Skipped optimistic replacement when a user message_start matches another recorded local submission, preserving the pending prompt bubble until its own expanded event arrives.
Added coverage for the queued-message drain race between startPendingSubmission and prompt dispatch.
Fixes#3199
Updated optimistic replay to track the replacement component handles created during transcript rebuilds, so expanded slash prompts still replace the raw replayed message.
Extended the regression test to cover the rebuild window called out in review.
Fixes#3199
Replaced raw optimistic slash-command transcript entries with the canonical user message emitted by AgentSession when prompt expansion changes the text.
Added coverage for prompt-template expansion reconciliation so the transcript keeps one expanded user message.
Fixes#3199
Pressing Ctrl+T (toggle thinking-block visibility) — or any other path that calls rebuildChatFromMessages, e.g. the theme/preset selector — during the pre-streaming window after a submission cleared the just-submitted user message until the first assistant token arrived.
startPendingSubmission paints the user message optimistically before session.prompt(...) lands it in session entries; rebuildChatFromMessages reads buildTranscriptSessionContext() which has no record of it yet, so the rebuild erased it and the streaming-component re-add block in toggleThinkingBlockVisibility was a no-op (no stream yet).
Centralize the fix in rebuildChatFromMessages: after rendering the transcript, replay the in-flight optimistic user submission. EventController#handleMessageStart already clears optimisticUserMessageSignature once the real user message_start lands, so the replay is a no-op post-streaming and cannot duplicate. cancelPendingSubmission clears the signature and marks the submission cancelled before its own rebuild, so cancellation paths still wipe the message.
Fixes#2372
roboomp