fix: harden resume and SSE replay

frontend/src/shell/AppShell.tsx

@@ -100,22 +100,29 @@ export function AppShell() {
     });
   }
   async function doResume(id: string) {
-    const s = sessions.find((x) => x.id === id) ?? null;
     const reconnectSameSession = activeSessionId === id;
-    // Optimistic switch: change to the session view IMMEDIATELY so the click feels
-    // instant (the resume POST spawns a Pi process and can take seconds). The
-    // working spinner shows straight away; the backend calls run after.
-    setPanelSession(null);
-    resetSession();
-    recordLifecycle("Resuming session");
-    setAwaitingQuestion(false);
-    // Optimistic: the resume POST is about to hand the ball to the harness.
-    setAgentActive(true);
-    setActiveSessionId(id);
     try {
-      await resumeSession(id);
-      if (reconnectSameSession) setStreamGeneration((value) => value + 1);
-      // Optimistic phase paint: colour the re-entry phase before the first gate.
+      const result = await resumeSession(id);
+      setPanelSession(null);
+      setAwaitingQuestion(false);
+
+      // A running/waiting runtime for the currently selected session is already bound to this
+      // store and EventSource. Reopening it would replay state and can lose in-flight delivery.
+      if (result.alreadyActive && reconnectSameSession) return;
+
+      resetSession();
+      if (!result.alreadyActive) {
+        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.
+      if (!result.alreadyActive && reconnectSameSession) {
+        setStreamGeneration((value) => value + 1);
+      }
+
+      // Paint the persisted re-entry phase while the replacement stream starts replaying.
       // The manifest's `phase` is the 1-based current phase (1..8).
       try {
         const m = (await getSession(id)) as { phase?: number };
@@ -126,10 +133,6 @@ export function AppShell() {
         /* non-fatal: the first gate will set the phase */
       }
     } catch {
-      // Revert the optimistic switch and restore the panel.
-      resetSession();
-      setActiveSessionId(null);
-      if (s) setPanelSession(s);
       toast.error("Failed to resume session.");
     }
   }