__vitest_browser_runner__.runTests is not a function when browser mode x customzied root

Describe the bug

when the root is specified to an outside dir x browser mode
https://github.com/Jinjiang/reproductions/tree/vitest-browser-20250408

(not sure whether it's same to #6674)

Reproduction

https://github.com/Jinjiang/reproductions/tree/vitest-browser-20250408

cd tool
pnpm install

# it works
pnpm test
# it works
pnpm test2
# it doesn't work
pnpm test3

error:

Vitest caught 1 unhandled error during the test run.
This might cause false positive tests. Resolve unhandled errors to make sure your tests are not affected.

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TypeError: __vitest_browser_runner__.runTests is not a function
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System Info

System:
    OS: macOS 15.5
    CPU: (8) arm64 Apple M1
    Memory: 98.09 MB / 8.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - ~/.local/share/mise/installs/node/22/bin/node
    Yarn: 1.22.18 - ~/.yarn/bin/yarn
    npm: 10.9.2 - ~/.local/share/mise/installs/node/22/bin/npm
    pnpm: 10.7.1 - ~/Library/pnpm/pnpm
    bun: 1.1.36 - ~/.local/share/mise/installs/bun/latest/bin/bun
  Browsers:
    Safari: 18.5
  npmPackages:
    @vitest/browser: ^3.1.1 => 3.1.1 
    playwright: ^1.51.1 => 1.51.1 
    vitest: ^3.1.1 => 3.1.1

Used Package Manager

pnpm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Start with the linked reproduction at Jinjiang/reproductions, enter its tool directory, install dependencies, and compare pnpm test, pnpm test2, and pnpm test3. Trace how browser mode handles a root outside the project directory and identify why the failing case exposes a missing runTests function. Done means the custom-root browser-mode case runs without the reported TypeError.