fix(desktop): scope branch-opens-primary to the currently selected session

forkBranch was unconditionally routing every branched session into the
main pane via resumeSession, including sidebar/background branches of
a session the user isn't currently viewing. That reintroduces the
#69750 focus-stealing bug for that path: branching a different session
from the sidebar yanked the active view away from whatever was open.
Only take over the main pane when the branch's parent is the session
already selected; otherwise keep opening it as its own tile.

apps/desktop/src/app/session/hooks/use-session-actions.test.tsx

@@ -1437,6 +1437,7 @@ describe('branchStoredSession desktop source tagging', () => {
         navigate={navigate}
         onReady={branch => (branchStoredSession = branch)}
         requestGateway={requestGateway}
+        selectedStoredSessionId="stored-parent"
       />
     )
     await waitFor(() => expect(branchStoredSession).not.toBeNull())
@@ -1451,6 +1452,56 @@ describe('branchStoredSession desktop source tagging', () => {
     expect($sessionTiles.get().some(tile => tile.storedSessionId === 'branch-stored')).toBe(false)
   })
 
+  it('keeps the current view when branching a different session from the sidebar (does not reintroduce #69750)', async () => {
+    const requestGateway = vi.fn(async (method: string) => {
+      if (method === 'session.create') {
+        return { session_id: 'branch-runtime', stored_session_id: 'branch-stored' } as never
+      }
+
+      if (method === 'session.resume') {
+        return {
+          info: {},
+          message_count: 0,
+          messages: [],
+          resumed: 'branch-stored',
+          session_id: 'branch-runtime',
+          session_key: 'branch-stored'
+        } as never
+      }
+
+      return {} as never
+    })
+
+    // The user is looking at "stored-other" and right-clicks a DIFFERENT,
+    // unrelated session ("stored-parent") in the sidebar to branch it.
+    setSessions([storedSession({ id: 'stored-parent', message_count: 1 })])
+    setSelectedStoredSessionId('stored-other')
+    vi.mocked(getAllSessionMessages).mockResolvedValue({
+      messages: [{ content: 'branch me', role: 'user', timestamp: 1 }],
+      session_id: 'stored-parent'
+    } as never)
+
+    const navigate = vi.fn()
+    let branchStoredSession: ((storedSessionId: string) => Promise<boolean>) | null = null
+    render(
+      <BranchHarness
+        navigate={navigate}
+        onReady={branch => (branchStoredSession = branch)}
+        requestGateway={requestGateway}
+        selectedStoredSessionId="stored-other"
+      />
+    )
+    await waitFor(() => expect(branchStoredSession).not.toBeNull())
+
+    await expect(branchStoredSession!('stored-parent')).resolves.toBe(true)
+
+    // Branching a session that is not the one currently open must not steal
+    // the user's active view — "stored-other" stays selected.
+    expect($selectedStoredSessionId.get()).toBe('stored-other')
+    // The branch instead opens as its own tile.
+    expect($sessionTiles.get().some(tile => tile.storedSessionId === 'branch-stored')).toBe(true)
+  })
+
   it('tags desktop branch sessions as desktop sessions', async () => {
     let createParams: Record<string, unknown> | undefined
 

apps/desktop/src/app/session/hooks/use-session-actions/index.ts

@@ -1656,11 +1656,20 @@ export function useSessionActions({
           updateSessionState(branched.session_id, state => ({ ...state, ...runtimeInfo }), routedSessionId)
         }
 
-        // Load the branch as the primary session so it opens in the main
-        // workspace, not just a sidebar row. resumeSession reuses the runtime
-        // warm-cached above (ensureSessionState/updateSessionState) instead of
-        // an extra resume RPC.
-        await resumeSession(routedSessionId)
+        // Only take over the main pane when the chat being branched is the one
+        // already open there — branching a background/sidebar session must
+        // not yank the user's current view away from what they're looking at
+        // (the #69750 focus-stealing bug, reintroduced if this fires
+        // unconditionally). resumeSession reuses the runtime warm-cached above
+        // (ensureSessionState/updateSessionState) instead of an extra resume RPC.
+        if (parentStoredId !== null && selectedStoredSessionIdRef.current === parentStoredId) {
+          await resumeSession(routedSessionId)
+        } else {
+          openSessionTile(routedSessionId, 'center')
+          patchSessionTile(routedSessionId, { runtimeId: branched.session_id })
+          revealTreePane(`session-tile:${routedSessionId}`)
+        }
+
         broadcastSessionsChanged()
 
         return true
@@ -1674,7 +1683,15 @@ export function useSessionActions({
         }, 0)
       }
     },
-    [copy, creatingSessionRef, ensureSessionState, requestGateway, resumeSession, updateSessionState]
+    [
+      copy,
+      creatingSessionRef,
+      ensureSessionState,
+      requestGateway,
+      resumeSession,
+      selectedStoredSessionIdRef,
+      updateSessionState
+    ]
   )
 
   // Branch the open chat — optionally from a specific message — off its live transcript.