[select][combobox] Fix record items label lookup reading from Object.prototype

[XionWCFM]

Fixes #5517

Summary

When items is provided as a plain record map, resolveSelectedLabel read the label with (items as any)[value] ?? fallback(). The bracket lookup walks the prototype chain, so a selected value matching an Object.prototype member (constructor, toString, hasOwnProperty, __proto__, …) resolved to the inherited function instead of the fallback label. Since functions are never null/undefined, the ?? fallback never ran, and rendering the value via Select.Value / Combobox.Value crashed with "Functions are not valid as a React child."

The lookup now only reads own keys via Object.hasOwn, preserving the documented null placeholder key behavior (coerced to "null").

Why this is the established pattern in the codebase

This is the same prototype-chain trap that CheckboxGroup already guards against — checkbox values are consumer data, so a value like constructor must not read off Object.prototype:

• checkbox-group/useCheckboxGroupParent.ts:17-18 uses a Map instead of an object for its value registry, with an explicit comment:

  A Map rather than an object: checkbox values are consumer data, and a value like constructor would otherwise read straight off Object.prototype.

• checkbox-group/useCheckboxGroupParent.test.tsx:258 has a dedicated test, "does not read aria-controls ids off Object.prototype".

Form errors are keyed by field name (also consumer data), so the same collision is guarded with Object.hasOwn in form/Form.tsx:150, field/root/FieldRoot.tsx:94, and field/error/FieldError.tsx:41. This change brings the remaining record lookup in resolveSelectedLabel in line with those.

Test plan

• Added regression tests in resolveValueLabel.test.ts covering:
  • prototype-member values falling back to the stringified label
  • an own record key that happens to match a prototype member still resolving
  • the null placeholder key in a record still resolving
• pnpm test:jsdom resolveValueLabel --no-watch, SelectValue, ComboboxValue — all pass
• pnpm eslint, pnpm typescript, pnpm prettier — clean

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5518
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5518
  

commit: 344d3bc

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+27B(+0.01%)	🔺+6B(0.00%)

Details of bundle changes

Performance

Total duration: 1,180.54 ms -7.26 ms^(-0.6%) | Renders: 76 ⁽⁺⁰⁾ | Paint: 1,866.52 ms +13.73 ms^(+0.7%)

Test	Duration	Renders
Slider mount (300 instances)	128.66 ms 🔺+22.20 ms(+20.9%)	2 (+0)
Tooltip mount (300 contained roots)	55.93 ms 🔺+10.36 ms(+22.7%)	1 (+0)
Mixed surface mount (app-like density)	64.01 ms ▼-25.66 ms(-28.6%)	5 (+0)

12 tests within noise — details

Metric alarms

Test	Metric	Change
Slider mount (300 instances)	bench:paint	🔺 +43.39 ms

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[atomiks]

Thanks for the fix!