Summary

Fixes a bug where rapidly creating several chats (Ctrl-N → type → send, ~3×) and then waiting for one to finish caused the other still-running chats to disappear from the sidebar.

Root cause. A new session's first user message isn't flushed to the SessionDB until its turn is persisted (_persist_session at turn end). The gateway lazily creates the DB row on prompt.submit with message_count = 0 (_ensure_session_db_row), so a session that's still mid-first-response has message_count = 0 in the DB.

refreshSessions() lists with min_messages=1 and then hard-replaces $sessions. Every message.complete triggers a refresh (completeAssistantMessage → refreshSessions()), so the instant one session finished, the others — still at message_count = 0 — were filtered out of the server page and dropped from the list.

Fix. Merge instead of replace. New mergeWorkingSessions() preserves any session that is still in $workingSessionIds but missing from the server page, so concurrent new chats stay visible until their own turn persists and the server starts returning them. Optimistic deletes/archives already drop the row from the previous list, so a removed session can't be resurrected by the merge. The sidebar re-sorts by recency, so merge order is cosmetic; the Math.max(sessionsTotal, length) footer math already tolerates the extra rows.

UX follow-up. The send path created the optimistic row with a null preview, so an in-flight chat read "Untitled session" until its turn persisted and auto-title ran. Now that concurrent in-flight chats are preserved, several "Untitled session" rows could appear at once. Seed the optimistic preview with the user's first message (the branch path already does this) so each row is labeled immediately; the server's preview/title supersedes it once the turn persists.

Changes

• store/session.ts — add pure, testable mergeWorkingSessions(previous, incoming, workingIds) helper.
• app/desktop-controller.tsx — refreshSessions() merges working sessions instead of hard-replacing.
• session/hooks/use-session-actions.ts + use-prompt-actions.ts — seed optimistic sidebar preview with the first message.
• store/session.test.ts — 4 unit tests (preserve omitted working sessions, no duplication, never resurrect deleted, no-op when nothing working).

Test plan

• vitest run src/store/session.test.ts — 6/6 pass
• npm run type-check — clean
• eslint on changed files — 0 new problems (pre-existing warnings/import-sort error already on main)
• Full src/ suite — 232 pass; the single failure (pane-shell width test) is pre-existing on main and unrelated (verified via git stash)
• Manual: Ctrl-N → message ×3, let one finish, confirm the other two remain (labeled with their message) and finish normally