Ensure padding on tab headers is only applied when they overflow

There is no need for the padding if all tabs can fit inside the visible region.

Author: nicolaihenriksen

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

@@ -33,19 +33,42 @@ public TabControl TabControl
         DependencyProperty.Register(nameof(TabControl), typeof(TabControl),
             typeof(TabControlHeaderScrollBehavior), new PropertyMetadata(null, OnTabControlChanged));
 
+
     private static void OnTabControlChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
     {
         var behavior = (TabControlHeaderScrollBehavior)d;
         if (e.OldValue is TabControl oldTabControl)
         {
             oldTabControl.SelectionChanged -= behavior.OnTabChanged;
+            oldTabControl.SizeChanged -= behavior.OnTabControlSizeChanged;
         }
         if (e.NewValue is TabControl newTabControl)
         {
             newTabControl.SelectionChanged += behavior.OnTabChanged;
+            newTabControl.SizeChanged += behavior.OnTabControlSizeChanged;
         }
     }
 
+    public FrameworkElement ScrollableContent
+    {
+        get => (FrameworkElement)GetValue(ScrollableContentProperty);
+        set => SetValue(ScrollableContentProperty, value);
+    }
+
+    public static readonly DependencyProperty ScrollableContentProperty =
+        DependencyProperty.Register(nameof(ScrollableContent), typeof(FrameworkElement),
+            typeof(TabControlHeaderScrollBehavior), new PropertyMetadata(null, OnScrollableContentChanged));
+
+    private static void OnScrollableContentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+    {
+        var behavior = (TabControlHeaderScrollBehavior)d;
+        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;
 
@@ -70,10 +93,30 @@ bool IsMovingForward()
         int GetItemIndex(object? item) => tabControl.Items.IndexOf(item);
     }
 
+    private void OnTabControlSizeChanged(object sender, SizeChangedEventArgs _) => AddPaddingToScrollableContentIfWiderThanViewPort();
+    private void AssociatedObject_SizeChanged(object sender, SizeChangedEventArgs _) => AddPaddingToScrollableContentIfWiderThanViewPort();
+
+    private void AddPaddingToScrollableContentIfWiderThanViewPort()
+    {
+        if (ScrollableContent is null)
+            return;
+
+        if (ScrollableContent.ActualWidth > TabControl.ActualWidth)
+        {
+            ScrollableContent.Margin = ScrollableContentPadding;
+        }
+        else
+        {
+            ScrollableContent.Margin = NoScrollableContentPadding;
+        }
+    }
+
     protected override void OnAttached()
     {
         base.OnAttached();
         AssociatedObject.ScrollChanged += AssociatedObject_ScrollChanged;
+        AssociatedObject.SizeChanged += AssociatedObject_SizeChanged;
+        Dispatcher.BeginInvoke(() => AddPaddingToScrollableContentIfWiderThanViewPort());
     }
 
     protected override void OnDetaching()
@@ -82,6 +125,7 @@ protected override void OnDetaching()
         if (AssociatedObject is { } ao)
         {
             ao.ScrollChanged -= AssociatedObject_ScrollChanged;
+            ao.SizeChanged -= AssociatedObject_SizeChanged;
         }
     }
 

src/MaterialDesignThemes.Wpf/Internal/BringIntoViewHijackingStackPanel.cs

@@ -24,8 +24,8 @@ private void OnRequestBringIntoView(object sender, RoutedEventArgs e)
         {
             e.Handled = true;
             double offset = TabScrollDirection switch {
-                TabScrollDirection.Backward => -52,
-                TabScrollDirection.Forward => 52,
+                TabScrollDirection.Backward => -TabControlHeaderScrollBehavior.ScrollOffset,
+                TabScrollDirection.Forward => TabControlHeaderScrollBehavior.ScrollOffset,
                 _ => 0
             };
             var point = child.TranslatePoint(new Point(), this);

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

@@ -40,9 +40,9 @@
                               HorizontalScrollBarVisibility="Hidden"
                               VerticalScrollBarVisibility="Hidden">
           <b:Interaction.Behaviors>
-            <behaviorsInternal:TabControlHeaderScrollBehavior TabControl="{Binding RelativeSource={RelativeSource TemplatedParent}}" />
+            <behaviorsInternal:TabControlHeaderScrollBehavior TabControl="{Binding RelativeSource={RelativeSource TemplatedParent}}" ScrollableContent="{Binding ElementName=ScrollableContent}" />
           </b:Interaction.Behaviors>
-          <internal:BringIntoViewHijackingStackPanel TabScrollDirection="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(behaviorsInternal:TabControlHeaderScrollBehavior.TabScrollDirection)}">
+          <internal:BringIntoViewHijackingStackPanel x:Name="ScrollableContent" TabScrollDirection="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(behaviorsInternal:TabControlHeaderScrollBehavior.TabScrollDirection)}">
             <UniformGrid x:Name="CenteredHeaderPanel"
                                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                      Margin="{Binding Path=(wpf:TabAssist.HeaderPanelMargin), RelativeSource={RelativeSource TemplatedParent}}"