diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 9e5e476393..bbf391d4ec 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -18,7 +18,7 @@ on:
         description: 'JDK version'
         required: false
         type: string
-        default: "17"
+        default: "21"
       jdk_distribution:
         description: 'JDK distribution'
         required: false
diff --git a/app/build.gradle b/app/build.gradle
index 94695fcc10..de49844436 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -171,7 +171,7 @@ android {
 
     java {
         toolchain {
-            languageVersion = JavaLanguageVersion.of(17)
+            languageVersion = JavaLanguageVersion.of(21)
         }
     }
 
diff --git a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/RuntimeImpl.java b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/RuntimeImpl.java
index 5d5966ddcb..a68e8fc2f7 100644
--- a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/RuntimeImpl.java
+++ b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/RuntimeImpl.java
@@ -33,9 +33,6 @@
 import org.chromium.base.PathUtils;
 import org.chromium.base.library_loader.LibraryLoader;
 import org.chromium.base.library_loader.LibraryProcessType;
-import org.chromium.components.signin.AccountManagerFacadeImpl;
-import org.chromium.components.signin.AccountManagerFacadeProvider;
-import org.chromium.components.signin.SystemAccountManagerDelegate;
 import org.chromium.content_public.browser.BrowserStartupController;
 import org.chromium.content_public.browser.DeviceUtils;
 import org.chromium.ui.base.ResourceBundle;
@@ -191,27 +188,32 @@ private void initBrowserProcess(Context context) {
         CommandLine.init(new String[] {});
         if (BuildConfig.DEBUG)
             CommandLine.getInstance().appendSwitchWithValue("enable-logging", "stderr");
+
+        // Disable AndroidUseCorrectWindowBounds (enabled by the field-trial testing config in
+        // M14x): it makes GetBoundsInRootWindow / window.outerWidth/Height report the physical
+        // Android panel instead of the content surface. Wolvic renders to an offscreen VR
+        // texture larger than the panel, and YouTube sizes its fullscreen <video> against
+        // window.outerWidth/Height -- so the panel value shrinks the video. The view (surface)
+        // bounds are the correct "window" bounds for us (the pre-M14x behaviour).
+        String disableFeatures = "AndroidUseCorrectWindowBounds";
         if (BuildConfig.FLAVOR_abi == "x64")
-            CommandLine.getInstance().appendSwitchWithValue("disable-features", "Vulkan");
+            disableFeatures += ",Vulkan";
+        CommandLine.getInstance().appendSwitchWithValue("disable-features", disableFeatures);
 
         // Enable WebXR Hand Input, which is disabled by default in blink (experimental)
         CommandLine.getInstance().appendSwitchWithValue("enable-features", "WebXRHandInput");
 
         setupWebGLMSAA();
-        DeviceUtils.addDeviceSpecificUserAgentSwitch();
+        DeviceUtils.updateDeviceSpecificUserAgentSwitch(context);
         LibraryLoader.getInstance().ensureInitialized();
 
-        // Initialize the AccountManagerFacade with the correct AccountManagerDelegate. Must be done
-        // only once and before AccountManagerFacadeProvider.getInstance() is invoked.
-        AccountManagerFacadeProvider.setInstance(
-                new AccountManagerFacadeImpl(new SystemAccountManagerDelegate()));
-
         BrowserStartupController.getInstance().startBrowserProcessesAsync(
                 LibraryProcessType.PROCESS_BROWSER, true /* startGpuProcess */, false /* startMinimalBrowser */,
+                false /* singleProcess */, false /* scheduleFlushStartupTasks */,
                 new BrowserStartupController.StartupCallback() {
                     @Override
-                    public void onSuccess() {
-                        Log.i(LOGTAG, "The browser process started!");
+                    public void onSuccess(BrowserStartupController.StartupMetrics metrics) {
+                        Log.i(LOGTAG, "The browser process started!" + metrics.getTotalDurationOfPostedTasksMs());
                         mIsReady = true;
                         mCallbacks.forEach(callback -> {
                             callback.onReady();
diff --git a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/SessionImpl.java b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/SessionImpl.java
index f72689546b..6ec3b7dd4b 100644
--- a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/SessionImpl.java
+++ b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/SessionImpl.java
@@ -20,6 +20,7 @@
 import com.igalia.wolvic.browser.api.WSessionState;
 import com.igalia.wolvic.browser.api.WTextInput;
 import com.igalia.wolvic.browser.api.WWebResponse;
+import org.chromium.content_public.browser.Visibility;
 import org.chromium.content_public.browser.WebContents;
 import org.chromium.wolvic.DownloadManagerBridge;
 import org.chromium.wolvic.PasswordForm;
@@ -130,9 +131,9 @@ public void setActive(boolean active) {
         assert mTab.getActiveWebContents() != null;
         WebContents webContents = mTab.getActiveWebContents();
         if (active) {
-            webContents.onShow();
+            webContents.updateWebContentsVisibility(Visibility.VISIBLE);
         } else {
-            webContents.onHide();
+            webContents.updateWebContentsVisibility(Visibility.HIDDEN);
             webContents.suspendAllMediaPlayers();
         }
         webContents.setAudioMuted(!active);
diff --git a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabImpl.java b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabImpl.java
index c116ac7a1d..35b0e51c85 100644
--- a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabImpl.java
+++ b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabImpl.java
@@ -5,7 +5,7 @@
 import androidx.annotation.NonNull;
 
