cv5lq {
    color: #888;
  }

2. Enter any character in the Counter.svelte file, and the triggered new build will generate the corrupt CSS:

dist/assets/index-DvP653D8.css

:root {
  font-family: system-ui, Avenir, Helvetica, Arial, sans-serif;
  line-height: 1.5;
  font-weight: 400;

  color-scheme: light dark;
  color: rgba(255, 255, 255, 0.87);
  background-color: #242424;

  font-synthesis: none;
  text-rendering: optimizeLegibility;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

a {
  font-weight: 500;
  color: #646cff;
  text-decoration: inherit;
}
a:hover {
  color: #535bf2;
}

body {
  margin: 0;
  display: flex;
  place-items: center;
  min-width: 320px;
  min-height: 100vh;
}

h1 {
  font-size: 3.2em;
  line-height: 1.1;
}

.card {
  padding: 2em;
}

#app {
  max-width: 1280px;
  margin: 0 auto;
  padding: 2rem;
  text-align: center;
}

button {
  border-radius: 8px;
  border: 1px solid transparent;
  padding: 0.6em 1.2em;
  font-size: 1em;
  font-weight: 500;
  font-family: inherit;
  background-color: #1a1a1a;
  cursor: pointer;
  transition: border-color 0.25s;
}
button:hover {
  border-color: #646cff;
}
button:focus,
button:focus-visible {
  outline: 4px auto -webkit-focus-ring-color;
}

@media (prefers-color-scheme: light) {
  :root {
    color: #213547;
    background-color: #ffffff;
  }
  a:hover {
    color: #747bff;
  }
  button {
    background-color: #f9f9f9;
  }
}
<script>
  import svelteLogo from './assets/svelte.svg'
  import viteLogo from '/vite.svg'
  import Counter from './lib/Counter.svelte'
</script>

<main>
  <div>
    <a href="https://vite.dev" target="_blank" rel="noreferrer">
      <img src={viteLogo} class="logo" alt="Vite Logo" />
    </a>
    <a href="https://svelte.dev" target="_blank" rel="noreferrer">
      <img src={svelteLogo} class="logo svelte" alt="Svelte Logo" />
    </a>
  </div>
  <h1>Vite + Svelte</h1>

  <div class="card">
    <Counter />
  </div>

  <p>
    Check out <a href="https://github.com/sveltejs/kit#readme" target="_blank" rel="noreferrer">SvelteKit</a>, the official Svelte app framework powered by Vite!
  </p>

  <p class="read-the-docs">
    Click on the Vite and Svelte logos to learn more
  </p>
</main>

<style>
  .logo {
    height: 6em;
    padding: 1.5em;
    will-change: filter;
    transition: filter 300ms;
  }
  .logo:hover {
    filter: drop-shadow(0 0 2em #646cffaa);
  }
  .logo.svelte:hover {
    filter: drop-shadow(0 0 2em #ff3e00aa);
  }
  .read-the-docs {
    color: #888;
  }
</style>

Note: Corrupted CSS will be generated in non-initial builds, i.e., the second or subsequent builds generated by the watch mode.

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/vite-plugin-svelte: ^6.1.0 => 6.1.0 
    svelte: ^5.37.3 => 5.37.3 
    vite: ^7.0.6 => 7.0.6