v2 Fixes #2810. Pressing Alt key on a Toplevel with only a MenuBar throws System.InvalidOperationException.

Terminal.Gui/Application.cs

@@ -832,7 +832,8 @@ public static void End (RunState runState)
 					OverlappedTop.OnAllChildClosed ();
 				} else {
 					SetCurrentOverlappedAsTop ();
-					Current.OnEnter (Current);
+					runState.Toplevel.OnLeave (Current);
+					Current.OnEnter (runState.Toplevel);
 				}
 				Refresh ();
 			}

Terminal.Gui/View/ViewSubViews.cs

@@ -383,7 +383,7 @@ public override bool CanFocus {
 						SuperView?.EnsureFocus ();
 						if (SuperView != null && SuperView.Focused == null) {
 							SuperView.FocusNext ();
-							if (SuperView.Focused == null) {
+							if (SuperView.Focused == null && Application.Current != null) {
 								Application.Current.FocusNext ();
 							}
 							Application.BringOverlappedTopToFront ();

Terminal.Gui/Views/Toplevel.cs

@@ -553,6 +553,7 @@ void FocusNearestView (IEnumerable<View> views, Direction direction)
 		///<inheritdoc/>
 		public override void Add (View view)
 		{
+			CanFocus = true;
 			AddMenuStatusBar (view);
 			base.Add (view);
 		}

Terminal.Gui/Views/Window.cs

@@ -56,29 +56,5 @@ void SetInitialProperties ()
 			ColorScheme = Colors.Base; // TODO: make this a theme property
 			BorderStyle = DefaultBorderStyle;
 		}
-
-		// TODO: Are these overrides really needed? 
-		/// <inheritdoc/>
-		public override void Add (View view)
-		{
-			base.Add (view);
-			if (view.CanFocus) {
-				CanFocus = true;
-			}
-			AddMenuStatusBar (view);
-		}
-
-		/// <inheritdoc/>
-		public override void Remove (View view)
-		{
-			if (view == null) {
-				return;
-			}
-
-			SetNeedsDisplay ();
-			base.Remove (view);
-			RemoveMenuStatusBar (view);
-
-		}
 	}
 }

UnitTests/View/NavigationTests.cs

@@ -764,11 +764,25 @@ public void CanFocus_Sets_To_False_On_Single_View_Focus_View_On_Another_Toplevel
 			Assert.True (view1.CanFocus);
 			Assert.True (view1.HasFocus);
 			Assert.True (view2.CanFocus);
-			Assert.False (view2.HasFocus);
+			Assert.False (view2.HasFocus); // Only one of the most focused toplevels view can have focus
+
+			Assert.True (Application.Top.ProcessKey (new KeyEvent (Key.Tab, new KeyModifiers ())));
+			Assert.True (view1.CanFocus);
+			Assert.False (view1.HasFocus); // Only one of the most focused toplevels view can have focus
+			Assert.True (view2.CanFocus);
+			Assert.True (view2.HasFocus);
+
+			Assert.True (Application.Top.ProcessKey (new KeyEvent (Key.Tab, new KeyModifiers ())));
+			Assert.True (view1.CanFocus);
+			Assert.True (view1.HasFocus);
+			Assert.True (view2.CanFocus);
+			Assert.False (view2.HasFocus); // Only one of the most focused toplevels view can have focus
 
