Addresses the PR #138 review's architecture note (the deferred 'non-blocking'
item). The brand-new -> persisted chat identity was spread across two separate
useState slots (threadKey + liveThreadChatId) plus a render-phase guard, so the
mount key and the live thread's chat id could in principle diverge.
Consolidate them into ONE atomic state object { key, chatId } with pure,
unit-tested transitions in thread-identity.ts:
- newThread(newKey) — a brand-new id-less chat (fresh session key);
- switchThread(chatId) — switch to an existing chat (key := chat id) -> remount;
- adoptThread(prev, id) — a new chat learns its real id IN PLACE (key unchanged
-> no remount, live useChat store preserved).
The 'key vs chatId diverged' state is now unrepresentable. The render-phase
reconciliation stays (the atom is also set externally, e.g. page-history's
history-item opening a referenced chat), but adoption vs switch is now explicit.
Behavior is unchanged; verified live: new-chat adopt + 2nd turn in the same row,
no mid-conversation remount, the two-tab race stays leak-free, switch-to-existing
remounts + reseeds, and reload restores full history. Adds thread-identity.test.ts.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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