first commit

Nitrox.Launcher/Models/Behaviors/FocusOnViewShowBehavior.cs

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+using Avalonia.Controls;
+using Avalonia.Xaml.Interactivity;
+using CommunityToolkit.Mvvm.Messaging;
+
+namespace Nitrox.Launcher.Models.Behaviors;
+
+/// <summary>
+///     Focuses the <see cref="Behavior.AssociatedObject" /> when its parent view is shown.
+/// </summary>
+public class FocusOnViewShowBehavior : Behavior<Control>
+{
+    protected override void OnAttached()
+    {
+        WeakReferenceMessenger.Default.Register<ViewShownMessage>(this, static (obj, _) => (obj as FocusOnViewShowBehavior)?.Focus());
+        base.OnAttached();
+    }
+
+    protected override void OnDetaching()
+    {
+        WeakReferenceMessenger.Default.UnregisterAll(this);
+        base.OnDetaching();
+    }
+
+    protected override void OnAttachedToVisualTree() => Focus();
+
+    private void Focus()
+    {
+        if (AssociatedObject == null)
+        {
+            return;
+        }
+        if (!AssociatedObject.IsEffectivelyVisible)
+        {
+            return;
+        }
+
+        AssociatedObject.Focus();
+        if (AssociatedObject is TextBox textBox)
+        {
+            textBox.SelectAll();
+        }
+    }
+}