fix(client): flow occupancy is a subscribed source; an unloading occupant withdraws the open flow

hasDirectoryFlow was a plain per-render read: a flow plugin unloading (HMR)
while its dialog was open left flowOpen stuck with nobody to cancel,
permanently disabling the workspace actions. Occupancy now rides
useSyncExternalStore over the hole's registration subscription, and an
empty hole withdraws an active flow; the menu entry also reacts to
activation without a reopen (ds-review-bot).

packages/client/ui-workspace/tests/apply.spec.ts

@@ -91,11 +91,16 @@ describe('ui-workspace apply', () => {
     expect(browser.hasDirectoryFlow()).toBe(false)
     expect(picker.hasDirectoryFlow()).toBe(false)
     // A flow occupant flips exactly its own surface.
+    const notified = vi.fn()
+    const unsubscribe = browser.subscribeDirectoryFlow(notified)
     const dispose = b.slots.register({ name: 'sidebar.workspaces.directoryFlow' } as never, () => null)
     expect(browser.hasDirectoryFlow()).toBe(true)
     expect(picker.hasDirectoryFlow()).toBe(false)
+    await Promise.resolve()
+    expect(notified).toHaveBeenCalled()
     dispose()
     expect(browser.hasDirectoryFlow()).toBe(false)
+    unsubscribe()
   })
 
   it('unregisters every entry on teardown', async () => {

packages/client/ui-workspace/tests/workspace-browser.spec.tsx

@@ -60,6 +60,7 @@ function mount(overrides: Partial<WorkspaceBrowserProps> = {}) {
     insertSessionBefore: vi.fn(async () => {}),
     createWorkspace: vi.fn(async () => workspace('created', [])),
     hasDirectoryFlow: () => true,
+    subscribeDirectoryFlow: () => () => {},
     renderSlot: ((_name: string, owner: { open: boolean }) => (owner.open ? <div data-testid="directory-flow" /> : null)) as never,
     ...overrides,
   }

packages/client/ui-workspace/tests/workspace-picker.spec.tsx

@@ -50,10 +50,27 @@ function flowProbe() {
   return { probe, renderSlot }
 }
 
+/** Manual occupancy source: flip() drives the uSES subscription like a real registration change. */
+function occupancySource(initial = true) {
+  let occupied = initial
+  const listeners = new Set<() => void>()
+  return {
+    hasDirectoryFlow: () => occupied,
+    subscribeDirectoryFlow: (listener: () => void) => {
+      listeners.add(listener)
+      return () => { listeners.delete(listener) }
+    },
+    flip: (next: boolean) => {
+      occupied = next
+      for (const listener of [...listeners]) listener()
+    },
+  }
+}
+
 function mount(
   items: readonly WorkspaceView[] = [workspace('alpha', 'Alpha')],
   createWorkspace = vi.fn(),
-  hasDirectoryFlow: () => boolean = () => true,
+  occupancy = occupancySource(),
 ) {
   const onPick = vi.fn()
   const onClose = vi.fn()
@@ -68,7 +85,8 @@ function mount(
       onPick={onPick}
       onClose={onClose}
       createWorkspace={createWorkspace}
-      hasDirectoryFlow={hasDirectoryFlow}
+      hasDirectoryFlow={occupancy.hasDirectoryFlow}
+      subscribeDirectoryFlow={occupancy.subscribeDirectoryFlow}
       renderSlot={renderSlot}
     />
   )
@@ -76,7 +94,7 @@ function mount(
     renderPicker(items),
   )
   return {
-    view, onPick, onClose, createWorkspace, probe,
+    view, onPick, onClose, createWorkspace, probe, occupancy,
     rerenderItems: (nextItems: readonly WorkspaceView[]) => { view.rerender(renderPicker(nextItems)) },
   }
 }
@@ -247,7 +265,7 @@ describe('WorkspacePicker', () => {
       <WorkspacePicker
         open useSessions={hook(sessions)} useWorkspaces={hook(workspaceState([]))}
         onPick={vi.fn()} onClose={vi.fn()} createWorkspace={vi.fn()}
-        hasDirectoryFlow={() => true} renderSlot={renderSlot}
+        hasDirectoryFlow={() => true} subscribeDirectoryFlow={() => () => {}} renderSlot={renderSlot}
       />,
     )
     expect(screen.queryByRole('menu')).toBeNull()
@@ -262,26 +280,35 @@ describe('WorkspacePicker', () => {
       <WorkspacePicker
         open anchorRef={anchor()} useSessions={hook(sessions)} useWorkspaces={hook(state)}
         onPick={vi.fn()} onClose={vi.fn()} createWorkspace={vi.fn()}
-        hasDirectoryFlow={() => true} renderSlot={renderSlot}
+        hasDirectoryFlow={() => true} subscribeDirectoryFlow={() => () => {}} renderSlot={renderSlot}
       />,
     )
     expect(screen.getByRole('status').textContent).toBe('Loading workspaces…')
   })
 
   it('hides the folder entry while the directory-flow hole is empty', () => {
-    mount([], vi.fn(), () => false)
+    mount([], vi.fn(), occupancySource(false))
     expect(screen.getByRole('menuitem', { name: 'Create a new workspace' })).toBeTruthy()
     expect(screen.queryByRole('menuitem', { name: 'Open local folder…' })).toBeNull()
   })
 
-  it('shows the folder entry once the hole reports an occupant on a later render', () => {
-    let occupied = false
-    const b = mount([], vi.fn(), () => occupied)
+  it('shows the folder entry when a flow package activates after the first paint', () => {
+    const b = mount([], vi.fn(), occupancySource(false))
     expect(screen.queryByRole('menuitem', { name: 'Open local folder…' })).toBeNull()
-    // A flow package activating after the first paint is observed on the
-    // next render — the same cadence as reopening the menu.
-    occupied = true
-    b.rerenderItems([])
+    // Registration changes flow through the subscription, no re-render needed.
+    act(() => { b.occupancy.flip(true) })
     expect(screen.getByRole('menuitem', { name: 'Open local folder…' })).toBeTruthy()
   })
+
+  it('withdraws an open flow when its occupant unloads, re-enabling the menu actions', () => {
+    const b = mount([])
+    chooseItem('Open local folder…')
+    expect(screen.getByTestId('directory-flow')).toBeTruthy()
+    // The flow plugin unloads mid-interaction (HMR): nobody is left to
+    // cancel, so the owner withdraws and the actions come back.
+    act(() => { b.occupancy.flip(false) })
+    expect(b.probe.owner!.open).toBe(false)
+    expect(screen.getByRole<HTMLButtonElement>('menuitem', { name: 'Create a new workspace' }).disabled).toBe(false)
+    expect(screen.queryByRole('menuitem', { name: 'Open local folder…' })).toBeNull()
+  })
 })