fix(flow-chat): preserve side-question composer target

Validate target cleanup against live session and pane state so a stale
composer snapshot does not clear a newly selected side conversation.

Add regression coverage for selection before the session snapshot catches
up, including guards against sending to the main session.

Author: wsp1911

src/web-ui/src/flow_chat/hooks/useChatInputTargets.test.tsx

@@ -15,10 +15,10 @@ describe('composer target lifecycle through production stores', () => {
   let root: Root;
   let container: HTMLDivElement;
   let target: ReturnType<typeof useChatInputTargets>;
-  function Probe() {
+  function Probe({ sessions }: { sessions?: ReadonlyMap<string, Session> }) {
     const state = flowChatStore.getState();
     target = useChatInputTargets({ currentSessionId: state.activeSessionId,
-      sessions: state.sessions, auxiliaryEnabled: true,
+      sessions: sessions ?? state.sessions, auxiliaryEnabled: true,
       activeChild: selectActiveBtwSessionTab(useAgentCanvasStore.getState())?.content.data as BtwSessionPanelData | undefined });
     return null;
   }
@@ -61,6 +61,24 @@ describe('composer target lifecycle through production stores', () => {
     expect(target.isTargetCurrent('a')).toBe(false);
   });
 
+  it('keeps an explicit side-question target while the composer session snapshot catches up', () => {
+    render();
+    const previousSessions = flowChatStore.getState().sessions;
+    setSession('draft', { sessionKind: 'btw', parentSessionId: 'main',
+      isTransient: true, agentBackedTransient: true });
+    act(() => {
+      useAgentCanvasStore.getState().addTab(buildBtwSessionPanelContent('draft', 'main', {}), 'active');
+      target.selectTargetSession('draft');
+      root.render(<Probe sessions={previousSessions} />);
+    });
+    expect(target.isTargetCurrent('draft')).toBe(true);
+    expect(target.isTargetCurrent('main')).toBe(false);
+
+    render();
+    expect(target.effectiveSessionId).toBe('draft');
+    expect(target.inputTarget).toBe('btw');
+  });
+
   it('does not transfer selection to a newly opened sibling or revive old selection', () => {
     openChild('a');
     act(() => target.selectTargetSession('a'));

src/web-ui/src/flow_chat/hooks/useChatInputTargets.ts

@@ -66,17 +66,6 @@ export function useChatInputTargets(options: {
     candidates: candidatesFor(options.currentSessionId, options.activeChild, options.sessions),
   });
 
-  // Resolve synchronously in render; cleanup only prevents a removed selection
-  // from reviving when a closed pane or an unavailable child reappears later.
-  useLayoutEffect(() => {
-    if (requested && (requested.surfaceId !== surfaceId
-      || requested.parentSessionId !== options.currentSessionId
-      || requested.sessionId !== resolution.effectiveSessionId)) {
-      requestedRef.current = null;
-      setRequested(null);
-    }
-  }, [options.currentSessionId, requested, resolution.effectiveSessionId, surfaceId]);
-
   const resolveLive = useCallback(() => {
     const current = optionsRef.current;
     const state = FlowChatStore.getInstance().getState();
@@ -91,6 +80,18 @@ export function useChatInputTargets(options: {
     });
   }, []);
 
+  // A new side-question pane can render before ChatInput's session snapshot
+  // catches up. Only authoritative state may discard an explicit selection.
+  // Closed panes and unavailable children must still release their selection.
+  useLayoutEffect(() => {
+    if (requested && (requested.surfaceId !== surfaceId
+      || requested.parentSessionId !== options.currentSessionId
+      || requested.sessionId !== resolveLive().effectiveSessionId)) {
+      requestedRef.current = null;
+      setRequested(null);
+    }
+  }, [options.currentSessionId, requested, resolution.effectiveSessionId, resolveLive, surfaceId]);
+
   const selectTargetSession = useCallback((sessionId: string) => {
     const current = optionsRef.current;
     if (!resolveLive().targets.some(target => target.sessionId === sessionId)) return;