 			view1.CanFocus = false;
 			Assert.False (view1.CanFocus);
 			Assert.False (view1.HasFocus);
+			Assert.True (view2.CanFocus);
+			Assert.True (view2.HasFocus);
 			Assert.Equal (win2, Application.Current.Focused);
 			Assert.Equal (view2, Application.Current.MostFocused);
 		}
@@ -790,11 +804,26 @@ public void CanFocus_Sets_To_False_With_Two_Views_Focus_Another_View_On_The_Same
 			Assert.True (view1.CanFocus);
 			Assert.True (view1.HasFocus);
 			Assert.True (view2.CanFocus);
-			Assert.False (view2.HasFocus);
+			Assert.False (view2.HasFocus); // Only one of the most focused toplevels view can have focus
+
+			Assert.True (Application.Top.ProcessKey (new KeyEvent (Key.Tab | Key.CtrlMask, new KeyModifiers ())));
+			Assert.True (Application.Top.ProcessKey (new KeyEvent (Key.Tab | Key.CtrlMask, new KeyModifiers ())));
+			Assert.True (view1.CanFocus);
+			Assert.False (view1.HasFocus); // Only one of the most focused toplevels view can have focus
+			Assert.True (view2.CanFocus);
+			Assert.True (view2.HasFocus);
+
+			Assert.True (Application.Top.ProcessKey (new KeyEvent (Key.Tab | Key.CtrlMask, new KeyModifiers ())));
+			Assert.True (view1.CanFocus);
+			Assert.True (view1.HasFocus);
+			Assert.True (view2.CanFocus);
+			Assert.False (view2.HasFocus); // Only one of the most focused toplevels view can have focus
 
 			view1.CanFocus = false;
 			Assert.False (view1.CanFocus);
 			Assert.False (view1.HasFocus);
+			Assert.True (view2.CanFocus);
+			Assert.False (view2.HasFocus);
 			Assert.Equal (win1, Application.Current.Focused);
 			Assert.Equal (view12, Application.Current.MostFocused);
 		}
@@ -815,13 +844,27 @@ public void CanFocus_Sets_To_False_On_Toplevel_Focus_View_On_Another_Toplevel ()
 			Assert.True (view1.CanFocus);
 			Assert.True (view1.HasFocus);
 			Assert.True (view2.CanFocus);
-			Assert.False (view2.HasFocus);
+			Assert.False (view2.HasFocus); // Only one of the most focused toplevels view can have focus
+
+			Assert.True (Application.Top.ProcessKey (new KeyEvent (Key.Tab | Key.CtrlMask, new KeyModifiers ())));
+			Assert.True (view1.CanFocus);
+			Assert.False (view1.HasFocus); // Only one of the most focused toplevels view can have focus
+			Assert.True (view2.CanFocus);
+			Assert.True (view2.HasFocus);
+
+			Assert.True (Application.Top.ProcessKey (new KeyEvent (Key.Tab | Key.CtrlMask, new KeyModifiers ())));
+			Assert.True (view1.CanFocus);
+			Assert.True (view1.HasFocus);
+			Assert.True (view2.CanFocus);
+			Assert.False (view2.HasFocus); // Only one of the most focused toplevels view can have focus
 
 			win1.CanFocus = false;
 			Assert.False (view1.CanFocus);
 			Assert.False (view1.HasFocus);
 			Assert.False (win1.CanFocus);
 			Assert.False (win1.HasFocus);
+			Assert.True (view2.CanFocus);
+			Assert.True (view2.HasFocus);
 			Assert.Equal (win2, Application.Current.Focused);
 			Assert.Equal (view2, Application.Current.MostFocused);
 		}

UnitTests/Views/ToplevelTests.cs

@@ -962,7 +962,98 @@ public void OnEnter_OnLeave_Triggered_On_Application_Begin_End ()
 			Application.End (rs);
 
