fix(desktop-client): prevent duplicate window creation and serialize mode switches

- Add creation promise guards (mainWindowCreatePromise etc.) so concurrent
  ensureXxxWindow() calls share a single in-flight BrowserWindow, avoiding
  duplicate windows on rapid IPC
- Add windowModeSwitchQueue to serialize desktop:set-window-mode calls and
  the initial startup switch, preventing races when login/logout fire quickly
- Fix retireInactiveAppWindows: also retire existing login windows when
  switching to login mode, so stale login windows don't accumulate

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/desktop-client/src/main/bootstrap.ts

@@ -97,6 +97,10 @@ let loginWindow: ElectronBrowserWindow | null = null;
 let settingsWindow: ElectronBrowserWindow | null = null;
 let mainRendererContents: ElectronWebContents | null = null;
 let quitReleaseInFlight = false;
+let mainWindowCreatePromise: Promise<ElectronBrowserWindow> | null = null;
+let loginWindowCreatePromise: Promise<ElectronBrowserWindow> | null = null;
+let settingsWindowCreatePromise: Promise<ElectronBrowserWindow> | null = null;
+let windowModeSwitchQueue: Promise<void> = Promise.resolve();
 const forceCloseWindows = new WeakSet<ElectronBrowserWindow>();
 const appWindowModes = new WeakMap<ElectronBrowserWindow, RendererWindowMode>();
 
@@ -281,7 +285,7 @@ function retireInactiveAppWindows(mode: WindowMode, target: ElectronBrowserWindo
     const candidateMode = rendererModeFromWindow(candidate);
     if (
       (mode === "main" && (candidateMode === "login" || candidateMode === "main"))
-      || (mode === "login" && (candidateMode === "main" || candidateMode === "settings"))
+      || (mode === "login" && (candidateMode === "login" || candidateMode === "main" || candidateMode === "settings"))
     ) {
       retireWindowAfterIpcReturn(candidate);
     }
@@ -333,9 +337,17 @@ async function ensureMainWindow(): Promise<ElectronBrowserWindow> {
   if (existing) {
     return existing;
   }
-  const window = await createAppWindow("main");
-  mainWindow = window;
-  return window;
+  if (!mainWindowCreatePromise) {
+    mainWindowCreatePromise = createAppWindow("main")
+      .then((window) => {
+        mainWindow = window;
+        return window;
+      })
+      .finally(() => {
+        mainWindowCreatePromise = null;
+      });
+  }
+  return mainWindowCreatePromise;
 }
 
 async function ensureLoginWindow(): Promise<ElectronBrowserWindow> {
@@ -343,9 +355,17 @@ async function ensureLoginWindow(): Promise<ElectronBrowserWindow> {
   if (existing) {
     return existing;
   }
-  const window = await createAppWindow("login");
-  loginWindow = window;
-  return window;
+  if (!loginWindowCreatePromise) {
+    loginWindowCreatePromise = createAppWindow("login")
+      .then((window) => {
+        loginWindow = window;
+        return window;
+      })
+      .finally(() => {
+        loginWindowCreatePromise = null;
+      });
+  }
+  return loginWindowCreatePromise;
 }
 
 async function ensureSettingsWindow(): Promise<ElectronBrowserWindow> {
@@ -353,9 +373,17 @@ async function ensureSettingsWindow(): Promise<ElectronBrowserWindow> {
   if (existing) {
     return existing;
   }
-  const window = await createSettingsWindow();
-  settingsWindow = window;
-  return window;
+  if (!settingsWindowCreatePromise) {
+    settingsWindowCreatePromise = createSettingsWindow()
+      .then((window) => {
+        settingsWindow = window;
+        return window;
+      })
+      .finally(() => {
+        settingsWindowCreatePromise = null;
+      });
+  }
+  return settingsWindowCreatePromise;
 }
 
 async function ensureWindow(mode: WindowMode): Promise<ElectronBrowserWindow> {
@@ -424,6 +452,18 @@ async function switchWindowMode(
   retireInactiveAppWindows(mode, target);
 }
 
+function queueWindowModeSwitch(
+  mode: WindowMode,
+  request: WindowModeRequest = {},
+  sourceWindow: ElectronBrowserWindow | null = null,
+): Promise<void> {
+  const nextSwitch = windowModeSwitchQueue
+    .catch(() => undefined)
+    .then(() => switchWindowMode(mode, request, sourceWindow));
+  windowModeSwitchQueue = nextSwitch.catch(() => undefined);
+  return nextSwitch;
+}
+
 async function openSettingsWindow(): Promise<void> {
   const window = await ensureSettingsWindow();
   await revealWindow(window);
@@ -722,7 +762,7 @@ function registerBridgeHandlers(): void {
   });
   ipcMain.handle("desktop:set-window-mode", async (event, mode: WindowMode, request?: unknown) => {
     if (mode === "login" || mode === "main") {
-      await switchWindowMode(mode, normalizeWindowModeRequest(request), windowFromIpcEvent(event));
+      await queueWindowModeSwitch(mode, normalizeWindowModeRequest(request), windowFromIpcEvent(event));
     }
     return null;
   });
@@ -755,7 +795,7 @@ if (!hasSingleInstanceLock) {
       }
       mainRendererContents.send("desktop:runtime-invalidated", event);
     });
-    await switchWindowMode(currentWindowMode);
+    await queueWindowModeSwitch(currentWindowMode);
     initTray(() => {
       revealActiveWindowSafely("tray");
     });