test: Cannot create proxy with a non-object as target or handler

[peterpeterparker]

Describe the bug

I'm facing a similar issue with Svelte v5 in testing context to the one someone reported on SO today. The unit test works fine until I add a <style></style> to the component.

As soon as a component contains a style, the following error starts appearing when running npm run test:

FAIL src/tests/lib/components/Back.spec.ts [ src/tests/lib/components/Back.spec.ts ]
TypeError: Error while preprocessing src/lib/components/Back.svelte - Cannot create proxy with a non-object as target or handler
❯ new PartialEnvironment node_modules/vite/dist/node/chunks/dep-yUJfKD1i.js:16766:19
❯ preprocessCSS node_modules/vite/dist/node/chunks/dep-yUJfKD1i.js:48447:23
❯ node_modules/@sveltejs/vite-plugin-svelte/src/preprocess.js:114:10
❯ style node_modules/@sveltejs/vite-plugin-svelte/src/preprocess.js:77:37
❯ process_single_tag node_modules/svelte/src/compiler/preprocess/index.js:283:21
❯ replace_in_code node_modules/svelte/src/compiler/preprocess/replace_in_code.js:70:23
❯ process_tag node_modules/svelte/src/compiler/preprocess/index.js:300:26
❯ Module.preprocess node_modules/svelte/src/compiler/preprocess/index.js:363:25
❯ compileSvelte node_modules/@sveltejs/vite-plugin-svelte/src/utils/compile.js:85:20

I'm facing an issue in our design kit, which I started using for migrating our existing app to Svelte v5 (PR).

Note that I originally reported the issue to Svelte Testing (here), but since I can reproduce the issue with a Svelte mount, we can rule out that library as the root cause.

Reproduction

git clone https://github.com/peterpeterparker/svelte-vite-testing-library-css
cd svelte-vite-testing-library-css
npm ci
npm run test

Logs

No response

System Info

System:
    OS: macOS 15.1.1
    CPU: (8) arm64 Apple M2
    Memory: 105.69 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.12.0 - ~/.n/bin/node
    Yarn: 1.22.22 - ~/.n/bin/yarn
    npm: 10.9.2 - ~/.n/bin/npm
    pnpm: 9.15.0 - ~/.n/bin/pnpm
  Browsers:
    Brave Browser: 131.1.73.97
    Chrome: 131.0.6778.109
    Safari: 18.1.1
  npmPackages:
    svelte: ^5.0.0 => 5.9.0

Severity

blocking an upgrade

[paoloricciuti]

This is a duplicate of sveltejs/vite-plugin-svelte#1043

and you can see a why in this comment sveltejs/vite-plugin-svelte#1043 (comment)