Fixes #4920. Re-enter RunLoop when End is cancelled by IsRunningChanging (#4921)

* Fix StopRequested not reset when IsRunningChanging cancels a stop

When an IsRunningChanging handler vetoed a stop (e.g. 'Are you sure?'
pattern), End() returned early but Run() also returned - leaving
IsRunning=true, StopRequested=true, and no event loop running, causing
an application freeze.

Fix: wrap the try/finally in ApplicationImpl.Run() in a while(true) loop.
After End() returns, check runnable.IsRunning:
- false -> End succeeded, break (no behaviour change)
- true  -> End was cancelled, reset StopRequested=false and re-enter RunLoop

Also adds regression test Run_WhenStopCancelledByIsRunningChanging_RunLoopResumes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: YourRobotOverlord

Terminal.Gui/App/ApplicationImpl.Run.cs

@@ -227,15 +227,30 @@ public void Invoke (Action action)
             return null;
         }
 
-        try
+        // Loop to handle the case where End is cancelled by an IsRunningChanging handler.
+        // When End is cancelled, IsRunning remains true; we reset StopRequested and re-run the loop.
+        while (true)
         {
-            // All runnables block until RequestStop() is called
-            RunLoop (runnable, errorHandler);
-        }
-        finally
-        {
-            // End the session (raises IsRunningChanging/IsRunningChanged, pops from stack)
-            End (token);
+            try
+            {
+                // All runnables block until RequestStop() is called
+                RunLoop (runnable, errorHandler);
+            }
+            finally
+            {
+                // End the session (raises IsRunningChanging/IsRunningChanged, pops from stack)
+                End (token);
+            }
+
+            // If End succeeded IsRunning is now false — we are done
+            if (!runnable.IsRunning)
+            {
+                break;
+            }
+
+            // End was cancelled by an IsRunningChanging handler (e.g., "Are you sure?" veto).
+            // Reset StopRequested so RunLoop can re-enter its while condition correctly.
+            runnable.StopRequested = false;
         }
 
         return token.Result;

Tests/UnitTestsParallelizable/Application/ApplicationImplTests.cs

@@ -392,6 +392,34 @@ public void Run_IsRunningChanging_Cancel_IsRunningChanged_Not_Raised ()
         Assert.Equal (0, isRunningChanged);
     }
 
+    // Copilot
+    /// <summary>
+    ///     Regression test for: https://github.com/gui-cs/Terminal.Gui/issues/4920
+    ///     StopRequested not reset when IsRunningChanging cancels a stop.
+    ///     When a handler vetoes the stop (by cancelling IsRunningChanging), Run() must re-enter
+    ///     the RunLoop instead of returning prematurely with IsRunning still true.
+    /// </summary>
+    [Fact]
+    public void Run_WhenStopCancelledByIsRunningChanging_RunLoopResumes ()
+    {
+        IApplication app = NewMockedApplicationImpl ()!;
+        app.Init (DriverRegistry.Names.ANSI);
+        app.StopAfterFirstIteration = true;
+
+        OnceCancelStopRunnable runnable = new ();
+
+        // Act — must complete without hanging
+        app.Run (runnable);
+
+        // The first stop attempt was cancelled (IsRunning stayed true temporarily).
+        // After the fix, StopRequested was reset and RunLoop re-entered; the second stop
+        // attempt succeeded so IsRunning is now false.
+        Assert.False (runnable.IsRunning);
+        Assert.Equal (1, runnable.CancelCount);
+
+        app.Dispose ();
+    }
+
     private bool IdleExit (IApplication app)
     {
         if (app.TopRunnableView != null)
@@ -490,4 +518,29 @@ public void ApplicationImpl_UsesInstanceFields_NotStaticReferences ()
         Assert.Null (v2.TopRunnableView);
         Assert.Empty (v2.SessionStack!);
     }
+
+    /// <summary>
+    ///     A runnable that cancels only the first stop attempt.
+    ///     Used to simulate a "Are you sure?" veto pattern.
+    /// </summary>
+    private class OnceCancelStopRunnable : Runnable<int>
+    {
+        private bool _firstStop = true;
+
+        /// <summary>Gets how many times the stop was cancelled.</summary>
+        public int CancelCount { get; private set; }
+
+        protected override bool OnIsRunningChanging (bool oldIsRunning, bool newIsRunning)
+        {
+            if (!newIsRunning && _firstStop)
+            {
+                _firstStop = false;
+                CancelCount++;
+
+                return true; // Cancel this stop attempt
+            }
+
+            return base.OnIsRunningChanging (oldIsRunning, newIsRunning);
+        }
+    }
 }