fix: apply browser CLI options only if the project has the browser set in the config already

[sheremet-va]

Description

This PR changes how --browser CLI flags (and API options) affect configuration. If user config doesn't have the browser setup already (an object without enabled: false), then they don't affect the config at all. Vitest will throw an error if no projects define the browser config, but the --browser CLI option was set. The --browser.enabled still works the same way and will override options even if the config has enabled: false.

Hopefully, we can make simplify this when Vite 7 is out.

This PR also fixes a regression from 3.1.3 when extend: true wouldn't correctly extend the browser configuration.

Fixes #7944
Fixes #7964

Note

This PR needs to be backported to 3.1.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

[netlify]

✅ Deploy Preview for vitest-dev ready!

Name	Link
🔨 Latest commit	3459559
🔍 Latest deploy log	https://app.netlify.com/projects/vitest-dev/deploys/68263c5b2fbfdb0008647d7d
😎 Deploy Preview	https://deploy-preview-7984--vitest-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[AriPerkkio]

If user config doesn't have the browser setup already (an object without enabled: false), then they don't affect the config at all. Vitest will throw an error [...]

Is this intentional breaking change? Browser mode is experimental, so yes, it is allowed within non-major semver update.

So for example previously vitest --browser --browser.name=chromium --browser.provider=playwright without any config file worked, now it throws an error.

[AriPerkkio]

/ecosystem-ci run

[vitest-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
nuxt-test-utils	✅ success
elk	✅ success
nuxt	✅ success
effect	❌ failure
lerna-lite	✅ success
vue	✅ success
sveltejs-cli	✅ success
vitest-sonar-reporter	✅ success
zustand	✅ success
vitest-browser-examples	✅ success
vitest-coverage-large	✅ success
aria-live-capture	✅ success
vite	✅ success
vitest-benchmark-large	✅ success
vitest-in-webcontainer	✅ success
vitest-reporters-large	✅ success
webcontainer-test	❌ failure
vitest-vscode	✅ success

[sheremet-va]

Is this intentional breaking change? Browser mode is experimental, so yes, it is allowed within non-major semver update.

So for example previously vitest --browser --browser.name=chromium --browser.provider=playwright without any config file worked, now it throws an error.

Yes, this is intentional breaking change.