PR #224 fixed an AI-chat streaming-render regression by moving the React.memo
content signature into the parent: MessageList now snapshots
messageSignature(message) per render and passes it to MessageItem as the
immutable `signature` prop. The existing memo tests only SIMULATED that
parent half by hardcoding `signature={messageSignature(message)}` in their
harness; the real MessageList was never exercised (chat-thread.test.tsx mocks
it out, and there was no message-list.test).
Add message-list.test.tsx that mounts the REAL MessageList (without mocking
MessageItem or messageSignature) and asserts that an in-place mutation of a
reused message object surfaces on re-render. This guards the parent-side
contract: re-caching the signature on message identity (stable across deltas
while parts mutate) would refreeze the row, and this test would fail.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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