Fixed issues @bdisp noted:

Terminal.Gui/View/Orientation/IOrientation.cs

@@ -29,15 +29,14 @@ public interface IOrientation
     public bool OnOrientationChanging (Orientation currentOrientation, Orientation newOrientation) { return false; }
 
     /// <summary>
-    /// 
+    ///     Raised when <see cref="Orientation"/> has changed.
     /// </summary>
-    public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+    public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 
     /// <summary>
     ///     Called when <see cref="Orientation"/> has been changed.
     /// </summary>
-    /// <param name="oldOrientation"></param>
     /// <param name="newOrientation"></param>
     /// <returns></returns>
-    public void OnOrientationChanged (Orientation oldOrientation, Orientation newOrientation) { return; }
+    public void OnOrientationChanged (Orientation newOrientation) { return; }
 }

Terminal.Gui/View/Orientation/OrientationHelper.cs

@@ -29,7 +29,7 @@
 ///     }
 /// 
 ///     public event EventHandler<CancelEventArgs<Orientation>> OrientationChanging;
-///     public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+///     public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 /// 
 ///     public bool OnOrientationChanging (Orientation currentOrientation, Orientation newOrientation)
 ///     {
@@ -37,7 +37,7 @@
 ///        return false; // Return true to cancel the change
 ///     }
 /// 
-///     public void OnOrientationChanged (Orientation oldOrientation, Orientation newOrientation)
+///     public void OnOrientationChanged (Orientation newOrientation)
 ///     {
 ///         // Custom logic after orientation has changed
 ///     }
@@ -98,11 +98,10 @@ public class OrientationHelper
             }
 
             // Best practice is to invoke the virtual method first.
-            _owner?.OnOrientationChanged (old, _orientation);
+            _owner?.OnOrientationChanged (_orientation);
 
             // Even though Changed is not cancelable, it is still a good practice to raise the event after.
-            args = new (in old, ref _orientation);
-            OrientationChanged?.Invoke (_owner, args);
+            OrientationChanged?.Invoke (_owner, new (in _orientation));
         }
     }
 
@@ -135,5 +134,5 @@ public class OrientationHelper
     ///         This event will be raised after the <see cref="IOrientation.OnOrientationChanged"/> method is called.
     ///     </para>
     /// </remarks>
-    public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+    public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 }

Terminal.Gui/Views/Bar.cs

@@ -75,12 +75,11 @@ public class Bar : View, IOrientation, IDesignable
     public event EventHandler<CancelEventArgs<Orientation>> OrientationChanging;
 
     /// <inheritdoc/>
-    public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+    public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 
     /// <summary>Called when <see cref="Orientation"/> has changed.</summary>
-    /// <param name="oldOrientation"></param>
     /// <param name="newOrientation"></param>
-    public void OnOrientationChanged (Orientation oldOrientation, Orientation newOrientation)
+    public void OnOrientationChanged (Orientation newOrientation)
     {
         SetNeedsLayout ();
     }

Terminal.Gui/Views/Line.cs

@@ -34,15 +34,14 @@ public class Line : View, IOrientation
     public event EventHandler<CancelEventArgs<Orientation>> OrientationChanging;
 
     /// <inheritdoc/>
-    public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+    public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 
     /// <summary>Called when <see cref="Orientation"/> has changed.</summary>
-    /// <param name="oldOrientation"></param>
     /// <param name="newOrientation"></param>
-    public void OnOrientationChanged (Orientation oldOrientation, Orientation newOrientation)
+    public void OnOrientationChanged (Orientation newOrientation)
     {
 
-        switch (Orientation)
+        switch (newOrientation)
         {
             case Orientation.Horizontal:
                 Height = 1;

Terminal.Gui/Views/RadioGroup.cs

@@ -338,12 +338,11 @@ public class RadioGroup : View, IDesignable, IOrientation
     public event EventHandler<CancelEventArgs<Orientation>> OrientationChanging;
 
     /// <inheritdoc/>
-    public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+    public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 
     /// <summary>Called when <see cref="Orientation"/> has changed.</summary>
-    /// <param name="oldOrientation"></param>
     /// <param name="newOrientation"></param>
-    public void OnOrientationChanged (Orientation oldOrientation, Orientation newOrientation)
+    public void OnOrientationChanged (Orientation newOrientation)
     {
         SetupKeyBindings ();
         SetContentSize ();

Terminal.Gui/Views/Shortcut.cs

@@ -167,12 +167,11 @@ public class Shortcut : View, IOrientation, IDesignable
     public event EventHandler<CancelEventArgs<Orientation>> OrientationChanging;
 
     /// <inheritdoc/>
-    public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+    public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 
     /// <summary>Called when <see cref="Orientation"/> has changed.</summary>
-    /// <param name="oldOrientation"></param>
     /// <param name="newOrientation"></param>
-    public void OnOrientationChanged (Orientation oldOrientation, Orientation newOrientation)
+    public void OnOrientationChanged (Orientation newOrientation)
     {
         // TODO: Determine what, if anything, is opinionated about the orientation.
         SetNeedsLayout ();

Terminal.Gui/Views/Slider.cs

@@ -242,10 +242,10 @@ public class Slider<T> : View, IOrientation
     public event EventHandler<CancelEventArgs<Orientation>> OrientationChanging;
 
     /// <inheritdoc />
-    public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+    public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 
     /// <inheritdoc />
-    public void OnOrientationChanged (Orientation oldOrientation, Orientation newOrientation)
+    public void OnOrientationChanged (Orientation newOrientation)
     {
         _config._sliderOrientation = newOrientation;
 

UnitTests/View/Orientation/OrientationHelperTests.cs

@@ -36,7 +36,7 @@ public class OrientationHelperTests
                         .Callback (() => onChangingOverrideCalled = true)
                         .Returns (false); // Ensure it doesn't cancel the change
 
-        mockIOrientation.Setup (x => x.OnOrientationChanged (It.IsAny<Orientation> (), It.IsAny<Orientation> ()))
+        mockIOrientation.Setup (x => x.OnOrientationChanged (It.IsAny<Orientation> ()))
                         .Callback (() => onChangedOverrideCalled = true);
 
         var orientationHelper = new OrientationHelper (mockIOrientation.Object);

UnitTests/View/Orientation/OrientationTests.cs

@@ -21,7 +21,7 @@ public class OrientationTests
         }
 
         public event EventHandler<CancelEventArgs<Orientation>> OrientationChanging;
-        public event EventHandler<CancelEventArgs<Orientation>> OrientationChanged;
+        public event EventHandler<EventArgs<Orientation>> OrientationChanged;
 
         public bool CancelOnOrientationChanging { get; set; }
 
@@ -35,7 +35,7 @@ public class OrientationTests
             return CancelOnOrientationChanging; // Return true to cancel the change
         }
 
-        public void OnOrientationChanged (Orientation oldOrientation, Orientation newOrientation)
+        public void OnOrientationChanged (Orientation newOrientation)
         {
             OnOrientationChangedCalled = true;
             // Custom logic after orientation has changed