Create FocusAutoCompleteBoxTextBoxBehavior.cs

src/Avalonia.Xaml.Interactions.Custom/AutoCompleteBox/FocusAutoCompleteBoxTextBoxBehavior.cs

@@ -0,0 +1,33 @@
+using System.Linq;
+using System.Reactive.Disposables;
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+
+namespace Avalonia.Xaml.Interactions.Custom;
+
+/// <summary>
+/// 
+/// </summary>
+public class FocusAutoCompleteBoxTextBoxBehavior : AttachedToVisualTreeBehavior<AutoCompleteBox>
+{
+    /// <inheritdoc />
+    protected override void OnAttachedToVisualTree(CompositeDisposable disposable)
+    {
+        if (AssociatedObject is null)
+        {
+            return;
+        }
+
+        AssociatedObject.GotFocus += AssociatedObjectOnGotFocus;
+
+        Disposable.Create(() => AssociatedObject.GotFocus -= AssociatedObjectOnGotFocus);
+    }
+
+    private void AssociatedObjectOnGotFocus(object? sender, GotFocusEventArgs e)
+    {
+        var textBox = AssociatedObject?.GetVisualDescendants().OfType<TextBox>().FirstOrDefault();
+        Dispatcher.UIThread.Post(() => textBox?.Focus());
+    }
+}