fix(controls): Fix Separator layout and add Orientation attached prop…

src/Wpf.Ui/Controls/Separator/Separator.xaml

@@ -5,28 +5,54 @@
     All Rights Reserved.
 -->
 
-<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+<ResourceDictionary
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:controls="clr-namespace:Wpf.Ui.Controls">
 
-    <Style TargetType="{x:Type Separator}">
-        <Setter Property="BorderBrush" Value="{DynamicResource SeparatorBorderBrush}" />
-        <Setter Property="Background" Value="Transparent" />
-        <Setter Property="Margin" Value="0" />
-        <Setter Property="BorderThickness" Value="1,1,0,0" />
-        <Setter Property="Focusable" Value="False" />
-        <Setter Property="SnapsToDevicePixels" Value="True" />
-        <Setter Property="OverridesDefaultStyle" Value="True" />
+    <!--
+        Provides two default resource keys:
+        - Horizontal: DefaultSeparatorHorizontalMargin  
+        - Vertical: DefaultSeparatorVerticalMargin
+
+        Allows developers to customize default margins for each orientation.
+    -->
+    <Thickness x:Key="DefaultSeparatorHorizontalMargin">0,2,0,2</Thickness>
+    <Thickness x:Key="DefaultSeparatorVerticalMargin">2,0,2,0</Thickness>
+
+    <Style x:Key="DefaultSeparatorStyle" TargetType="{x:Type Separator}">
+        <!--
+            Note: Any existing code referencing the "SeparatorBorderBrush" resource key will
+            need to be updated to "SeparatorBackgroundBrush".
+        -->
+        <Setter Property="Background" Value="{DynamicResource SeparatorBackgroundBrush}" />
+        <Setter Property="Margin" Value="{DynamicResource SeparatorHorizontalMargin}" />
+        <Setter Property="Focusable" Value="false" />
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type Separator}">
                     <Border
-                        Width="{TemplateBinding Width}"
-                        Margin="{TemplateBinding Margin}"
+                        x:Name="SeparatorBorder"
+                        Height="1"
+                        HorizontalAlignment="Stretch"
+                        VerticalAlignment="Center"
                         Background="{TemplateBinding Background}"
                         BorderBrush="{TemplateBinding BorderBrush}"
-                        BorderThickness="{TemplateBinding BorderThickness}" />
+                        BorderThickness="{TemplateBinding BorderThickness}"
+                        SnapsToDevicePixels="true" />
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="controls:SeparatorAttached.Orientation" Value="Vertical">
+                            <Setter TargetName="SeparatorBorder" Property="Width" Value="1" />
+                            <Setter TargetName="SeparatorBorder" Property="Height" Value="Auto" />
+                            <Setter TargetName="SeparatorBorder" Property="HorizontalAlignment" Value="Center" />
+                            <Setter TargetName="SeparatorBorder" Property="VerticalAlignment" Value="Stretch" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
                 </ControlTemplate>
             </Setter.Value>
         </Setter>
     </Style>
 
+    <Style BasedOn="{StaticResource DefaultSeparatorStyle}" TargetType="{x:Type Separator}" />
+
 </ResourceDictionary>

src/Wpf.Ui/Controls/Separator/SeparatorAttached.cs

@@ -0,0 +1,91 @@
+// This Source Code Form is subject to the terms of the MIT License.
+// If a copy of the MIT was not distributed with this file, You can obtain one at https://opensource.org/licenses/MIT.
+// Copyright (C) Leszek Pomianowski and WPF UI Contributors.
+// All Rights Reserved.
+
+using System.Windows.Controls;
+
+// ReSharper disable once CheckNamespace
+namespace Wpf.Ui.Controls;
+
+/// <summary>
+/// Provides <c>Orientation</c> property for Separator
+/// </summary>
+/// <example>
+/// <code lang="xml">
+/// &lt;Separator controls:SeparatorAttached.Orientation="Vertical" /&gt;
+/// </code>
+/// </example>
+public static class SeparatorAttached
+{
+    /// <summary>
+    /// Resource key for <see cref="Separator"/> horizontal margin.
+    /// </summary>
+    public const string HorizontalMarginKey = "DefaultSeparatorHorizontalMargin";
+
+    /// <summary>
+    /// Resource key for <see cref="Separator"/> vertical margin.
+    /// </summary>
+    public const string VerticalMarginKey = "DefaultSeparatorVerticalMargin";
+
+    public static readonly DependencyProperty OrientationProperty =
+        DependencyProperty.RegisterAttached(
+            "Orientation",
+            typeof(Orientation),
+            typeof(SeparatorAttached),
+            new FrameworkPropertyMetadata(Orientation.Horizontal, OnOrientationChanged)
+        );
+
+    /// <summary>Helper for getting <see cref="OrientationProperty"/> from <paramref name="obj"/>.</summary>
+    /// <param name="obj"><see cref="DependencyObject"/> to read <see cref="OrientationProperty"/> from.</param>
+    /// <returns>Orientation property value.</returns>
+    [AttachedPropertyBrowsableForType(typeof(Separator))]
+    public static Orientation GetOrientation(DependencyObject obj)
+    {
+        return (Orientation)obj.GetValue(OrientationProperty);
+    }
+
+    /// <summary>Helper for setting <see cref="OrientationProperty"/> on <paramref name="obj"/>.</summary>
+    /// <param name="obj"><see cref="DependencyObject"/> to set <see cref="OrientationProperty"/> on.</param>
+    /// <param name="value">Orientation property value.</param>
+    public static void SetOrientation(DependencyObject obj, Orientation value)
+    {
+        obj.SetValue(OrientationProperty, value);
+    }
+
+    private static void OnOrientationChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+    {
+        if (d is not FrameworkElement element)
+        {
+            return;
+        }
+
+        Thickness currentMargin = element.Margin;
+        Orientation newOrientation = (Orientation)e.NewValue;
+
+        // Smart margin logic:
+        // 1. When switching orientation, if Margin still equals the horizontal default,
+        //    we assume the user hasn't customized it and apply the new orientation's default.
+        // 2. Subsequent orientation switches only adjust the margin if it still matches
+        //    the previous orientation's default.
+        // 3. If the user coincidentally sets Margin to a value that matches one default,
+        //    switching to the other orientation will still apply the correct default.
+        // 4. Any non‑default value set by the user is treated as intentional and preserved.
+        if (newOrientation == Orientation.Vertical)
+        {
+            var horizontalMargin = element.TryFindResource(HorizontalMarginKey) as Thickness?;
+            if (currentMargin == horizontalMargin)
+            {
+                element.SetResourceReference(FrameworkElement.MarginProperty, VerticalMarginKey);
+            }
+        }
+        else if (newOrientation == Orientation.Horizontal)
+        {
+            var verticalMargin = element.TryFindResource(VerticalMarginKey) as Thickness?;
+            if (currentMargin == verticalMargin)
+            {
+                element.SetResourceReference(FrameworkElement.MarginProperty, HorizontalMarginKey);
+            }
+        }
+    }
+}

