Discovered serious issues with how HasFocus, OnEnter/OnLeave, etc... work in some edge cases.

This will require re-visiting the design at a deep level and fixing some long-standing but ignored issues such as how OnEnter/OnLeave don't follow proper cancelation design. Also, there's a need for keeping track of the old focus state of a tree of subviews when that tree loses focus; FocusDireciton is a hack that causes tons of confusion.

UnitTests/Application/Application.NavigationTests.cs

@@ -1,12 +1,37 @@
 using Moq;
 using Xunit.Abstractions;
+using Terminal.Gui;
 
-namespace Terminal.Gui.ApplicationTests;
+namespace Terminal.Gui.ApplicationTests.NavigationTests;
 
 public class ApplicationNavigationTests (ITestOutputHelper output)
 {
     private readonly ITestOutputHelper _output = output;
 
+    [Fact]
+    public void Focused_Change_Raises_FocusedChanged ()
+    {
+        bool raised = false;
+
+        ApplicationNavigation.FocusedChanged += ApplicationNavigationOnFocusedChanged;
+
+        ApplicationNavigation.Focused = new View();
+
+        Assert.True (raised);
+
+        ApplicationNavigation.Focused.Dispose ();
+        ApplicationNavigation.Focused = null;
+
+        ApplicationNavigation.FocusedChanged -= ApplicationNavigationOnFocusedChanged;
+
+        return;
+
+        void ApplicationNavigationOnFocusedChanged (object sender, EventArgs e)
+        {
+            raised = true;
+        }
+    }
+
     [Fact]
     public void GetDeepestFocusedSubview_ShouldReturnNull_WhenViewIsNull ()
     {