Checkbox: replace useLayoutEffect with ref callback for indeterminate prop

[alexus37]

Summary

The Checkbox component currently uses a useLayoutEffect to imperatively set the DOM .indeterminate property on the underlying <input> element. While this works, it can be replaced with a ref callback, which runs synchronously during commit (same timing guarantees as useLayoutEffect) without registering a separate effect.

Current behavior

useLayoutEffect(() => {
  if (checkboxRef.current) {
    checkboxRef.current.indeterminate = indeterminate || false
  }
}, [indeterminate, checked, checkboxRef])

Proposed change

Replace with a ref callback that sets .indeterminate inline:

const setIndeterminate = React.useCallback(
  (node: HTMLInputElement | null) => {
    if (node) {
      node.indeterminate = indeterminate || false
    }
  },
  [indeterminate, checked],
)

// merge with the forwarded ref
const mergedRef = useMergedRef(checkboxRef, setIndeterminate)

Additional cleanup

The useEffect that sets aria-checked can also be replaced with an inline JSX attribute:

'aria-checked': indeterminate ? 'mixed' : undefined

This avoids an unnecessary effect per Checkbox instance.

Motivation

• Ref callbacks are the idiomatic React pattern for imperative DOM properties with no HTML attribute equivalent
• Eliminates a layout effect per Checkbox render, reducing React commit phase work
• Minor but measurable improvement in pages that render many checkboxes (e.g., list views with bulk selection)

References

• React docs: Ref callbacks
• Source: Checkbox.tsx