Browser mode fails with `SyntaxError: Failed to construct 'WebSocket'`

[kossnocorp]

Describe the bug

After upgrading Vitest in the date-fns codebase (vitest@0.34.6 -> vitest@1.3.1 & @vitest/browser@0.34.6 -> @vitest/browser@1.3.1) I started getting many of these errors:

SyntaxError: Failed to construct 'WebSocket': The URL 'ws://localhost:undefined/' is invalid.
❯ setupWebSocket /@vite/client:505:20
❯ fallback /@vite/client:484:22
❯ WebSocket.<anonymous> /@vite/client:520:13

As the result npx vitest run --browser hangs forever, even though the tests pass when I open it with --ui.

Reproduction

git clone git@github.com:date-fns/date-fns.git
cd date-fns
npm install
npm install -D @vitest/ui
npx vitest --browser --ui

It will open the browser where you can see the errors:

System Info

System:
    OS: Linux 5.15 Ubuntu 20.04.1 LTS (Focal Fossa)
    CPU: (12) x64 AMD Ryzen 5 5600X 6-Core Processor
    Memory: 15.01 GB / 31.31 GB
    Container: Yes
    Shell: 5.8 - /usr/bin/zsh
  Binaries:
    Node: 20.10.0 - ~/.asdf/installs/nodejs/20.10.0/bin/node
    Yarn: 1.22.21 - ~/.asdf/installs/nodejs/20.10.0/bin/yarn
    npm: 10.2.3 - ~/.asdf/plugins/nodejs/shims/npm
    bun: 1.0.25 - ~/src/date-fns/date-fns/node_modules/.bin/bun
  Browsers:
    Chrome: 103.0.5060.114
  npmPackages:
    @vitest/browser: ^1.3.1 => 1.3.1 
    @vitest/coverage-v8: ^1.3.1 => 1.3.1 
    vitest: ^1.3.1 => 1.3.1

Used Package Manager

npm

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[hi-ogawa]

I tested locally and I can reproduce. I'm not really sure yet but maybe spwaning too many iframes is causing some problems.

I tweaked some config and isolate: false seems to work, but fileParallelims: false doesn't seem to help even though the tests look proceeding slower.
If no isolation is acceptable, then you may use npx vitest --browser --no-isolate instead for now.

[kossnocorp]

@hi-ogawa thank you! Yeah, isolate: false works for date-fns. Appreciate the tip!

[kunokareal]

a similar issue seems to be happening on a smaller number of tests (~1k) when running in firefox, but works fine in chrome. isolate: false does help

[underoot]

Also reproduced issue, noticed that it's begins only when number of files larger than 127
https://github.com/underoot/vitest-large-file-list-issue