 import org.chromium.components.embedder_support.view.ContentView;
-import org.chromium.content_public.browser.MediaSessionObserver;
+import org.chromium.content_public.browser.MediaSession;
 import org.chromium.content_public.browser.SelectionPopupController;
 import org.chromium.content_public.browser.WebContents;
 import org.chromium.wolvic.Tab;
@@ -15,6 +15,7 @@
  * Controls a single tab content in a browser for chromium backend.
  */
 public class TabImpl extends Tab {
+    private SessionImpl mSession;
     private TabMediaSessionObserver mTabMediaSessionObserver;
     private TabWebContentsDelegate mTabWebContentsDelegate;
     private TabWebContentsObserver mWebContentsObserver;
@@ -30,12 +31,21 @@ public TabImpl(@NonNull Context context, @NonNull SessionImpl session, WebConten
     }
 
     private void registerCallbacks(@NonNull SessionImpl session) {
-        mTabMediaSessionObserver = new TabMediaSessionObserver(mWebContents, session);
+        mSession = session;
+
         mTabWebContentsDelegate = new TabWebContentsDelegate(session, mWebContents);
         setWebContentsDelegate(mWebContents, mTabWebContentsDelegate);
 
         mWebContentsObserver = new TabWebContentsObserver(this, session);
 
+        // The native MediaSession is created lazily (on first media use), so it usually does not
+        // exist yet and MediaSession.fromWebContents() returns null. If it already exists, observe
+        // it now; otherwise TabWebContentsObserver.mediaSessionCreated() does it when the session
+        // appears. The session is created at most once per WebContents.
+        MediaSession mediaSession = MediaSession.fromWebContents(mWebContents);
+        if (mediaSession != null)
+            createMediaSessionObserver(mediaSession);
+
         SelectionPopupController controller =
                 SelectionPopupController.fromWebContents(mWebContents);
         controller.setDelegate(
@@ -43,12 +53,17 @@ private void registerCallbacks(@NonNull SessionImpl session) {
                         controller.getDelegateEventHandler(), session));
     }
 
+    /* package */ void createMediaSessionObserver(@NonNull MediaSession mediaSession) {
+        mTabMediaSessionObserver = new TabMediaSessionObserver(mediaSession, mWebContents, mSession);
+    }
+
     public void exitFullScreen() {
         mWebContents.exitFullscreen();
     }
 
     public void onMediaFullscreen(boolean isFullscreen) {
-        mTabMediaSessionObserver.onMediaFullscreen(isFullscreen);
+        if (mTabMediaSessionObserver != null)
+            mTabMediaSessionObserver.onMediaFullscreen(isFullscreen);
     }
 
     public void purgeHistory() {
diff --git a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabMediaSessionObserver.java b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabMediaSessionObserver.java
index dba10e8f39..10e50d22d8 100644
--- a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabMediaSessionObserver.java
+++ b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabMediaSessionObserver.java
@@ -43,8 +43,9 @@ public class TabMediaSessionObserver extends MediaSessionObserver implements Med
     private boolean mIsSuspended = false;
     private boolean mRunUpdatingPositionTask = false;
 
