feat(desktop): open a tab from the sidebar "+" when a chat is loaded

The sidebar "+" ran startFreshSessionDraft, so it replaced whatever was on
screen — ⌘N behavior. With a conversation already open (possibly mid-turn)
that discards it to make room for a blank draft, which is not what a create
affordance should do. The tab-strip "+" and ⌘T already stack a new tab.

Route the sidebar button through the same path once a session is loaded,
falling back to the fresh-draft path when the surface is empty and there is
no tab worth preserving. openNewSessionTile now takes an optional cwd so the
new tab stays anchored to the clicked project/worktree lane.

apps/desktop/src/app/contrib/wiring.tsx

@@ -89,7 +89,7 @@ import { useRouteResume } from '../session/hooks/use-route-resume'
 import { useSessionActions } from '../session/hooks/use-session-actions'
 import { useSessionListActions } from '../session/hooks/use-session-list-actions'
 import { useSessionStateCache } from '../session/hooks/use-session-state-cache'
-import { startWorkspaceSession } from '../session/workspace-session-target'
+import { newSessionOpensTab, startWorkspaceSession } from '../session/workspace-session-target'
 import { useOverlayRouting } from '../shell/hooks/use-overlay-routing'
 import { useWindowControlsOverlayWidth } from '../shell/hooks/use-window-controls-overlay-width'
 import { titlebarControlsPosition } from '../shell/titlebar'
@@ -484,8 +484,16 @@ export function ContribWiring({ children }: { children: ReactNode }) {
   // New session anchored to a workspace (sidebar "+" on a project/worktree).
   // Seeds cwd + branch from the clicked workspace; an explicit worktree path
   // also drills the sidebar into that project so the new lane is visible.
+  // Once a chat is loaded the button opens a TAB instead of replacing it —
+  // see newSessionOpensTab.
   const startSessionInWorkspace = useCallback(
     (path: null | string) => {
+      if (newSessionOpensTab(activeSessionIdRef.current, $selectedStoredSessionId.get())) {
+        void openNewSessionTile('center', { cwd: path, listed: false })
+
+        return
+      }
+
       startWorkspaceSession({
         activeSessionIdRef,
         followActiveSessionCwd,
@@ -495,7 +503,7 @@ export function ContribWiring({ children }: { children: ReactNode }) {
         startFreshSessionDraft
       })
     },
-    [activeSessionIdRef, requestGateway, startFreshSessionDraft]
+    [activeSessionIdRef, openNewSessionTile, requestGateway, startFreshSessionDraft]
   )
 
   // Composer "branch off into a new worktree": open a fresh session anchored

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

@@ -476,13 +476,14 @@ export function useSessionActions({
    *  list (Cursor-style draft tab); it surfaces on the next refresh once the
    *  first message persists a turn. "Open in split" keeps the listed behavior. */
   const openNewSessionTile = useCallback(
-    async (dir: TileDock = 'right', options?: { listed?: boolean }) => {
+    async (dir: TileDock = 'right', options?: { cwd?: null | string; listed?: boolean }) => {
       const listed = options?.listed ?? true
 
       try {
-        // Fresh tile → the resolved new-session cwd (project/default), not the
-        // primary composer's live cwd.
-        const params = await desktopSessionCreateParams(resolveNewSessionCwd().trim())
+        // Fresh tile → the caller's workspace when one was named (the sidebar
+        // "+" on a project/worktree lane), else the resolved new-session cwd
+        // (project/default) — never the primary composer's live cwd.
+        const params = await desktopSessionCreateParams((options?.cwd || resolveNewSessionCwd()).trim())
         const created = await requestGateway<SessionCreateResponse>('session.create', params)
         const stored = created.stored_session_id
 

apps/desktop/src/app/session/workspace-session-target.test.ts

@@ -9,7 +9,7 @@ import {
   setNewChatWorkspaceTarget
 } from '@/store/session'
 
-import { startWorkspaceSession } from './workspace-session-target'
+import { newSessionOpensTab, startWorkspaceSession } from './workspace-session-target'
 
 function deferred<T>() {
   let resolve!: (value: T) => void
@@ -21,6 +21,29 @@ function deferred<T>() {
   return { promise, resolve }
 }
 
+/**
+ * The sidebar "+" is a create affordance, not a discard one: with a chat
+ * already loaded it must stack a tab (⌘T) rather than replace the surface
+ * (⌘N), which would throw away a conversation that may still be mid-turn.
+ */
+describe('newSessionOpensTab', () => {
+  it('opens a tab once a conversation is on screen', () => {
+    expect(newSessionOpensTab('runtime-a', 'stored-a')).toBe(true)
+  })
+
+  it('opens a tab for a live runtime whose stored id has not landed yet', () => {
+    expect(newSessionOpensTab('runtime-a', null)).toBe(true)
+  })
+
+  it('opens a tab for a selected session still resuming into a runtime', () => {
+    expect(newSessionOpensTab(null, 'stored-a')).toBe(true)
+  })
+
+  it('replaces the empty surface when nothing is open', () => {
+    expect(newSessionOpensTab(null, null)).toBe(false)
+  })
+})
+
 describe('startWorkspaceSession', () => {
   afterEach(() => {
     setCurrentBranch('')

apps/desktop/src/app/session/workspace-session-target.ts

@@ -17,6 +17,19 @@ interface WorkspaceSessionOptions {
   startFreshSessionDraft: (options?: { workspaceTarget: string }) => void
 }
 
+/**
+ * Should the sidebar "+" open a NEW TAB rather than replace what's on screen?
+ *
+ * "+" is a create affordance, never a discard one. Once a conversation is
+ * loaded, replacing it with a blank draft would throw away a chat that may
+ * still be mid-turn, so the button stacks a tab beside it (⌘T) instead of
+ * taking over the surface (⌘N). With nothing open there is no tab worth
+ * preserving and the cheaper fresh-draft path applies.
+ */
+export function newSessionOpensTab(activeSessionId: null | string, selectedStoredSessionId: null | string): boolean {
+  return Boolean(activeSessionId || selectedStoredSessionId)
+}
+
 export function startWorkspaceSession({
   activeSessionIdRef,
   followActiveSessionCwd: followCwd = followActiveSessionCwd,