fix(desktop): Branch button silently does nothing inside a branched chat tile (#71969)

* fix: Branch button is a dead no-op inside a branched chat tile

session-tile.tsx wired onBranchInNewChat to () => undefined for
tiled/branched sessions (nested branching isn't supported there), but
the button in AssistantMessage's action bar rendered unconditionally
regardless of whether a real handler was supplied. The button looked
clickable but silently did nothing, with no visual feedback.

- AssistantMessage now only renders the Branch button when
  onBranchInNewChat is actually provided, matching the existing
  pattern used for onDismissError/onRestoreToMessage.
- session-tile.tsx no longer passes a no-op handler; the prop is
  simply omitted so the button doesn't render in tiles.
- onBranchInNewChat is now optional on ChatViewProps, and the
  latestChatActions passthrough wrapper uses the existing
  latestOptional helper instead of an unconditional call.

* test: assert Branch button visibility matches handler presence

Adds coverage for the bug #2 fix: renders Thread with and without an
onBranchInNewChat handler and asserts the Branch in new chat button
is shown only when a real handler is supplied, hidden otherwise -
covering both the normal open-chat case and the session-tile
(branched chat) case that used to leave a dead, clickable button.

apps/desktop/src/app/chat/index.tsx

@@ -71,7 +71,7 @@ interface ChatViewProps extends Omit<React.ComponentProps<'div'>, 'onSubmit'> {
   onCancel: () => Promise<void> | void
   onAddContextRef: (refText: string, label?: string, detail?: string) => void
   onAddUrl: (url: string) => void
-  onBranchInNewChat: (messageId: string) => void
+  onBranchInNewChat?: (messageId: string) => void
   maxVoiceRecordingSeconds?: number
   onAttachImageBlob: (blob: Blob) => Promise<boolean | void> | boolean | void
   onAttachDroppedItems: (candidates: DroppedFile[]) => Promise<boolean | void> | boolean | void

apps/desktop/src/app/chat/session-tile.tsx

@@ -170,7 +170,6 @@ function TileChat({
           onAddUrl={url => composer.addContextRefAttachment(`@url:${formatRefValue(url)}`, url)}
           onAttachDroppedItems={composer.attachDroppedItems}
           onAttachImageBlob={composer.attachImageBlob}
-          onBranchInNewChat={() => undefined}
           onCancel={actions.cancelRun}
           onDeleteSelectedSession={() => undefined}
           onDismissError={actions.dismissError}

apps/desktop/src/app/contrib/latest-actions.ts

@@ -34,7 +34,7 @@ export function latestChatActions(actions: ChatActions): ChatActions {
     onAddUrl: (...args) => actions.onAddUrl(...args),
     onAttachDroppedItems: (...args) => actions.onAttachDroppedItems(...args),
     onAttachImageBlob: (...args) => actions.onAttachImageBlob(...args),
-    onBranchInNewChat: (...args) => actions.onBranchInNewChat(...args),
+    onBranchInNewChat: latestOptional(() => actions.onBranchInNewChat),
     onCancel: (...args) => actions.onCancel(...args),
     onDeleteSelectedSession: (...args) => actions.onDeleteSelectedSession(...args),
     onDismissError: latestOptional(() => actions.onDismissError),

