feat(desktop): open any chat in its own window (#43219)

Pops a session into a standalone, focused window for side-by-side work.
A secondary window loads the renderer at the session route with a
?win=secondary flag (ahead of the HashRouter '#'); it drops the global
sidebar plus the install/onboarding overlays and renders a single chat,
sharing the one local gateway over WS (no backend duplication). The main
process keys windows by sessionId so re-opening focuses the existing one
and self-cleans on close.

Open it via:
- ⌘-click (mac) / ⌃-click (win/linux) a sidebar session — the universal
  "open in new window" gesture. Archive moves to the ⋯ / right-click menus
  only, off the easy-to-misfire modifier-click.
- "New window" in the session ⋯ and context menus (link-external icon,
  i18n'd across en/ja/zh/zh-hant).

A standalone window has no left rail, so AppShell treats its edge as
uncovered and applies the titlebar inset — the chat title clears the
macOS traffic lights instead of hiding behind them.

Co-authored-by: tim404x <tim404x@users.noreply.github.com>

apps/desktop/src/store/windows.test.ts

@@ -0,0 +1,93 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { canOpenSessionWindow, openSessionInNewWindow } from './windows'
+
+const desktopWindow = window as unknown as { hermesDesktop?: Window['hermesDesktop'] }
+const initialHermesDesktop = desktopWindow.hermesDesktop
+
+const notifyError = vi.fn()
+
+vi.mock('./notifications', () => ({
+  notifyError: (...args: unknown[]) => notifyError(...args)
+}))
+
+function installBridge(openSessionWindow?: Window['hermesDesktop']['openSessionWindow']) {
+  desktopWindow.hermesDesktop = {
+    ...(openSessionWindow ? { openSessionWindow } : {})
+  } as unknown as Window['hermesDesktop']
+}
+
+beforeEach(() => {
+  notifyError.mockClear()
+})
+
+afterEach(() => {
+  if (initialHermesDesktop) {
+    desktopWindow.hermesDesktop = initialHermesDesktop
+  } else {
+    delete desktopWindow.hermesDesktop
+  }
+})
+
+describe('canOpenSessionWindow', () => {
+  it('is false when the desktop bridge is absent', () => {
+    delete desktopWindow.hermesDesktop
+    expect(canOpenSessionWindow()).toBe(false)
+  })
+
+  it('is false when the bridge lacks openSessionWindow', () => {
+    installBridge(undefined)
+    expect(canOpenSessionWindow()).toBe(false)
+  })
+
+  it('is true when the bridge exposes openSessionWindow', () => {
+    installBridge(vi.fn().mockResolvedValue({ ok: true }))
+    expect(canOpenSessionWindow()).toBe(true)
+  })
+})
+
+describe('openSessionInNewWindow', () => {
+  it('no-ops without a session id', async () => {
+    const open = vi.fn().mockResolvedValue({ ok: true })
+    installBridge(open)
+
+    await openSessionInNewWindow('')
+
+    expect(open).not.toHaveBeenCalled()
+    expect(notifyError).not.toHaveBeenCalled()
+  })
+
+  it('no-ops gracefully when the bridge is absent (web fallback)', async () => {
+    delete desktopWindow.hermesDesktop
+
+    await openSessionInNewWindow('s1')
+
+    expect(notifyError).not.toHaveBeenCalled()
+  })
+
+  it('invokes the bridge with the session id', async () => {
+    const open = vi.fn().mockResolvedValue({ ok: true })
+    installBridge(open)
+
+    await openSessionInNewWindow('s1')
+
+    expect(open).toHaveBeenCalledWith('s1')
+    expect(notifyError).not.toHaveBeenCalled()
+  })
+
+  it('notifies on an ok:false result', async () => {
+    installBridge(vi.fn().mockResolvedValue({ ok: false, error: 'invalid-session-id' }))
+
+    await openSessionInNewWindow('s1')
+
+    expect(notifyError).toHaveBeenCalledTimes(1)
+  })
+
+  it('notifies when the bridge throws', async () => {
+    installBridge(vi.fn().mockRejectedValue(new Error('boom')))
+
+    await openSessionInNewWindow('s1')
+
+    expect(notifyError).toHaveBeenCalledTimes(1)
+  })
+})

apps/desktop/src/store/windows.ts

@@ -0,0 +1,52 @@
+import { notifyError } from './notifications'
+
+// Window flag set by the Electron main process when it opens a standalone
+// session window (see electron/main.cjs buildSessionWindowUrl). It rides in the
+// query string BEFORE the HashRouter '#', so we read it from location.search,
+// never from the router. A "secondary" window renders a single chat without the
+// global session sidebar or the install / onboarding overlays.
+const SECONDARY_WINDOW_FLAG = 'secondary'
+
+let secondaryWindowCache: boolean | null = null
+
+export function isSecondaryWindow(): boolean {
+  if (secondaryWindowCache !== null) {
+    return secondaryWindowCache
+  }
+
+  let result = false
+
+  try {
+    result = new URLSearchParams(window.location.search).get('win') === SECONDARY_WINDOW_FLAG
+  } catch {
+    result = false
+  }
+
+  secondaryWindowCache = result
+
+  return result
+}
+
+// True when running inside the Electron desktop shell (the preload bridge is
+// present). The "open in new window" affordance is desktop-only.
+export function canOpenSessionWindow(): boolean {
+  return typeof window !== 'undefined' && typeof window.hermesDesktop?.openSessionWindow === 'function'
+}
+
+// Open (or focus) a standalone OS window for a single chat session. No-ops
+// gracefully outside Electron so callers can wire it unconditionally.
+export async function openSessionInNewWindow(sessionId: string): Promise<void> {
+  if (!sessionId || !canOpenSessionWindow()) {
+    return
+  }
+
+  try {
+    const result = await window.hermesDesktop.openSessionWindow(sessionId)
+
+    if (!result?.ok) {
+      notifyError(new Error(result?.error || 'unknown error'), 'Could not open chat in a new window')
+    }
+  } catch (err) {
+    notifyError(err, 'Could not open chat in a new window')
+  }
+}