Autocomplete: use blur relatedTarget to control menu visibility

[Copilot]

Split out from #7644 (see thread) so it can ship independently of the wider useProvidedRefOrCreate → useMergedRefs refactor.

AutocompleteInput's blur handler decided whether to close the menu by comparing document.activeElement against the input's ref. That inference can't distinguish "focused, then focus moved away" from "never focused", and it depends on how the input's ref is wired up — so migrating TextInput to merged refs (in #7644) breaks the blur logic.

The handler now reads the blur event's relatedTarget to detect whether focus is moving into the menu, closing the menu only when it isn't. This tracks the actual focus transition rather than post-interaction DOM state, and is independent of the ref setup.

safeSetTimeout(() => {
  const nextFocusedElement = event.relatedTarget as Node | null
  const menuElement = document.getElementById(`${id}-listbox`)
  if (!nextFocusedElement || (nextFocusedElement !== menuElement && !menuElement?.contains(nextFocusedElement))) {
    setShowMenu(false)
    // ...reset input value
  }
}, 0)

Changelog

New

• Test: menu stays open when focus moves into the menu.

Changed

• Autocomplete: input blur handler uses the blur event's relatedTarget to decide whether to close the menu.

Removed

• Reliance on document.activeElement/inputRef for blur-driven menu dismissal.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Run npx vitest run packages/react/src/Autocomplete/Autocomplete.test.tsx. The new "keeps the menu open when focus moves into the menu" test exercises the relatedTarget path. Note: browser-mode synthetic focus makes a strict old-vs-new discriminator unreliable (the fragility called out in the source thread), so the test locks in the intended relatedTarget contract rather than reproducing the original failure.

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Latest commit: 64f9a95

The changes in this PR will be included in the next version bump.

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