[combobox] Avoid clearing selected value if item is not present in items array

[atomiks]

• I have followed (at least) the PR section of the contributing guide.

This avoids surprises with value getting cleared (to null) when the selected item isn't present in items, particularly in async combobox implementations. Native <select>s retain the previous value in state when the <option> gets removed from the list, with the exception that it automatically selects a fallback value which is a bit confusing here

Fixes #3818
Fixes #3502

[pkg-pr-new]

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@3824
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@3824
  

commit: dec9923

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	▼-348B(-0.08%)	▼-118B(-0.09%)

Details of bundle changes

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[greptile-apps]

Greptile Summary

This PR removes the automatic value reconciliation logic that was clearing selected values when the items array changed. The previous behavior was clearing controlled values to null (or filtering out items in multiple mode) when the selected item wasn't present in the items array, which caused issues for async combobox implementations and paginated data scenarios.

• Removed 46 lines of reconciliation logic from AriaCombobox.tsx that was forcing value clearing
• Updated test expectations to reflect values persisting when items change
• Added new test case validating that controlled object values not in items array still display their labels correctly using itemToStringLabel
• Removed two tests that were validating the old reconciliation behavior

The change aligns with native <select> behavior which retains the previous value when an <option> is removed. The stringifyAsLabel utility already handles displaying labels for values not in the items array by using itemToStringLabel or extracting labels from object structures, ensuring the input displays correctly even when the selected item isn't in the current items list.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change cleanly removes problematic reconciliation logic that was causing regressions. The implementation is well-supported by the existing stringifyAsLabel utility which handles values not in the items array. Comprehensive tests validate the new behavior, and the approach aligns with native HTML form control behavior.
• No files require special attention

Important Files Changed

Filename	Overview
packages/react/src/combobox/root/AriaCombobox.tsx	Removed automatic value reconciliation logic that was clearing selected values when items changed, allowing controlled values to persist even when not in items array
packages/react/src/combobox/root/ComboboxRoot.test.tsx	Removed tests for the old reconciliation behavior and added test for showing labels of controlled values not in items array, updated test expectations to match new behavior

[sarthakmalik0810]

Thank you for the quick fix @atomiks! Since this might have broken async Comboboxes, do you think the release for this fix could be expedited? Many thanks!