perf(web): the browser panel's host loads in the desktop app only

apps/web/src/components/ChatView.desktop-window.browser.tsx

@@ -94,4 +94,25 @@ describe("ChatView (full app): the desktop app's window", () => {
       root.classList.remove("electron", "wco");
     }
   });
+
+  it("mounts the layer that holds every thread's browser pages", async () => {
+    // Its code is a chunk only the desktop app fetches (routes/__root.tsx), so
+    // the window has to load it and mount it by itself, with no panel open.
+    const mounted = await mountChatView({
+      viewport: DEFAULT_VIEWPORT,
+      snapshot: createSnapshotForTargetUser({
+        targetMessageId: "msg-user-desktop-browser-host" as MessageId,
+        targetText: "desktop browser host",
+      }),
+    });
+
+    try {
+      await waitForElement(
+        () => document.querySelector<HTMLElement>('[data-testid="browser-host-layer"]'),
+        "The desktop app should mount the browser host layer at its root.",
+      );
+    } finally {
+      await mounted.cleanup();
+    }
+  });
 });

apps/web/src/routes/__root.tsx

@@ -16,15 +16,14 @@ import {
   useNavigate,
   useRouter,
 } from "@tanstack/react-router";
-import { useCallback, useEffect, useEffectEvent, useRef, useState } from "react";
+import { Suspense, useCallback, useEffect, useEffectEvent, useRef, useState } from "react";
 import { QueryClient, useQueryClient } from "@tanstack/react-query";
 
 import { APP_DISPLAY_NAME } from "../branding";
 import { AppSidebarLayout } from "../components/AppSidebarLayout";
 import { CommandPalette, preloadCommandPaletteDialog } from "../components/CommandPalette";
 import { preloadDestinationLists } from "../components/sidebar/destinationLists";
 import { PushAlertsRuntime } from "../pushAlerts/PushAlertsRuntime";
-import { BrowserHostLayer } from "../components/browser/BrowserHostLayer";
 import { BrowserProfileReconciler } from "../components/desktop/BrowserProfileReconciler";
 import { DesktopUpdateInstallDialog } from "../components/desktop/DesktopUpdateInstallDialog";
 import { SshPasswordPromptDialog } from "../components/desktop/SshPasswordPromptDialog";
@@ -163,6 +162,17 @@ import {
   type StaleBuildRecovery,
 } from "../staleBuild";
 
+// Every thread's browser pages and this window's browser connection for
+// agents: the desktop app only, since the web build has no `<webview>`. A
+// chunk of its own, so a browser tab or a phone never downloads it; the
+// desktop app fetches it as it starts, beside everything else.
+const BrowserHostLayerLazy = lazyWithPreload(() =>
+  import("../components/browser/BrowserHostLayer").then((module) => module.BrowserHostLayer),
+);
+if (isElectron) {
+  void BrowserHostLayerLazy.preload();
+}
+
 // The theme library's sync across computers: after first paint, never at startup.
 const ThemeLibrarySyncLazy = lazyWithPreload(() =>
   import("../themeLibrary/themeLibraryState").then((module) => module.ThemeLibrarySync),
@@ -276,7 +286,12 @@ function RootRouteView() {
         {/* The Keep bar every theme trial shows, the import dialog, theme file drops. */}
         <ThemeTrialBar />
         {isElectron ? <BrowserProfileReconciler /> : null}
-        {isElectron ? <BrowserHostLayer /> : null}
+        {/* Plain Suspense: the browser panel needs it, so a failure shows. */}
+        {isElectron ? (
+          <Suspense fallback={null}>
+            <BrowserHostLayerLazy.Component />
+          </Suspense>
+        ) : null}
         <SshPasswordPromptDialog />
         <QuitConfirmationDialog />
         <DesktopUpdateInstallDialog />

scripts/check-web-bundle-size.ts

@@ -51,9 +51,11 @@ const DEFAULT_ENTRY_GZIP_BUDGET_BYTES = 255_000;
 // Raised to 915 KB (2026-11-13, Will's OK): main sat 49 B under the budget
 // (911,951 B in CI), so the browser's search engine setting (#907), about
 // 200 B of Settings catalog and schema once its address rules and search URLs
-// were moved behind the browser panel, read 912,165 B. Worth trimming next:
-// the browser panel's host and automation load at startup on the web and
-// phones too, which have no browser panel.
+// were moved behind the browser panel, read 912,165 B.
+// Measured 890,863 B here (2026-11-14) once the browser panel's host and
+// automation, which only the desktop app has a use for, became a chunk only
+// it fetches (routes/__root.tsx): about 22 KB back. The budget stays, as
+// headroom.
 const DEFAULT_STARTUP_GZIP_BUDGET_BYTES = 915_000;
 
 const entryBudgetBytes = Number(