fix: clear stale inflight UI state (closes #2795, squashed from 5 commits)

Cherry-pick of PR #2796 by @ai-ag2026, squashed from 5 author commits onto current master:
- dcee0563 fix: drop stale optimistic sidebar rows
- 3a73400d fix: clear stale busy state before send
- 46c3b902 fix: preserve server idle rows during optimistic merge
- de51d271 fix: let chat start survive pre-start UI errors
- d2f5c906 fix: hide nonfatal pre-start send warnings

Authorship preserved via --author. Code-only squash (no CHANGELOG).

tests/test_inflight_send_start_race.py

@@ -24,7 +24,7 @@ def _function_body(src: str, name: str) -> str:
 def test_send_preserves_optimistic_messages_across_chat_start_await():
     """send() must not dereference INFLIGHT[activeSid] after await without a fallback."""
     body = _function_body(MESSAGES_JS, "send")
-    setup_idx = body.index("const optimisticMessages=[...S.messages];")
+    setup_idx = body.index("optimisticMessages=[...S.messages];")
     inflight_idx = body.index("INFLIGHT[activeSid]={messages:optimisticMessages")
     await_idx = body.index("const startData=await api('/api/chat/start'")
     save_idx = body.index("saveInflightState(activeSid,{streamId", await_idx)
@@ -49,3 +49,90 @@ def test_stale_inflight_purge_preserves_current_send_before_stream_id_exists():
     skip_idx = body.index("_sendInProgress")
     delete_idx = body.index("delete INFLIGHT[sid];")
     assert skip_idx < delete_idx, "the current-send skip must run before any purge deletion"
+
+
+def test_send_clears_stale_busy_state_before_queue_branch():
+    """A stale client-only busy flag must not divert a new user turn into the invisible queue."""
+    body = _function_body(MESSAGES_JS, "send")
+
+    assert "_clearStaleBusyStateBeforeSend" in body, (
+        "send() should reconcile client-only stale busy state before deciding busy/queue mode"
+    )
+    reconcile_idx = body.index("_clearStaleBusyStateBeforeSend")
+    busy_branch_idx = body.index("if(S.busy||compressionRunning)")
+    chat_start_idx = body.index("api('/api/chat/start'")
+    assert reconcile_idx < busy_branch_idx < chat_start_idx, (
+        "stale busy reconciliation must run before the queue branch and before /api/chat/start"
+    )
+
+
+def test_pre_start_optimistic_ui_helpers_cannot_block_chat_start():
+    """Optional optimistic UI helpers must not strand a local bubble before /api/chat/start."""
+    body = _function_body(MESSAGES_JS, "send")
+    helper_body = _function_body(MESSAGES_JS, "_runOptionalPreStartUiStep")
+
+    optimistic_idx = body.index("S.messages.push(userMsg);renderMessages();appendThinking('',{pending:true});setBusy(true);")
+    chat_start_idx = body.index("api('/api/chat/start'")
+    pre_start = body[optimistic_idx:chat_start_idx]
+
+    assert "try" in helper_body and "catch" in helper_body, (
+        "optional pre-start UI helper wrapper must catch errors before /api/chat/start"
+    )
+    assert "setStatus(`UI warning before send:" not in helper_body, (
+        "non-fatal pre-start UI helper failures should stay in the console; visible status flashes "
+        "look like real send errors even though /api/chat/start continues"
+    )
+    assert "_runOptionalPreStartUiStep" in pre_start, (
+        "send() should wrap optimistic sidebar/title/polling helpers before /api/chat/start"
+    )
+    assert "upsertActiveSessionForLocalTurn" in pre_start and "applySessionTitleUpdate" in pre_start
+
+
+def test_pre_start_optimistic_block_cannot_prevent_chat_start():
+    """Any pre-start UI/storage exception must still fall through to /api/chat/start."""
+    body = _function_body(MESSAGES_JS, "send")
+    optimistic_idx = body.index("S.messages.push(userMsg);renderMessages();appendThinking('',{pending:true});setBusy(true);")
+    chat_start_idx = body.index("api('/api/chat/start'")
+    pre_start = body[optimistic_idx:chat_start_idx]
+
+    assert "}catch(preStartError){" in pre_start, (
+        "The whole optimistic pre-start block needs a catch, not only individual optional helpers"
+    )
+    assert "continuing to /api/chat/start" in pre_start, (
+        "The recovery path should document that chat/start must still execute"
+    )
+    assert pre_start.rindex("}catch(preStartError){") < chat_start_idx, (
+        "pre-start catch must be before the /api/chat/start call"
+    )
+
+
+def test_server_absent_optimistic_first_turn_rows_are_not_kept_forever():
+    """A local first-turn sidebar row must expire when /api/chat/start never persisted it."""
+    body = _function_body(SESSIONS_JS, "_mergeOptimisticFirstTurnSessions")
+
+    assert "_shouldKeepLocalOnlyOptimisticSessionRow(local)" in body, (
+        "server-absent optimistic rows need an explicit keep/drop gate"
+    )
+    keep_idx = body.index("if(_shouldKeepLocalOnlyOptimisticSessionRow(local))")
+    append_idx = body.index("merged.push({...local,is_streaming:true});")
+    drop_idx = body.index("_dropStaleOptimisticSessionRow(sid);", append_idx)
+    assert keep_idx < append_idx < drop_idx, (
+        "local optimistic rows may only be appended inside the explicit keep gate"
+    )
+    drop_body = _function_body(SESSIONS_JS, "_dropStaleOptimisticSessionRow")
+    assert "clearInflightState(sid)" in drop_body, (
+        "dropping a phantom row should also clear persisted browser recovery state"
+    )
+
+
+def test_server_idle_row_wins_over_stale_optimistic_count():
+    """If the server says the row is idle, stale local message_count/title must not win."""
+    body = _function_body(SESSIONS_JS, "_mergeOptimisticFirstTurnSessions")
+
+    assert "const keepLocalOptimistic=" in body
+    assert "message_count:keepLocalOptimistic?Math.max(localCount,fetchedCount):fetchedCount" in body, (
+        "stale optimistic message_count must not override a confirmed idle server row"
+    )
+    assert "title:keepLocalOptimistic?(local.title||fetched.title):fetched.title" in body, (
+        "stale optimistic provisional title must not override a confirmed idle server row"
+    )