diff --git a/chrome/browser/web_applications/extensions/extension_status_utils.cc b/chrome/browser/web_applications/extensions/extension_status_utils.cc
index 64c84164c082b..0e2cd102f0c20 100644
--- a/chrome/browser/web_applications/extensions/extension_status_utils.cc
+++ b/chrome/browser/web_applications/extensions/extension_status_utils.cc
@@ -96,6 +96,10 @@ bool IsExternalExtensionUninstalled(content::BrowserContext* context,
 #if BUILDFLAG(IS_WIN) || BUILDFLAG(IS_MAC) || BUILDFLAG(IS_LINUX)
 bool IsExtensionUnsupportedDeprecatedApp(content::BrowserContext* context,
                                          const std::string& extension_id) {
+  // POC: allow platform apps so the <webview> POC app can launch on Linux.
+  if (!testing::g_enable_chrome_apps_for_testing) {
+    return false;
+  }
   if (testing::g_enable_chrome_apps_for_testing) {
     return false;
   }
diff --git a/content/browser/preloading/prefetch/prefetch_streaming_url_loader.cc b/content/browser/preloading/prefetch/prefetch_streaming_url_loader.cc
index 065c5503df617..2a25fe1069074 100644
--- a/content/browser/preloading/prefetch/prefetch_streaming_url_loader.cc
+++ b/content/browser/preloading/prefetch/prefetch_streaming_url_loader.cc
@@ -380,6 +380,10 @@ void PrefetchStreamingURLLoader::StartServiceWorkerInterceptor(
       static_cast<ServiceWorkerContextWrapper*>(
           browser_context->GetDefaultStoragePartition()
               ->GetServiceWorkerContext());
+  LOG(ERROR) << "[POC] PrefetchStreamingURLLoader: SW interceptor for prefetch "
+                "url=" << request.url
+             << " using DEFAULT partition sw_context="
+             << static_cast<void*>(service_worker_context);
   service_worker_handle_ = std::make_unique<ServiceWorkerMainResourceHandle>(
       service_worker_context, base::DoNothing(),
       std::move(fetch_event_client_id));
diff --git a/content/browser/service_worker/service_worker_main_resource_handle.cc b/content/browser/service_worker/service_worker_main_resource_handle.cc
index 4cd3f0044247e..d335ab6729978 100644
--- a/content/browser/service_worker/service_worker_main_resource_handle.cc
+++ b/content/browser/service_worker/service_worker_main_resource_handle.cc
@@ -6,6 +6,7 @@
 
 #include <utility>
 
+#include "base/debug/stack_trace.h"
 #include "base/functional/bind.h"
 #include "content/browser/preloading/prefetch/prefetch_features.h"
 #include "content/browser/renderer_host/policy_container_host.h"
@@ -109,6 +110,35 @@ bool ServiceWorkerMainResourceHandle::InitializeForRequest(
         client_for_prefetch->key() != key) {
       return false;
     }
+    // === POC INSTRUMENTATION ===================================
+    // The top_frame_origin/key check above just passed. Now verify whether the
+    // prefetch's ServiceWorkerClient lives in the same StoragePartition as this
+    // navigation's handle. They MUST match for correct isolation; if they
+    // differ, a cross-partition ServiceWorker controller is about to be
+    // installed on the navigation's client below.
+    ServiceWorkerContextCore* nav_ctx = context_wrapper_->context();
+    ServiceWorkerContextCore* prefetch_ctx =
+        client_for_prefetch->context().get();
+    LOG(ERROR) << "[POC] InitializeForRequest url=" << url
+               << " key=" << key.GetDebugString()
+               << " nav_partition="
+               << static_cast<void*>(context_wrapper_->storage_partition())
+               << " nav_sw_context=" << static_cast<void*>(nav_ctx)
+               << " prefetch_sw_context=" << static_cast<void*>(prefetch_ctx);
+    if (nav_ctx != prefetch_ctx) {
+      LOG(ERROR)
+          << "[POC] *** CROSS-PARTITION SERVICEWORKER CONTROLLER ADOPTION ***";
+      LOG(ERROR)
+          << "[POC] The prefetch's ServiceWorkerClient belongs to a different "
+             "ServiceWorkerContextCore (i.e. a different StoragePartition) "
+             "than the navigation handle. About to install a "
+             "default-partition ServiceWorkerRegistration as the controller "
+             "of a non-default-partition client.";
+      base::debug::StackTrace().Print();
+      // Intentionally fall through so the cross-partition adoption proceeds
+      // and its impact is observable in the guest WebView.
+    }
+    // === END POC INSTRUMENTATION ===============================
   }
 
   isolation_info_ = std::move(new_isolation_info);