[utils] Fix UA-CH browser detection

[lyzno1]

This fixes detectBrowser so navigator.userAgentData.brands is no longer converted into a synthetic user-agent string before browser checks.

UA Client Hints expose structured brand data, so Edge detection now checks the Microsoft Edge brand directly and keeps the legacy navigator.userAgent fallback for browsers without UA-CH support.

References:

• https://developer.chrome.com/docs/privacy-security/user-agent-client-hints
• https://learn.microsoft.com/en-us/microsoft-edge/web-platform/user-agent-guidance
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Browser_detection_using_the_user_agent

Verification:

• Added focused tests for UA-CH brands and legacy Edge UA fallback
• TypeScript and lint pass for the touched utils files

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4900
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4900
  

commit: 89dff1b

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	▼-42B(-0.01%)	🔺+12B(+0.01%)

Details of bundle changes

Performance

Total duration: 1,143.38 ms -105.63 ms^(-8.5%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,734.59 ms -160.80 ms^(-8.5%)

Test	Duration	Renders
Slider mount (300 instances)	147.87 ms ▼-53.90 ms(-26.7%)	3 (+0)

11 tests within noise — details

---

Check out the code infra dashboard for more information about this PR.

[netlify]

✅ Deploy Preview for base-ui ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	89dff1b
🔍 Latest deploy log	https://app.netlify.com/projects/base-ui/deploys/6a12a5ca511ff600077932be
😎 Deploy Preview	https://deploy-preview-4900--base-ui.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

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

[romgrk]

What's the motivation for this change? As much as I enjoy getting rid of legacy APIs, if it's working, I wouldn't touch it. The UA string is not going away.

[lyzno1]

What's the motivation for this change? As much as I enjoy getting rid of legacy APIs, if it's working, I wouldn't touch it. The UA string is not going away.

That’s fair.

The existing behavior works for current mainstream UA values. The motivation here wasn’t to remove the legacy UA fallback — this still keeps navigator.userAgent for browsers without UA-CH support.

The narrow issue I was trying to address is that userAgentData.brands is structured data, but the current helper turns it into a synthetic UA string and then runs the existing browser regexes against it. That can make UA-CH GREASE/arbitrary brands participate in checks that were originally written for the legacy UA string.

That said, I agree this is more of a correctness hardening than a user-visible bug. If you’d rather avoid touching this while the current behavior works, I’m happy to close the PR.