-    public TabMediaSessionObserver(@NonNull WebContents webContents, @NonNull SessionImpl session) {
-        super(MediaSession.fromWebContents(webContents));
+    public TabMediaSessionObserver(@NonNull MediaSession mediaSession,
+                                   @NonNull WebContents webContents, @NonNull SessionImpl session) {
+        super(mediaSession);
 
         mSession = session;
         mMediaImageManager =
diff --git a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabWebContentsDelegate.java b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabWebContentsDelegate.java
index 6a2f13adec..ead2661e22 100644
--- a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabWebContentsDelegate.java
+++ b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabWebContentsDelegate.java
@@ -17,10 +17,8 @@
 import org.chromium.components.find_in_page.FindMatchRectsDetails;
 import org.chromium.components.find_in_page.FindNotificationDetails;
 import org.chromium.content_public.browser.InvalidateTypes;
-import org.chromium.content_public.browser.LoadUrlParams;
+import org.chromium.content_public.browser.RenderFrameHost;
 import org.chromium.content_public.browser.WebContents;
-import org.chromium.url.GURL;
-import org.chromium.wolvic.Tab;
 import org.chromium.wolvic.WolvicWebContentsDelegate;
 import org.json.JSONException;
 import org.json.JSONObject;
@@ -55,7 +53,7 @@ public boolean takeFocus(boolean reverse) {
    }
 
     @Override
-    public void enterFullscreenModeForTab(boolean prefersNavigationBar, boolean prefersStatusBar) {
+    public void enterFullscreenModeForTab(RenderFrameHost renderFrameHost, boolean prefersNavigationBar, boolean prefersStatusBar, long displayId) {
         @Nullable WSession.ContentDelegate delegate = mSession.getContentDelegate();
         if (delegate == null) return;
 
@@ -148,26 +146,6 @@ public void closeContents() {
         }, 0);
     }
 
-    @Override
-    public void onUpdateUrl(GURL url) {
-        String newUrl = YoutubeUrlHelper.maybeRewriteYoutubeURL(url);
-        // If mobile Youtube URL is detected, redirect to the desktop version.
-        if (!url.getSpec().equals(newUrl)) {
-            LoadUrlParams params = new LoadUrlParams(newUrl);
-            mWebContents.getNavigationController().setEntryExtraData(
-                    mWebContents.getNavigationController().getLastCommittedEntryIndex(),
-                    Tab.NAVIGATION_ENTRY_MARKED_AS_SKIPPED_KEY,
-                    Tab.NAVIGATION_ENTRY_MARKED_AS_SKIPPED_VALUE);
-            mWebContents.getNavigationController().loadUrl(params);
-            return;
-        }
-
-        WSession.NavigationDelegate delegate = mSession.getNavigationDelegate();
-        if (delegate != null) {
-            delegate.onLocationChange(mSession, mWebContents.getVisibleUrl().getSpec());
-        }
-    }
-
     @Override
     public void showRepostFormWarningDialog() {
         mSession.getChromiumPromptDelegate().onRepostConfirmWarningDialog().then(result -> {
diff --git a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabWebContentsObserver.java b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabWebContentsObserver.java
index e37fbbac75..4b7b733567 100644
--- a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabWebContentsObserver.java
+++ b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabWebContentsObserver.java
@@ -10,9 +10,12 @@
 import com.igalia.wolvic.browser.api.WWebRequestError;
 
 import org.chromium.base.ContextUtils;
+import org.chromium.base.task.PostTask;
+import org.chromium.base.task.TaskTraits;
 import org.chromium.components.embedder_support.view.ContentView;
 import org.chromium.content_public.browser.LifecycleState;
 import org.chromium.content_public.browser.LoadUrlParams;
+import org.chromium.content_public.browser.MediaSession;
 import org.chromium.content_public.browser.NavigationHandle;
 import org.chromium.content_public.browser.WebContents;
 import org.chromium.content_public.browser.WebContentsObserver;
@@ -63,6 +66,25 @@ public void didRedirectNavigation(NavigationHandle navigationHandle) {
     public void didStartNavigationInPrimaryMainFrame(NavigationHandle navigationHandle) {
         super.didStartNavigationInPrimaryMainFrame(navigationHandle);
 
