ReplyContextExpandable was declared inside MailboxModal's render, making it a new
element type on every render. Any parent update remounted the whole recursive
reply thread, so expanding one reply row collapsed the others and discarded their
DOM identity, focus, and scroll position.
Hoist it to module scope and pass the parent's reply state and handlers through an
explicit env prop so the element type stays stable. Adds a regression test that
expands one row, expands a second, and asserts the first keeps both its node
identity and aria-expanded state — same defect class as the FN-8606 ModalShell fix.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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