[field] Fix stale filled and focused state on control replacement

[atomiks]

Two Field lifecycle states could go stale across control replacement. A filled uncontrolled control replaced by a fresh empty one under the same Field.Root kept data-filled set, and a control removed while focused never fires blur, leaving data-focused stuck on the root.

filled now follows last-attach-wins ownership of the shared input ref: the owning control publishes unconditionally on attach, a superseded control can't clear its successor's state, and a survivor reclaims a null ref after the owner unmounts. Each control also tracks whether it holds the field's focused state and releases it on unmount only in that case. Form registration reads the control's own element rather than the mutable shared ref, so field values stay readable when overlapping control lifetimes clear that ref.

Native label association is deliberately left on the existing commit-driven lifecycle (tests now pin it, including labels in other containers, htmlFor retargeting, and OTP slots mounting late). Label changes with no control rerender at all remain untracked in v1: useAriaLabelledBy patches over the hidden-input-inside-label layout, which is slated for rework in v2, and observer-based tracking measurably regressed mount performance for what it covered.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5345
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5345
  

commit: 69c0549

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+107B(+0.02%)	🔺+22B(+0.02%)

Details of bundle changes

Performance

Total duration: 1,250.90 ms +99.64 ms^(+8.7%) | Renders: 78 ⁽⁺⁰⁾ | Paint: 1,976.31 ms +176.62 ms^(+9.8%)

Test	Duration	Renders
Menu mount (300 instances)	148.33 ms 🔺+31.96 ms(+27.5%)	2 (+0)
Mixed surface mount (app-like density)	83.82 ms 🔺+18.83 ms(+29.0%)	5 (+0)
Menu open (500 items)	87.23 ms 🔺+17.19 ms(+24.5%)	12 (+0)
Select open (500 options)	44.85 ms +4.69 ms(+11.7%)	14 (+0)

11 tests within noise — details

Metric alarms

Test	Metric	Change
Menu mount (300 instances)	bench:paint	🔺 +46.40 ms
Mixed surface mount (app-like density)	bench:paint	🔺 +26.85 ms
Menu open (500 items)	bench:paint	🔺 +24.24 ms
Menu open (500 items)	bench:paint#menu-open	🔺 +24.24 ms
Select open (500 options)	bench:paint	🔺 +19.61 ms

…and 1 more metric alarm — details

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