+        // Rewrite mobile YouTube watch URLs to their desktop variant before the request
+        // is initiated. The load is deferred because starting a navigation synchronously
+        // from a navigation callback can corrupt the native navigation objects (same
+        // reason onCreateNewWindow/closeContents post their work).
+        GURL url = navigationHandle.getUrl();
+        String rewrittenUrl = YoutubeUrlHelper.maybeRewriteYoutubeURL(url);
+        if (!url.getSpec().equals(rewrittenUrl)) {
+            WebContents webContents = getWebContents();
+            if (webContents != null) {
+                PostTask.postDelayedTask(TaskTraits.UI_DEFAULT, () -> {
+                    if (!webContents.isDestroyed()) {
+                        webContents.getNavigationController().loadUrl(
+                                new LoadUrlParams(rewrittenUrl));
+                    }
+                }, 0);
+            }
+            return;
+        }
+
         WSession.NavigationDelegate delegate = mSession.getNavigationDelegate();
         if (delegate == null)
             return;
@@ -130,7 +152,7 @@ public void onCreateNewPaymentHandler(final WebContents newWebContents) {
             return;
         }
 
-        Context context = mWebContents.get().getTopLevelNativeWindow().getContext().get();
+        Context context = getWebContents().getTopLevelNativeWindow().getContext().get();
         PaymentRequestUI paymentHandler = new PaymentRequestUI(context, newWebContents, null);
         final TabCompositorView compositorView = paymentHandler.getCompositorView();
         assert newWebContents.getViewAndroidDelegate() != null
@@ -154,16 +176,16 @@ public void onCreateNewPaymentHandler(final WebContents newWebContents) {
         // Show Compositor View after attaching to the parent view.
         compositorView.setCurrentWebContents(newWebContents);
 
-        mPaymentWebContentsObserver = new WebContentsObserver(newWebContents) {
+        mPaymentWebContentsObserver = new WebContentsObserver() {
             @Override
-            public void destroy() {
+            public void webContentsDestroyed() {
                 mSession.releaseOverlayDisplay(compositorView);
                 mTab.setPaymentWebContents(null, null, null);
 
                 contentDelegate.onHidePaymentHandler(mSession);
-                newWebContents.removeObserver(this);
             }
         };
+        mPaymentWebContentsObserver.observe(newWebContents);
     }
 
     @Override
@@ -201,7 +223,7 @@ public X509Certificate certificate() {
     private void dispatchCanGoBackOrForward() {
         @Nullable WSession.NavigationDelegate delegate = mSession.getNavigationDelegate();
         if (delegate != null) {
-            WebContents webContents = mWebContents.get();
+            WebContents webContents = getWebContents();
             if (webContents == null)
                 return;
 
@@ -230,4 +252,9 @@ public void didFirstVisuallyNonEmptyPaint() {
     public void hasEffectivelyFullscreenVideoChange(boolean isFullscreen) {
         mSession.getTab().onMediaFullscreen(isFullscreen);
     }
+
+    @Override
+    public void mediaSessionCreated(MediaSession mediaSession) {
+        mTab.createMediaSessionObserver(mediaSession);
+    }
 }
diff --git a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/YoutubeUrlHelper.java b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/YoutubeUrlHelper.java
index 1c2ee5c3fb..b84073d024 100644
--- a/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/YoutubeUrlHelper.java
+++ b/app/src/common/chromium/com/igalia/wolvic/browser/api/impl/YoutubeUrlHelper.java
@@ -26,7 +26,14 @@ public static String maybeRewriteYoutubeURL(GURL url) {
             return url.getSpec();
         }
 
-        return ensureAppIsSetToDesktop(Uri.parse(url.getSpec())).toString();
+        // Already in desktop mode: return the spec verbatim so callers can rely on
+        // this being idempotent and never re-trigger a navigation.
+        Uri uri = Uri.parse(url.getSpec());
+        if ("desktop".equals(uri.getQueryParameter("app"))) {
+            return url.getSpec();
+        }
+
+        return ensureAppIsSetToDesktop(uri).toString();
     }
 
     private static Uri ensureAppIsSetToDesktop(Uri uri) {