fix(preview): Browser pane navigates when the desktop backend has no browser channel

apps/server/src/preview/ServerBrowser.test.ts

@@ -742,6 +742,26 @@ it.effect("a tab the desktop does not draw is marked headless, so the desktop st
   ).pipe(Effect.provide(layer)),
 );
 
+it.effect("a server started without the desktop browser channel marks its tabs headless", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const { tabId } = yield* ready;
+      const manager = yield* Manager.PreviewManager;
+      const { sessions } = yield* manager.list({ threadId: scope.thread.threadId });
+
+      expect(sessions.find((session) => session.tabId === tabId)?.headless).toBe(true);
+    }),
+  ).pipe(
+    // A WSL-only desktop starts its backend without the browser fds.
+    Effect.provide(
+      ServerBrowser.layer.pipe(
+        Layer.provide(DesktopChannel.layer),
+        Layer.provideMerge(dependencies),
+      ),
+    ),
+  ),
+);
+
 it.effect("an agent's tab the reloaded desktop dropped goes back to the desktop", () =>
   Effect.scoped(
     Effect.gen(function* () {

apps/server/src/preview/ServerBrowser.ts

@@ -887,8 +887,8 @@ const make = Effect.gen(function* () {
     if (desktop) opened.push(desktop.close);
     // Attaching over CDP emulates a light scheme; a tab on System follows the desktop's.
     if (desktop) await desktop.page.emulateMedia({ colorScheme: null });
-    // Otherwise a desktop that attaches late shows its own page, and the agent drives another.
-    if (!desktop && desktopChannel.available) {
+    // Otherwise a desktop that attaches late, or has no channel to draw it, shows its own page.
+    if (!desktop) {
       await runPromise(
         manager
           .markHeadless({ threadId: ThreadId.make(snapshot.threadId), tabId: snapshot.tabId })