fix: make session resume atomic across restarts

frontend/src/shell/AppShell.tsx

@@ -32,7 +32,8 @@ import { useEffect, useMemo, useRef, useState } from "react";
  */
 export function AppShell() {
   const [activeSessionId, setActiveSessionId] = useState<string | null>(null);
-  const [streamGeneration, setStreamGeneration] = useState(0);
+  const activeSessionIdRef = useRef<string | null>(null);
+  const [streamCursorResetEpoch, setStreamCursorResetEpoch] = useState(0);
   const [creatingSession, setCreatingSession] = useState(false);
   const [awaitingQuestion, setAwaitingQuestion] = useState(false);
   const { data: sessions = [] } = useQuery<SessionSummary[]>({
@@ -65,6 +66,12 @@ export function AppShell() {
   const selectedSessions = sessions.filter((session) => selectedSessionIds.has(session.id));
   const allSessionsSelected = sessions.length > 0 && selectedSessions.length === sessions.length;
 
+  function selectActiveSession(id: string | null) {
+    // Keep async Resume completions synchronized before React commits the state update.
+    activeSessionIdRef.current = id;
+    setActiveSessionId(id);
+  }
+
   // A background refresh can remove a session (for example from another browser).
   // Keep the local selection aligned with the authoritative list.
   useEffect(() => {
@@ -100,9 +107,9 @@ export function AppShell() {
     });
   }
   async function doResume(id: string) {
-    const reconnectSameSession = activeSessionId === id;
     try {
       const result = await resumeSession(id);
+      const reconnectSameSession = activeSessionIdRef.current === id;
       setPanelSession(null);
       setAwaitingQuestion(false);
 
@@ -115,11 +122,11 @@ export function AppShell() {
         recordLifecycle("Resuming session");
         setAgentActive(true);
       }
-      setActiveSessionId(id);
-      // The backend has now completed clear/rebind. Recreate a same-id source with its cursor;
-      // switching ids naturally creates a new source and resets the cursor in the stream hook.
+      selectActiveSession(id);
+      // A cold runtime starts a fresh SSE id sequence. Recreate a same-id source only after
+      // Resume succeeds, and explicitly discard the old runtime's cursor.
       if (!result.alreadyActive && reconnectSameSession) {
-        setStreamGeneration((value) => value + 1);
+        setStreamCursorResetEpoch((value) => value + 1);
       }
 
       // Paint the persisted re-entry phase while the replacement stream starts replaying.
@@ -182,7 +189,7 @@ export function AppShell() {
         targets.filter((_, index) => results[index].status === "fulfilled").map((session) => session.id),
       );
       if (deletedIds.has(panelSession?.id ?? "")) setPanelSession(null);
-      if (deletedIds.has(activeSessionId ?? "")) { resetSession(); setActiveSessionId(null); }
+      if (deletedIds.has(activeSessionId ?? "")) { resetSession(); selectActiveSession(null); }
       setSelectedSessionIds((current) => new Set([...current].filter((id) => !deletedIds.has(id))));
       refresh();
       if (deletedIds.size !== targets.length) {
@@ -226,7 +233,7 @@ export function AppShell() {
   // session sits idle or a gate awaits the reviewer (pendingWidget).
   const running = working && !finalized;
 
-  useSessionStream(activeSessionId, streamGeneration);
+  useSessionStream(activeSessionId, 0, streamCursorResetEpoch);
 
   // A backend "session_exit" system event (e.g. the replay server emitting it
   // when the reviewer picks "Esci") asks us to leave the live session view and
@@ -240,7 +247,7 @@ export function AppShell() {
       // Never let a streamed event terminate the managed Pi child. Only the
       // explicit “Stop & save” action is allowed to call /close.
       resetSession();
-      setActiveSessionId(null);
+      selectActiveSession(null);
       setAwaitingQuestion(false);
     }
     // The final workflow turn ends with the session already finalized on disk:
@@ -254,7 +261,7 @@ export function AppShell() {
     resetSession();
     setAwaitingQuestion(true);
     setCreatingSession(false);
-    setActiveSessionId(null);
+    selectActiveSession(null);
     // Best effort only: session creation keeps the authoritative readiness gate.
     // Composer focus is deliberately independent of this network request.
     void prewarmRuntime().catch(() => undefined);
@@ -269,7 +276,7 @@ export function AppShell() {
   function finishSessionCreation(id: string) {
     // React batches these updates, preserving the provisional view and timer
     // while useSessionStream opens the durable session's SSE channel.
-    setActiveSessionId(id);
+    selectActiveSession(id);
     setCreatingSession(false);
     setAwaitingQuestion(false);
     refresh();
@@ -287,7 +294,7 @@ export function AppShell() {
       await closeSession(activeSessionId);
     } finally {
       resetSession();
-      setActiveSessionId(null);
+      selectActiveSession(null);
       setAwaitingQuestion(false);
     }
   }