 			Assert.True (isEnter);
-			Assert.False (isLeave);
+			Assert.False (isLeave);  // Leave event cannot be trigger because it v.Enter was performed and v is focused
+			Assert.True (v.HasFocus);
+		}
+
+		[Fact, AutoInitShutdown]
+		public void OnEnter_OnLeave_Triggered_On_Application_Begin_End_With_More_Toplevels ()
+		{
+			var iterations = 0;
+			var steps = new int [5];
+			var isEnterTop = false;
+			var isLeaveTop = false;
+			var vt = new View ();
+			var top = Application.Top;
+			var diag = new Dialog ();
+
+			vt.Enter += (s, e) => {
+				iterations++;
+				isEnterTop = true;
+				if (iterations == 1) {
+					steps [0] = iterations;
+					Assert.Null (e.View);
+				} else {
+					steps [4] = iterations;
+					Assert.Equal (diag, e.View);
+				}
+			};
+			vt.Leave += (s, e) => {
+				iterations++;
+				steps [1] = iterations;
+				isLeaveTop = true;
+				Assert.Equal (diag, e.View);
+			};
+			top.Add (vt);
+
+			Assert.False (vt.CanFocus);
+			var exception = Record.Exception (() => top.OnEnter (top));
+			Assert.Null (exception);
+			exception = Record.Exception (() => top.OnLeave (top));
+			Assert.Null (exception);
+
+			vt.CanFocus = true;
+			Application.Begin (top);
+
+			Assert.True (isEnterTop);
+			Assert.False (isLeaveTop);
+
+			isEnterTop = false;
+			var isEnterDiag = false;
+			var isLeaveDiag = false;
+			var vd = new View ();
+			vd.Enter += (s, e) => {
+				iterations++;
+				steps [2] = iterations;
+				isEnterDiag = true;
+				Assert.Null (e.View);
+			};
+			vd.Leave += (s, e) => {
+				iterations++;
+				steps [3] = iterations;
+				isLeaveDiag = true;
+				Assert.Equal (top, e.View);
+			};
+			diag.Add (vd);
+
+			Assert.False (vd.CanFocus);
+			exception = Record.Exception (() => diag.OnEnter (diag));
+			Assert.Null (exception);
+			exception = Record.Exception (() => diag.OnLeave (diag));
+			Assert.Null (exception);
+
+			vd.CanFocus = true;
+			var rs = Application.Begin (diag);
+
+			Assert.True (isEnterDiag);
+			Assert.False (isLeaveDiag);
+			Assert.False (isEnterTop);
+			Assert.True (isLeaveTop);
+
+			isEnterDiag = false;
+			isLeaveTop = false;
+			Application.End (rs);
+
+			Assert.False (isEnterDiag);
+			Assert.True (isLeaveDiag);
+			Assert.True (isEnterTop);
+			Assert.False (isLeaveTop);  // Leave event cannot be trigger because it v.Enter was performed and v is focused
+			Assert.True (vt.HasFocus);
+			Assert.Equal (1, steps [0]);
+			Assert.Equal (2, steps [1]);
+			Assert.Equal (3, steps [2]);
+			Assert.Equal (4, steps [3]);
+			Assert.Equal (5, steps [^1]);
 		}
 
 		[Fact, AutoInitShutdown]
@@ -1494,5 +1585,22 @@ void Current_DrawContentComplete (object sender, DrawEventArgs e)
 
 			Application.End (rs);
 		}
+
+		[Fact, AutoInitShutdown]
+		public void Activating_MenuBar_By_Alt_Key_Does_Not_Throw ()
+		{
+			var menu = new MenuBar (new MenuBarItem [] {
+				new MenuBarItem ("Child", new MenuItem [] {
+					new MenuItem ("_Create Child", "", null)
+				})
+			});
+			var topChild = new Toplevel ();
+			topChild.Add (menu);
+			Application.Top.Add (topChild);
+			Application.Begin (Application.Top);
+
+			var exception = Record.Exception (() => topChild.ProcessHotKey (new KeyEvent (Key.AltMask, new KeyModifiers { Alt = true })));
+			Assert.Null (exception);
+		}
 	}
 }

UnitTests/Views/WindowTests.cs

@@ -193,5 +193,22 @@ public void OnCanFocusChanged_Only_Must_ContentView_Forces_SetFocus_After_IsInit
 			Assert.False (win2.HasFocus);
 			Assert.False (view2.HasFocus);
 		}
+
+		[Fact, AutoInitShutdown]
+		public void Activating_MenuBar_By_Alt_Key_Does_Not_Throw ()
+		{
+			var menu = new MenuBar (new MenuBarItem [] {
+				new MenuBarItem ("Child", new MenuItem [] {
+					new MenuItem ("_Create Child", "", null)
+				})
+			});
+			var win = new Window ();
+			win.Add (menu);
+			Application.Top.Add (win);
+			Application.Begin (Application.Top);
+
+			var exception = Record.Exception (() => win.ProcessHotKey (new KeyEvent (Key.AltMask, new KeyModifiers { Alt = true })));
+			Assert.Null (exception);
+		}
 	}
 }