fix(components): disable Side Chat until there is an assistant turn to fork

packages/components/src/components/sessions/session-detail.tsx

@@ -1077,6 +1077,18 @@ const SessionDetail = ({
       draftTabs.some((draft) => draft.id === activeTabSessionIdRaw);
     return stillExists ? activeTabSessionIdRaw : sessionId;
   }, [activeTabSessionIdRaw, visibleChildSessions, draftTabs, sessionId]);
+  /**
+   * The active conversation's fork target, as a value a RENDER can read.
+   *
+   * `chatRefsMap` is a ref, so `getLastAssistantTurnId()` answers only when
+   * somebody asks — which is fine for a click and useless for a disabled state.
+   * `setChatTabRef` mirrors it into state below; this ref is how that callback
+   * knows which tab is the active one without taking a dependency and changing
+   * its identity on every tab switch.
+   */
+  const activeTabSessionIdForForkRef = useRef<string>(activeTabSessionId);
+  activeTabSessionIdForForkRef.current = activeTabSessionId;
+  const [activeTabAssistantTurnId, setActiveTabAssistantTurnId] = useState<string | null>(null);
   const activeSessionTabId = useMemo<SessionId | null>(() => {
     if (activeTabSessionId === sessionId) return sessionId;
     return visibleChildSessions.find((s) => s.id === activeTabSessionId)?.id ?? null;
@@ -1689,6 +1701,17 @@ const SessionDetail = ({
   const setChatTabRef = useCallback(
     (tabId: string, ref: SessionChatInterfaceHandle | DraftSessionChatInterfaceHandle | null) => {
       chatRefsMap.current.set(tabId, ref);
+      // A DETACH IS IGNORED, and that is what keeps this from looping. Every
+      // render of this page hands each surface a fresh ref arrow, so React
+      // calls it with `null` and then with the handle inside one commit; taking
+      // the `null` would queue a state change on every commit for ever.
+      // `useImperativeHandle` re-attaches whenever the handle's own
+      // dependencies change — `lastCompletedAssistantMessageId` among them — so
+      // the attach alone carries every value this needs.
+      if (ref === null || tabId !== activeTabSessionIdForForkRef.current) return;
+      setActiveTabAssistantTurnId(
+        'getLastAssistantTurnId' in ref ? ref.getLastAssistantTurnId() : null
+      );
     },
     []
   );
@@ -3355,10 +3378,17 @@ const SessionDetail = ({
       label: t('sessions.detailTabs.sideSession', 'Side Chat'),
       kind: 'session',
       pending: isCreatingSideSession,
-      disabled: launcherState === 'disabled' || isCreatingSideSession,
+      // The third reason there is nothing to launch, beside an offline machine
+      // and a launch already in flight: a fork needs a completed assistant turn,
+      // and `forkActiveConversation` refuses with a toast without one.
+      disabled:
+        launcherState === 'disabled' ||
+        isCreatingSideSession ||
+        activeTabAssistantTurnId === null,
     };
   }, [
     activeDraftTab,
+    activeTabAssistantTurnId,
     activeTabSession,
     activeTabSessionMachineOnlineStatus,
     canForkSession,