fix: ApplicationImpl.Begin raises SessionBegun after state changes (#5162)

Terminal.Gui/App/ApplicationImpl.Run.cs

@@ -151,7 +151,7 @@ public void Invoke (Action action)
             TopRunnable = runnable;
 
             // Update cached state atomically - IsRunning and IsModal are now consistent
-            SessionBegun?.Invoke (this, new SessionTokenEventArgs (token));
+            // (CWP: state mutations happen BEFORE the SessionBegun notification below)
             runnable.SetIsRunning (true);
             runnable.SetIsModal (true);
 
@@ -161,6 +161,10 @@ public void Invoke (Action action)
 
         // END CRITICAL SECTION - IsRunning/IsModal now thread-safe
 
+        // CWP: Raise SessionBegun AFTER all state mutations so subscribers
+        // observe a fully-consistent session token (IsRunning/IsModal == true).
+        SessionBegun?.Invoke (this, new SessionTokenEventArgs (token));
+
         // Fire events AFTER lock released (avoid deadlocks in event handlers)
         previousTop?.RaiseIsModalChangedEvent (false);
 

Tests/UnitTests.NonParallelizable/Application/SessionBegunCwpTests.cs

@@ -0,0 +1,58 @@
+#nullable enable
+namespace UnitTests.NonParallelizable.ApplicationTests;
+
+/// <summary>
+///     Regression tests for issue #5162. Ensures that <see cref="IApplication.SessionBegun"/>
+///     is raised AFTER all session state mutations, per the Cancellable Workflow Pattern (CWP):
+///     subscribers must observe a fully-consistent <see cref="SessionToken"/> with
+///     <see cref="IRunnable.IsRunning"/> and <see cref="IRunnable.IsModal"/> already set to
+///     <see langword="true"/>.
+/// </summary>
+public class SessionBegunCwpTests
+{
+    // Claude - Opus 4.7
+    [Fact]
+    public void SessionBegun_Raised_After_IsRunning_And_IsModal_Set_True ()
+    {
+        ApplicationImpl.ResetModelUsageTracking ();
+
+        IApplication app = Application.Create ();
+        Runnable? runnable = null;
+
+        bool? observedIsRunning = null;
+        bool? observedIsModal = null;
+
+        try
+        {
+            runnable = new ();
+
+            EventHandler<SessionTokenEventArgs> handler = (_, e) =>
+                                                          {
+                                                              IRunnable? r = e.State.Runnable;
+
+                                                              if (r is null) { return; }
+
+                                                              observedIsRunning = r.IsRunning;
+                                                              observedIsModal = r.IsModal;
+                                                          };
+
+            app.SessionBegun += handler;
+
+            SessionToken? token = app.Begin (runnable);
+
+            app.SessionBegun -= handler;
+
+            Assert.NotNull (token);
+            Assert.True (observedIsRunning, "SessionBegun fired before SetIsRunning(true).");
+            Assert.True (observedIsModal, "SessionBegun fired before SetIsModal(true).");
+
+            app.End (token);
+        }
+        finally
+        {
+            runnable?.Dispose ();
+            app.Dispose ();
+            ApplicationImpl.ResetModelUsageTracking ();
+        }
+    }
+}