[combobox] Support primitive item values

[atomiks]

Partially addresses #3594.
Related to #4419.

This lets Combobox use primitive values with supported object items, such as items={[{ value: "banana", label: "Banana" }]} with defaultValue="banana" or <Combobox.Item value="banana" />.

The public API stays the same: Combobox infers primitive values from supported item shapes instead of adding an itemValue prop. Select is unchanged in this PR.

Changes

• Support primitive Combobox values for { value, label } items.
• Use inferred values for selection, highlight, scroll into view, typeahead, autofill, and labels.
• Keep Combobox item values available while filtering, closing, using inline lists, or rendering without items.
• Add Combobox tests for primitive values, custom equality, autofill, virtualized items, typeahead, and inline lists.

Notes

• This only infers values for the supported { value, label } item shape.
• Other object item shapes still need matching values or isItemEqualToValue.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4686
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4686
  

commit: 0fd9141

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+1.42KB(+0.31%)	🔺+538B(+0.36%)

Details of bundle changes

Performance

Total duration: 1,222.22 ms -112.48 ms^(-8.4%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,831.45 ms -210.60 ms^(-10.3%)

Test	Duration	Renders
Select open (500 options)	42.71 ms ▼-15.21 ms(-26.3%)	14 (+0)

11 tests within noise — details

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[Copilot]

Pull request overview

This PR refactors Select and Combobox to stop storing selectedIndex as synchronized store state and instead derive it from snapshot arrays of item values, enabling closed programmatic value updates without requiring item registration/mounting.

Changes:

• Replace stored selectedIndex with derived selectors based on itemValues / allItemValues snapshots for Select and Combobox.
• Remove programmatic value-change force-mounting behavior while keeping autofill/interaction paths working.
• Update Combobox list navigation to use a derived selected item/index (avoiding ref reads during render), and add regression tests.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/react/src/select/store.ts	Replace stored selectedIndex with derived selectedIndex computed from itemValues.
packages/react/src/select/root/SelectRoot.tsx	Remove programmatic value-change force-mounting; add itemValues state and related behavior.
packages/react/src/select/root/SelectRoot.test.tsx	Add regression test ensuring programmatic value changes don’t force-mount the popup.
packages/react/src/select/positioner/SelectPositioner.tsx	Remove direct selectedIndex resets when clearing value (now derived).
packages/react/src/select/popup/SelectPopup.tsx	Switch selected-index checks to derived selector usage.
packages/react/src/select/item/SelectItem.tsx	Maintain itemValues snapshot updates on item mount/unmount; remove direct selectedIndex writes.
packages/react/src/combobox/store.ts	Add itemValues/allItemValues snapshots and derive selectedIndex from them.
packages/react/src/combobox/root/ComboboxRoot.test.tsx	Add regression tests for deriving/highlighting/scrolling selected item on first open without items.
packages/react/src/combobox/root/AriaCombobox.tsx	Remove stored selectedIndex sync logic; maintain snapshots; adjust navigation behavior to use derived selection.
packages/react/src/combobox/item/ComboboxItem.tsx	Populate and snapshot itemValues/allItemValues from registered items for derived selection/indexing.
packages/react/src/combobox/input/ComboboxInput.tsx	Remove selectedIndex updates from setIndices calls (now derived).
packages/react/src/combobox/clear/ComboboxClear.tsx	Remove selectedIndex updates from setIndices calls (now derived).
packages/react/src/combobox/chip/ComboboxChip.tsx	Remove selectedIndex updates from setIndices calls (now derived).

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