apps/desktop/src/components/assistant-ui/thread/assistant-message.test.tsx

@@ -0,0 +1,90 @@
+// Bug #2: the Branch-in-new-chat button used to render unconditionally even
+// when its handler was a no-op (session-tile.tsx passed `() => undefined`
+// for branched/tiled chats, where nested branching isn't supported). That
+// left a visibly clickable button that silently did nothing. The fix makes
+// AssistantMessage's action bar hide the button entirely when no handler is
+// supplied, matching how onDismissError/onRestoreToMessage already behave.
+import { AssistantRuntimeProvider, type ThreadMessage, useExternalStoreRuntime } from '@assistant-ui/react'
+import { cleanup, render, screen } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import { Thread } from '.'
+
+const createdAt = new Date('2026-05-01T00:00:00.000Z')
+
+class TestResizeObserver {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+vi.stubGlobal('ResizeObserver', TestResizeObserver)
+vi.stubGlobal('requestAnimationFrame', (callback: FrameRequestCallback) =>
+  window.setTimeout(() => callback(performance.now()), 0)
+)
+vi.stubGlobal('cancelAnimationFrame', (id: number) => window.clearTimeout(id))
+vi.stubGlobal('CSS', { escape: (str: string) => str })
+Element.prototype.scrollTo = function scrollTo() {}
+
+afterEach(() => {
+  cleanup()
+})
+
+function userMessage(): ThreadMessage {
+  return {
+    id: 'user-1',
+    role: 'user',
+    content: [{ type: 'text', text: 'question one' }],
+    attachments: [],
+    createdAt,
+    metadata: { custom: {} }
+  } as ThreadMessage
+}
+
+function assistantMessage(): ThreadMessage {
+  return {
+    id: 'assistant-1',
+    role: 'assistant',
+    content: [{ type: 'text', text: 'done' }],
+    status: { type: 'complete', reason: 'stop' },
+    createdAt,
+    metadata: {
+      unstable_state: null,
+      unstable_annotations: [],
+      unstable_data: [],
+      steps: [],
+      custom: {}
+    }
+  } as ThreadMessage
+}
+
+function Harness({ onBranchInNewChat }: { onBranchInNewChat?: (messageId: string) => void }) {
+  const runtime = useExternalStoreRuntime<ThreadMessage>({
+    messages: [userMessage(), assistantMessage()],
+    isRunning: false,
+    onNew: async () => {}
+  })
+  return (
+    <AssistantRuntimeProvider runtime={runtime}>
+      <Thread onBranchInNewChat={onBranchInNewChat} />
+    </AssistantRuntimeProvider>
+  )
+}
+
+describe('AssistantMessage branch button visibility (bug #2 fix)', () => {
+  it('shows the Branch in new chat button when a handler is provided (open chat)', async () => {
+    render(<Harness onBranchInNewChat={() => undefined} />)
+
+    expect(await screen.findByRole('button', { name: 'Branch in new chat' })).toBeTruthy()
+  })
+
+  it('hides the Branch in new chat button when no handler is provided (session-tile / branched chat)', async () => {
+    render(<Harness />)
+
+    // Wait for the assistant message to actually mount before asserting
+    // absence, so a missing button isn't just a false negative from an
+    // unrendered message.
+    await screen.findByText('done')
+
+    expect(screen.queryByRole('button', { name: 'Branch in new chat' })).toBeNull()
+  })
+})

apps/desktop/src/components/assistant-ui/thread/assistant-message.tsx

@@ -151,15 +151,17 @@ const AssistantActionBar: FC<MessageActionProps> = ({ messageId, getMessageText,
         data-slot="aui_msg-actions"
       >
         <MessageAge />
-        <TooltipIconButton
-          onClick={() => {
-            triggerHaptic('selection')
-            onBranchInNewChat?.(messageId)
-          }}
-          tooltip={copy.branchNewChat}
-        >
-          <GitForkIcon className="size-3.5" />
-        </TooltipIconButton>
+        {onBranchInNewChat && (
+          <TooltipIconButton
+            onClick={() => {
+              triggerHaptic('selection')
+              onBranchInNewChat(messageId)
+            }}
+            tooltip={copy.branchNewChat}
+          >
+            <GitForkIcon className="size-3.5" />
+          </TooltipIconButton>
+        )}
         <CopyButton appearance="icon" buttonSize="icon" label={copy.copy} text={getMessageText} />
         <ReadAloudButton getText={getMessageText} messageId={messageId} />
         <ActionBarPrimitive.Reload asChild>