fix(desktop): anchor project trunk + at first repo root when primary path unset

Clicking the project-group trunk + passed the null wire path through, which
downstream treats as the reserved Home/detached signal, silently creating a
global session. Fall back to the first repo root for path-less explicit
projects; Home keeps null. Covers click and new-session drag cwd.

apps/desktop/src/app/chat/sidebar/projects/overview-row.test.tsx

@@ -180,4 +180,26 @@ describe('ProjectOverviewRow', () => {
 
     expect(onNewSession).toHaveBeenCalledWith(null)
   })
+
+  // #124808: a real project whose primary_path was never set (multi-folder /
+  // path-less explicit project) still carries repo roots. Its trunk "+" must
+  // anchor the new session at the first repo root, not pass the null wire
+  // path through — null is the reserved Home/detached signal downstream, so
+  // the click silently created a global detached session.
+  it('anchors the trunk "+" at the first repo root when the project has no primary path', () => {
+    const multi = {
+      id: 'p_multi',
+      label: 'Multi',
+      path: null,
+      repos: [{ id: 'r1', label: 'app', path: '/work/app', groups: [], sessionCount: 0 }],
+      sessionCount: 0
+    } as unknown as SidebarProjectTree
+
+    const onNewSession = vi.fn()
+
+    render(<ProjectOverviewRow onNewSession={onNewSession} project={multi} />)
+    fireEvent.click(screen.getByRole('button', { name: 'New session in Multi' }))
+
+    expect(onNewSession).toHaveBeenCalledWith('/work/app')
+  })
 })

apps/desktop/src/app/chat/sidebar/projects/overview-row.tsx

@@ -147,6 +147,16 @@ export function ProjectOverviewRow({
   const hiddenCount = total - preview.length
   const offerShowAll = !showAllSessions && !expanded && preview.length > 0 && hiddenCount > 0
 
+  // #124808: a path-less explicit project (multi-folder, never assigned a
+  // primary_path) still carries repo roots. Its trunk "+" must anchor at
+  // the first repo root — passing the null wire path through would take the
+  // reserved Home/detached branch downstream and silently create a global
+  // session. Home itself keeps null ("no folder" is its contract).
+  const newSessionPath =
+    !project.isNoProject && !(project.path ?? '').trim()
+      ? ((project.repos ?? []).map(repo => repo.path).find(root => (root ?? '').trim()) ?? project.path)
+      : project.path
+
   const showAll = () => {
     // All-profiles view has no single backend to ask for one project's lanes;
     // drilling in is the reach there.
@@ -204,7 +214,7 @@ export function ProjectOverviewRow({
           {onNewSession && (
             <WorkspaceAddButton
               label={s.newSessionIn(project.label)}
-              onClick={() => onNewSession(project.path)}
+              onClick={() => onNewSession(newSessionPath)}
               onPointerDown={
                 onNewSessionSplit
                   ? event => {
@@ -217,11 +227,11 @@ export function ProjectOverviewRow({
                           onNewSessionSplit(placement.dir, {
                             anchor: placement.anchor,
                             before: placement.before,
-                            cwd: project.path
+                            cwd: newSessionPath
                           })
                         },
                         event,
-                        { cwd: project.path, label: s.newSessionIn(project.label) }
+                        { cwd: newSessionPath, label: s.newSessionIn(project.label) }
                       )
                     }
                   : undefined