src/Wpf.Ui/Resources/Theme/Dark.xaml

@@ -569,7 +569,7 @@
     <SolidColorBrush x:Key="RatingControlSelectedForeground" Color="{DynamicResource SystemAccentColorPrimary}" />
 
     <!--  Separator  -->
-    <SolidColorBrush x:Key="SeparatorBorderBrush" Color="{StaticResource DividerStrokeColorDefault}" />
+    <SolidColorBrush x:Key="SeparatorBackgroundBrush" Color="{StaticResource DividerStrokeColorDefault}" />
 
     <!--  Slider  -->
     <SolidColorBrush x:Key="SliderTrackFill" Color="{StaticResource ControlStrongFillColorDefault}" />

src/Wpf.Ui/Resources/Theme/HC1.xaml

@@ -454,7 +454,7 @@
     <SolidColorBrush x:Key="RatingControlSelectedForeground" Color="{StaticResource SystemColorHighlightColor}" />
 
     <!--  Separator  -->
-    <SolidColorBrush x:Key="SeparatorBorderBrush" Color="{StaticResource SystemColorWindowTextColor}" />
+    <SolidColorBrush x:Key="SeparatorBackgroundBrush" Color="{StaticResource SystemColorWindowTextColor}" />
 
     <!--  Slider  -->
     <SolidColorBrush x:Key="SliderTrackFill" Color="{StaticResource SystemColorWindowTextColor}" />

src/Wpf.Ui/Resources/Theme/HC2.xaml

@@ -453,7 +453,7 @@
     <SolidColorBrush x:Key="RatingControlSelectedForeground" Color="{StaticResource SystemColorHighlightColor}" />
 
     <!--  Separator  -->
-    <SolidColorBrush x:Key="SeparatorBorderBrush" Color="{StaticResource SystemColorWindowTextColor}" />
+    <SolidColorBrush x:Key="SeparatorBackgroundBrush" Color="{StaticResource SystemColorWindowTextColor}" />
 
     <!--  Slider  -->
     <SolidColorBrush x:Key="SliderTrackFill" Color="{StaticResource SystemColorWindowTextColor}" />

src/Wpf.Ui/Resources/Theme/HCBlack.xaml

@@ -453,7 +453,7 @@
     <SolidColorBrush x:Key="RatingControlSelectedForeground" Color="{StaticResource SystemColorHighlightColor}" />
 
     <!--  Separator  -->
-    <SolidColorBrush x:Key="SeparatorBorderBrush" Color="{StaticResource SystemColorWindowTextColor}" />
+    <SolidColorBrush x:Key="SeparatorBackgroundBrush" Color="{StaticResource SystemColorWindowTextColor}" />
 
     <!--  Slider  -->
     <SolidColorBrush x:Key="SliderTrackFill" Color="{StaticResource SystemColorWindowTextColor}" />

src/Wpf.Ui/Resources/Theme/HCWhite.xaml

@@ -453,7 +453,7 @@
     <SolidColorBrush x:Key="RatingControlSelectedForeground" Color="{StaticResource SystemColorHighlightColor}" />
 
     <!--  Separator  -->
-    <SolidColorBrush x:Key="SeparatorBorderBrush" Color="{StaticResource SystemColorWindowTextColor}" />
+    <SolidColorBrush x:Key="SeparatorBackgroundBrush" Color="{StaticResource SystemColorWindowTextColor}" />
 
     <!--  Slider  -->
     <SolidColorBrush x:Key="SliderTrackFill" Color="{StaticResource SystemColorWindowTextColor}" />

src/Wpf.Ui/Resources/Theme/Light.xaml

@@ -570,7 +570,7 @@
     <SolidColorBrush x:Key="RatingControlSelectedForeground" Color="{DynamicResource SystemAccentColorPrimary}" />
 
     <!--  Separator  -->
-    <SolidColorBrush x:Key="SeparatorBorderBrush" Color="{StaticResource DividerStrokeColorDefault}" />
+    <SolidColorBrush x:Key="SeparatorBackgroundBrush" Color="{StaticResource DividerStrokeColorDefault}" />
 
     <!--  Slider  -->
     <SolidColorBrush x:Key="SliderTrackFill" Color="{StaticResource ControlStrongFillColorDefault}" />