[autocomplete][combobox] Support external items mapping

[atomiks]

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

This adds support for the cmdk-style composition pattern without requiring items on Root, and without forcing the filtering/rendering to happen through a function child:

<Combobox.Root>
  <Combobox.Input />
  <Combobox.Portal>
    <Combobox.Positioner>
      <Combobox.Popup>
        <Combobox.List>
          {fruits.map((item) => (
            <Combobox.Item key={item} value={item}>
              <Combobox.ItemIndicator>
                <CheckIcon />
              </Combobox.ItemIndicator>
              <div>{item}</div>
            </Combobox.Item>
          ))}
        </Combobox.List>
      </Combobox.Popup>
    </Combobox.Positioner>
  </Combobox.Portal>
</Combobox.Root>

Before this, Autocomplete could support this shape reasonably well, but filtering had to happen externally. Combobox only had partial support and did not keep selection/highlight/empty-state behavior in sync.

Implementation

• Combobox.Item filters itself and returns null when it should not be shown.
• Combobox.Group uses hidden instead of unmounting its children so item registration effects can still run.
• The rebased version simplifies the bookkeeping a bit by reusing CompositeList metadata for item values instead of maintaining a separate DOM-node-to-value map.

Pros

• Works with a more composition-oriented API, including server-component-friendly usage where a function child would otherwise need to be hoisted into a client component.
• TS inference stays natural because the item type comes from the surrounding array.
• Rendering/order are fully controlled by user composition.

Tradeoffs

Real

• This is still more internal complexity than the items-driven path.
• For selection modes with no items, Combobox still needs to force-mount the popup on interaction so labels and selected indices can be registered from the rendered tree.
• Virtualization still requires items or filteredItems, because unrendered rows are otherwise unknowable.

Not really new / overstated

• Strings already work fine. Objects are only needed when the value/label are distinct, which is the same existing itemToStringLabel story rather than a new requirement introduced by this approach.
• Some of the implementation overhead was incidental rather than fundamental; the rebased version trims part of it by removing extra value-mapping plumbing.

Validation

• pnpm test:jsdom ComboboxRoot --no-watch
• pnpm test:jsdom AutocompleteRoot --no-watch
• pnpm typescript
• pnpm eslint docs/reference/generated/combobox-empty.json packages/react/src/autocomplete/root/AutocompleteRoot.test.tsx packages/react/src/combobox/empty/ComboboxEmpty.tsx packages/react/src/combobox/group/ComboboxGroup.tsx packages/react/src/combobox/group/ComboboxGroupContext.ts packages/react/src/combobox/input/ComboboxInput.tsx packages/react/src/combobox/item/ComboboxItem.tsx packages/react/src/combobox/list/ComboboxList.tsx packages/react/src/combobox/popup/ComboboxPopup.tsx packages/react/src/combobox/positioner/ComboboxPositioner.tsx packages/react/src/combobox/root/AriaCombobox.tsx packages/react/src/combobox/root/ComboboxRoot.test.tsx packages/react/src/combobox/root/ComboboxRootContext.tsx packages/react/src/combobox/store.ts packages/react/src/combobox/trigger/ComboboxTrigger.tsx packages/react/src/composite/list/useCompositeListItem.ts

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@3594
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@3594
  

commit: 9b79d6c

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+1.79KB(+0.39%)	🔺+615B(+0.42%)

Details of bundle changes

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[KingSora]

@atomiks do you think this would also cover the usecase described here: #3559 ?