[select] Fix the root id being ignored inside a Field

[atomiks]

An id on Select.Root is dropped when the Select sits inside a Field.Root. The trigger renders a generated id instead, so #my-id matches nothing and Field.Label points at the generated id. Outside a Field it works.

Select.Trigger registered idProp ?? rootId with the labelable provider, and rootId is the provider's own selected id read back through the Select store. The provider keeps its current selection while that id is still registered, so the generated id kept re-electing itself and the root's id could never take over. The trigger now registers only its own explicit id, which leaves the root as the sole registrant when the trigger has none.

Carved out of #5448, which fixed this through a preferId argument on registerControlId. That argument is not needed for this case.

#5448's notes also describe an exception in the unregister flush for Select.Trigger, because it registered an id adopted from the provider one commit behind. That registration no longer exists, so the exception may no longer be needed.

Still unfixed, and out of scope here: an explicit id on Select.Trigger does not move the label when it changes at runtime, and the equivalent Combobox cases are untouched.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5461
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5461
  

commit: a79e436

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	0B(0.00%)	0B(0.00%)

Details of bundle changes

Performance

Total duration: 1,010.40 ms ▼-391.99 ms^(-28.0%) | Renders: 76 ⁽⁺⁰⁾ | Paint: 1,586.56 ms ▼-615.44 ms^(-27.9%)

Test	Duration	Renders
Tabs mount (200 instances)	166.05 ms ▼-81.73 ms(-33.0%)	3 (+0)
Select mount (200 instances)	128.10 ms ▼-68.42 ms(-34.8%)	3 (+0)
Menu mount (300 instances)	83.45 ms ▼-59.62 ms(-41.7%)	1 (+0)
Menu open (500 items)	61.20 ms ▼-40.65 ms(-39.9%)	11 (+0)
Popover mount (300 instances)	51.53 ms ▼-34.09 ms(-39.8%)	1 (+0)

…and 3 more (+7 within noise) — details

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