CSS is leaked to JS when @vitejs/plugin-legacy is used

[jiyingzhi]

Describe the bug

Even if the CSS is split into separate files, it will still be injected into JS, using @vitejs/plugin-legacy.

Reproduction

Run the following command:

git clone git@github.com:jiyingzhi/vite-bug.git
npm i
npm run build

output:

dist/assets/vendor-legacy.60abe03a.js      51.65kb / brotli: 16.90kb
dist/assets/polyfills-legacy.df455098.js   73.89kb / brotli: 26.08kb
dist/assets/index-legacy.19f6a2d2.js       178.76kb / brotli: 42.35kb
dist/assets/logo.03d6d6da.png    6.69kb
dist/index.html                  1.05kb
dist/assets/index.0af28c7f.js    1.47kb / brotli: 0.71kb
dist/assets/vendor.e2d3fd7e.js   42.92kb / brotli: 15.53kb
dist/assets/index.b97d446c.css   177.38kb / brotli: 41.70kb

The size of dist/assets/index-legacy.19f6a2d2.js minus the size of dist/assets/index.0af28c7f.js is exactly the size of dist/assets/index.b97d446c.css.

System Info

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

System:
    OS: macOS 11.2.3
    CPU: (8) arm64 Apple M1
    Memory: 112.16 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
Binaries:
    Node: 15.14.0 - ~/.nvm/versions/node/v15.14.0/bin/node
    npm: 6.14.13 - ~/.nvm/versions/node/v15.14.0/bin/npm
    Watchman: 4.9.0 - /opt/homebrew/bin/watchman
Browsers:
    Chrome: 91.0.4472.101
    Edge: 90.0.818.41
    Safari: 14.0.3
npmPackages:
    @vitejs/plugin-vue: ^1.2.3 => 1.2.3 
    vite: ^2.3.7 => 2.3.7

Used package manager: npm

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[mila76]

same issue here

[bluwy]

Can confirm this is still happening with the latest version of Vite and plugin-legacy. I can't tell why or where this is happening though.

[bluwy]

@sapphi-red helped tracked this one down (thanks!), and it looks like a duplicate of #2062. There's a comment mentioning that it's intentional, but can be improved: #2062 (comment)