feat: add new event.

src/Ursa/Controls/NavMenu/NavMenu.cs

@@ -189,6 +189,16 @@ public class NavMenu : ItemsControl, ICustomKeyboardNavigation
         add => AddHandler(SelectionChangedEvent, value);
         remove => RemoveHandler(SelectionChangedEvent, value);
     }
+    
+    
+    public static readonly RoutedEvent<SelectionChangedEventArgs> SelectionChangingEvent =
+        RoutedEvent.Register<NavMenu, SelectionChangedEventArgs>(nameof(SelectionChanging), RoutingStrategies.Bubble);
+
+    public event EventHandler SelectionChanging
+    {
+        add => AddHandler(SelectionChangingEvent, value);
+        remove => RemoveHandler(SelectionChangingEvent, value);
+    }
 
     protected override bool NeedsContainerOverride(object? item, int index, out object? recycleKey)
     {
@@ -486,4 +496,22 @@ public class NavMenu : ItemsControl, ICustomKeyboardNavigation
 
         return null;
     }
+
+    internal bool CanChangeSelection(NavMenuItem item)
+    {
+        object? newSelection = null;
+        if (item.DataContext is not null && item.DataContext != DataContext)
+            newSelection = item.DataContext;
+        else
+            newSelection = item;
+        var args = new SelectionChangingEventArgs(
+            SelectionChangingEvent, 
+            new[] { SelectedItem },
+            new[] { newSelection })
+        {
+            Source = this,
+        };
+        RaiseEvent(args);
+        return args.CanSelect;
+    }
 }

src/Ursa/Controls/NavMenu/SelectionChangingEventArgs.cs

@@ -0,0 +1,24 @@
+using System.Collections;
+using Avalonia.Interactivity;
+
+namespace Ursa.Controls;
+
+public class SelectionChangingEventArgs: RoutedEventArgs
+{
+    /// <summary>Gets the items that were added to the selection.</summary>
+    public IList AddedItems { get; }
+
+    /// <summary>Gets the items that were removed from the selection.</summary>
+    public IList RemovedItems { get; }
+    
+    /// <summary>
+    /// Gets or sets a value indicating whether the selection can be changed. If set to <c>false</c>, the selection will not change.
+    /// </summary>
+    public bool CanSelect { get; set; } = true;
+    
+    public SelectionChangingEventArgs(RoutedEvent routedEvent, IList removedItems, IList addedItems): base(routedEvent)
+    {
+        RemovedItems = removedItems;
+        AddedItems = addedItems;
+    }
+}