Add TabAssist.TabScrollOffset to give control of the offset

Default value (40, although spec says 52) is set in style to allow easy override at the call site.

src/MaterialDesignThemes.Wpf/Behaviors/Internal/TabControlHeaderScrollBehavior.cs

@@ -65,10 +65,6 @@ private static void OnScrollableContentChanged(DependencyObject d, DependencyPro
         behavior.AddPaddingToScrollableContentIfWiderThanViewPort();
     }
 
-    internal const double ScrollOffset = 40;    // MD spec says 52 DP, but that seems a little excessive in practice
-    internal static readonly Thickness ScrollableContentPadding = new(ScrollOffset, 0, ScrollOffset, 0);
-    internal static readonly Thickness NoScrollableContentPadding = new(0);
-
     private double? _desiredScrollStart;
     private bool _isAnimatingScroll;
 
@@ -103,11 +99,12 @@ private void AddPaddingToScrollableContentIfWiderThanViewPort()
 
         if (ScrollableContent.ActualWidth > TabControl.ActualWidth)
         {
-            ScrollableContent.Margin = ScrollableContentPadding;
+            double offset = TabAssist.GetTabScrollOffset(TabControl);
+            ScrollableContent.Margin = new(offset, 0, offset, 0);
         }
         else
         {
-            ScrollableContent.Margin = NoScrollableContentPadding;
+            ScrollableContent.Margin = new();
         }
     }
 

src/MaterialDesignThemes.Wpf/Internal/BringIntoViewHijackingStackPanel.cs

@@ -15,6 +15,16 @@ public TabScrollDirection TabScrollDirection
         DependencyProperty.Register(nameof(TabScrollDirection), typeof(TabScrollDirection),
             typeof(BringIntoViewHijackingStackPanel), new PropertyMetadata(TabScrollDirection.Unknown));
 
+    public double TabScrollOffset
+    {
+        get => (double)GetValue(TabScrollOffsetProperty);
+        set => SetValue(TabScrollOffsetProperty, value);
+    }
+
+    public static readonly DependencyProperty TabScrollOffsetProperty =
+        DependencyProperty.Register(nameof(TabScrollOffset),
+            typeof(double), typeof(BringIntoViewHijackingStackPanel), new PropertyMetadata(0d));
+
     public BringIntoViewHijackingStackPanel()
         => AddHandler(FrameworkElement.RequestBringIntoViewEvent, new RoutedEventHandler(OnRequestBringIntoView), false);
 
@@ -26,8 +36,8 @@ private void OnRequestBringIntoView(object sender, RoutedEventArgs e)
 
             // TODO: Consider making the "TabScrollDirection" a destructive read (i.e. reset the value once it is read) to avoid leaving a Backward/Forward value that may be misinterpreted at a later stage.
             double offset = TabScrollDirection switch {
-                TabScrollDirection.Backward => -TabControlHeaderScrollBehavior.ScrollOffset,
-                TabScrollDirection.Forward => TabControlHeaderScrollBehavior.ScrollOffset,
+                TabScrollDirection.Backward => -TabScrollOffset,
+                TabScrollDirection.Forward => TabScrollOffset,
                 _ => 0
             };
             var point = child.TranslatePoint(new Point(), this);

src/MaterialDesignThemes.Wpf/TabAssist.cs

@@ -78,4 +78,14 @@ public static void SetAnimateTabScrolling(DependencyObject obj, bool value)
 
     public static readonly DependencyProperty AnimateTabScrollingProperty =
         DependencyProperty.RegisterAttached("AnimateTabScrolling", typeof(bool), typeof(TabAssist), new PropertyMetadata(false));
+
+    public static double GetTabScrollOffset(DependencyObject obj)
+        => (double)obj.GetValue(TabScrollOffsetProperty);
+
+    public static void SetTabScrollOffset(DependencyObject obj, double value)
+        => obj.SetValue(TabScrollOffsetProperty, value);
+
+    public static readonly DependencyProperty TabScrollOffsetProperty =
+        DependencyProperty.RegisterAttached("TabScrollOffset", typeof(double),
+            typeof(TabAssist), new PropertyMetadata(0d));
 }

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.TabControl.xaml

@@ -42,7 +42,9 @@
           <b:Interaction.Behaviors>
             <behaviorsInternal:TabControlHeaderScrollBehavior TabControl="{Binding RelativeSource={RelativeSource TemplatedParent}}" ScrollableContent="{Binding ElementName=ScrollableContent}" />
           </b:Interaction.Behaviors>
-          <internal:BringIntoViewHijackingStackPanel x:Name="ScrollableContent" TabScrollDirection="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(behaviorsInternal:TabControlHeaderScrollBehavior.TabScrollDirection)}">
+          <internal:BringIntoViewHijackingStackPanel x:Name="ScrollableContent"
+                                                     TabScrollDirection="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(behaviorsInternal:TabControlHeaderScrollBehavior.TabScrollDirection)}"
+                                                     TabScrollOffset="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:TabAssist.TabScrollOffset)}">
             <UniformGrid x:Name="CenteredHeaderPanel"
                                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                      Margin="{Binding Path=(wpf:TabAssist.HeaderPanelMargin), RelativeSource={RelativeSource TemplatedParent}}"
@@ -234,6 +236,8 @@
     <Setter Property="wpf:RippleAssist.Feedback" Value="{DynamicResource MaterialDesign.Brush.Button.Ripple}" />
     <Setter Property="wpf:TabAssist.HasUniformTabWidth" Value="False" />
     <Setter Property="wpf:TabAssist.AnimateTabScrolling" Value="True" />
+    <!-- MD spec says 52 DP, but that seems a little excessive in practice -->
+    <Setter Property="wpf:TabAssist.TabScrollOffset" Value="40" />
 
     <Style.Triggers>
       <Trigger Property="wpf:TabAssist.HeaderBehavior" Value="Wrapping">