Running browser tests gradually decreases parallelism

[nstepien]

Describe the bug

We've migrated our biggest codebase (>200 test suites) to using Vitest's Browser Mode.

I've noticed that, when running all the tests, at first Vitest runs as many test suites in parallel as possible, but the amount of parallel test suites gradually decreases as it's making progress, all the way down to only 1 test suite running at any one time.

This has a huge impact on overall test time, especially in CI.

Reproduction

Our codebase isn't public, and I don't know the exact root cause, so I'm not sure I can provide a useful reproducible repo.

I can narrow our config down to this:

    test: {
      include: ['src/**/*.test.{ts,tsx}'],
      globals: true,
      browser: {
        enabled: true,
        headless: true,
        provider: 'playwright',
        commands: { resizeViewport },
        viewport: { width: 1920, height: 1080 },
        instances: [{ browser: 'chromium' }]
      },
      setupFiles: [
        'src/styles.ts',
        'vitest/setup.ts',
        'vitest/hooks.ts',
        'vitest/failOnConsole.ts'
      ]
    }

System Info

System:
    OS: Windows 11 10.0.26100
    CPU: (64) x64 AMD Ryzen Threadripper 3970X 32-Core Processor
    Memory: 20.93 GB / 31.88 GB
  Binaries:
    Node: 23.9.0 - C:\Program Files\nodejs\node.EXE
    npm: 11.1.0 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Chromium (131.0.2903.99)
    Internet Explorer: 11.0.26100.1882
  npmPackages:
    @vitejs/plugin-react: ^4.3.4 => 4.3.4
    @vitest/browser: ^3.0.7 => 3.0.7
    @vitest/coverage-v8: ^3.0.7 => 3.0.7
    playwright: ^1.50.1 => 1.50.1
    vite: ^6.2.0 => 6.2.0
    vitest: ^3.0.7 => 3.0.7

Used Package Manager

npm

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[sheremet-va]

Yeah, currently it's kind of a fake parallelisation. It's doesn't use the same mechanism that Node.js tests use for parallelisation. It just starts specific tests in several tabs, and they don't communicate between each other when one is free to run other tests.