[select] Synchronize root props through the store

[flaviendelangle]

Select still sends root values such as disabled, readOnly, and required through a separate React context after #5582. Combobox sends the equivalent values through its selector-based store. This PR makes Select follow the Combobox model.

What changed

• Move disabled, readOnly, required, multiple, highlightItemOnHover, and itemProps into the Select store.
• Remove SelectRootPropsContext.
• Let the trigger, list, popup, and items subscribe only to the values they use.
• Seed itemProps before the first descendant render and synchronize later root updates in the existing atomic store transaction.
• Extend the store tests to cover disabled synchronization and its commit timing.

Trade-off

The removed context propagated new root props during the same render. Descendant ref callbacks therefore saw an updated disabled value during that commit.

store.useSyncedValues publishes changes from the root's layout effect, after descendant ref callbacks have run. Store subscribers then rerender before the browser paints, so the rendered DOM and interactions after the commit use the new value. There is one narrower window: if a consumer performs an imperative interaction from a descendant ref callback during the exact commit that changes disabled from false to true, the handler still sees false and can commit a selection. The updated characterization test pins this case.

This gives up Select's stronger same-commit guarantee in that ref-callback window. In return, changing one root value no longer invalidates every consumer of the combined context, including every mounted item, and Select now has the same propagation semantics as Combobox. Keeping synchronous ref-callback behavior would require retaining a separate context with its broader rerender fan-out, or changing the shared store synchronization model for both components.

Verification

• pnpm test:jsdom Select --no-watch (506 passed, 71 skipped)
• pnpm test:chromium Select --no-watch (558 passed, 19 skipped)
• pnpm typescript
• pnpm eslint
• pnpm prettier

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5589
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5589
  

commit: 1e00549

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+195B(+0.04%)	🔺+17B(+0.01%)

Details of bundle changes

Performance

Total duration: 1,075.57 ms -114.26 ms^(-9.6%) | Renders: 76 ⁽⁺⁰⁾ | Paint: 1,731.62 ms -218.59 ms^(-11.2%)

Test	Duration	Renders
Checkbox mount (500 instances)	59.91 ms ▼-17.25 ms(-22.4%)	1 (+0)
Dialog mount (300 instances)	21.95 ms ▼-11.05 ms(-33.5%)	1 (+0)

13 tests within noise — details

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

[flaviendelangle]

@atomiks maybe you have more insight about why the Select and the Combobox were not behaving the same way here.

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