Add DefaultFocusAdornerProperty and set for themes

src/Avalonia.Controls/Control.cs

@@ -403,7 +403,7 @@ namespace Avalonia.Controls
                 {
                     if (_focusAdorner == null)
                     {
-                        var template = GetValue(FocusAdornerProperty);
+                        var template = GetValue(FocusAdornerProperty) ?? adornerLayer.DefaultFocusAdorner;
 
                         if (template != null)
                         {

src/Avalonia.Controls/Primitives/AdornerLayer.cs

@@ -34,6 +34,12 @@ namespace Avalonia.Controls.Primitives
         public static readonly AttachedProperty<Control?> AdornerProperty =
             AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, Control?>("Adorner");
 
+        /// <summary>
+        /// Defines the <see cref="DefaultFocusAdorner"/> property.
+        /// </summary>
+        public static readonly StyledProperty<ITemplate<Control>?> DefaultFocusAdornerProperty =
+            AvaloniaProperty.Register<Control, ITemplate<Control>?>(nameof(DefaultFocusAdorner));
+        
         private static readonly AttachedProperty<AdornedElementInfo?> s_adornedElementInfoProperty =
             AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, AdornedElementInfo?>("AdornedElementInfo");
 
@@ -86,6 +92,15 @@ namespace Avalonia.Controls.Primitives
             visual.SetValue(AdornerProperty, adorner);
         }
 
+        /// <summary>
+        /// Gets or sets the default control's focus adorner.
+        /// </summary>
+        public ITemplate<Control>? DefaultFocusAdorner
+        {
+            get => GetValue(DefaultFocusAdornerProperty);
+            set => SetValue(DefaultFocusAdornerProperty, value);
+        }
+        
         private static void AdornerChanged(AvaloniaPropertyChangedEventArgs<Control?> e)
         {
             if (e.Sender is Visual visual)

src/Avalonia.Themes.Fluent/Controls/FocusAdorner.xaml → src/Avalonia.Themes.Fluent/Controls/AdornerLayer.xaml

@@ -1,14 +1,11 @@
-<Styles xmlns="https://github.com/avaloniaui"
-        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
-  <Styles.Resources>
-    <Thickness x:Key="SystemControlFocusVisualMargin">0</Thickness>
-    <Thickness x:Key="SystemControlFocusVisualPrimaryThickness">2</Thickness>
-    <Thickness x:Key="SystemControlFocusVisualSecondaryThickness">1</Thickness>
-  </Styles.Resources>
-
-  <!--  HighVisibility FocusAdorner  -->
-  <Style Selector=":is(Control)">
-    <Setter Property="FocusAdorner">
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+  <Thickness x:Key="SystemControlFocusVisualMargin">0</Thickness>
+  <Thickness x:Key="SystemControlFocusVisualPrimaryThickness">2</Thickness>
+  <Thickness x:Key="SystemControlFocusVisualSecondaryThickness">1</Thickness>
+  
+  <ControlTheme x:Key="{x:Type AdornerLayer}" TargetType="AdornerLayer">
+    <Setter Property="DefaultFocusAdorner">
       <FocusAdornerTemplate>
         <Border BorderThickness="{DynamicResource SystemControlFocusVisualPrimaryThickness}"
                 BorderBrush="{DynamicResource SystemControlFocusVisualPrimaryBrush}"
@@ -18,5 +15,5 @@
         </Border>
       </FocusAdornerTemplate>
     </Setter>
-  </Style>
-</Styles>
+  </ControlTheme>
+</ResourceDictionary>

src/Avalonia.Themes.Fluent/Controls/FluentControls.xaml

@@ -4,6 +4,7 @@
     <Styles.Resources>
         <ResourceDictionary>
             <ResourceDictionary.MergedDictionaries>
+                <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/AdornerLayer.xaml" />
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/Button.xaml" />
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/RadioButton.xaml" />
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/Expander.xaml" />
@@ -74,6 +75,5 @@
         </ResourceDictionary>
     </Styles.Resources>
 
-    <StyleInclude Source="avares://Avalonia.Themes.Fluent/Controls/FocusAdorner.xaml" />
     <StyleInclude Source="avares://Avalonia.Themes.Fluent/Controls/UserControl.xaml" />
 </Styles>

src/Avalonia.Themes.Simple/Controls/AdornerLayer.xaml

@@ -0,0 +1,13 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+  <ControlTheme x:Key="{x:Type AdornerLayer}" TargetType="AdornerLayer">
+    <Setter Property="DefaultFocusAdorner">
+      <FocusAdornerTemplate>
+        <Rectangle Margin="1"
+                   Stroke="Black"
+                   StrokeDashArray="1,2"
+                   StrokeThickness="1" />
+      </FocusAdornerTemplate>
+    </Setter>
+  </ControlTheme>
+</ResourceDictionary>

src/Avalonia.Themes.Simple/Controls/FocusAdorner.xaml

@@ -1,11 +0,0 @@
-<Style xmlns="https://github.com/avaloniaui"
-       Selector=":is(Control)">
-  <Setter Property="FocusAdorner">
-    <FocusAdornerTemplate>
-      <Rectangle Margin="1"
-                 Stroke="Black"
-                 StrokeDashArray="1,2"
-                 StrokeThickness="1" />
-    </FocusAdornerTemplate>
-  </Setter>
-</Style>

src/Avalonia.Themes.Simple/Controls/SimpleControls.xaml

@@ -3,6 +3,7 @@
   <Styles.Resources>
     <ResourceDictionary>
       <ResourceDictionary.MergedDictionaries>
+        <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/AdornerLayer.xaml" />
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/Button.xaml" />
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/RadioButton.xaml" />
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/Expander.xaml" />
@@ -72,6 +73,5 @@
     </ResourceDictionary>
   </Styles.Resources>
 
-  <StyleInclude Source="avares://Avalonia.Themes.Simple/Controls/FocusAdorner.xaml" />
   <StyleInclude Source="avares://Avalonia.Themes.Simple/Controls/UserControl.xaml" />
 </Styles>