Describe the bug

In 5.3.0 7b0a65e added custom logic that utilizes the proxyReq event to set the origin header. This throws an ERR_HTTP_HEADERS_SENT error for me when using using with https-proxy-agent (for example required in setups in which a corporate proxy is in place). I suspect the problem is the same as described in chimurai/http-proxy-middleware#957 and similar / related to the problem described in chimurai/http-proxy-middleware#472 (explanation provided in chimurai/http-proxy-middleware#472 (comment)).

When downgrading to 5.2.13 everything works again.

Reproduction

https://stackblitz.com/edit/github-sadccs?file=src%2FApp.vue,vite.config.js

Steps to reproduce

Run npm install
Replace https://example.org with an actual https proxy in vite.config.js (i don't know a public available one, sorry)
Run npm run dev

First click on the "Without proxy agent" button, you should see the expected result then.
Now click on the "With proxy agent" button, the ERR_HTTP_HEADERS_SENT should appear in the console.

System Info

Binaries:
    Node: 18.15.0
    npm: 9.5.0

Used Package Manager

npm

Logs

node:_http_outgoing:663
    throw new ERR_HTTP_HEADERS_SENT('set');
    ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at new NodeError (node:internal/errors:399:5)
    at ClientRequest.setHeader (node:_http_outgoing:663:11)
    at setOriginHeader (file:///-------/node_modules/vite/dist/node/chunks/dep-BcXSligG.js:61768:16)

Validations

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• The provided reproduction is a minimal reproducible example of the bug.