Use tab animation even when tab has focusable content

Reverts the "destructive read" commit because the event may fire twice now, and removing the scroll direction would lead the second event to scroll to a wrong (i.e. not offset) position. So we live with the extra event (in cases where there is no focusable content; probably not the common use-case), and live with the AP/DP value surviving.

Author: nicolaihenriksen

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

@@ -78,6 +78,10 @@ private void OnTabChanged(object sender, SelectionChangedEventArgs e)
         {
             _desiredScrollStart = AssociatedObject.ContentHorizontalOffset;
             SetScrollDirection(tabControl, (IsMovingForward() ? TabScrollDirection.Forward : TabScrollDirection.Backward));
+
+            // In case the TabItem has focusable content, the FrameworkElement.RequestBringIntoView won't fire. The lines below ensures that it fires.
+            var tab = tabControl.ItemContainerGenerator.ContainerFromItem(e.AddedItems[0]) as TabItem;
+            tab?.BringIntoView();
         }
 
         bool IsMovingForward()

src/MaterialDesignThemes.Wpf/Internal/PaddedBringIntoViewStackPanel.cs

@@ -13,7 +13,7 @@ public TabScrollDirection ScrollDirection
 
     public static readonly DependencyProperty ScrollDirectionProperty =
         DependencyProperty.Register(nameof(ScrollDirection), typeof(TabScrollDirection),
-            typeof(PaddedBringIntoViewStackPanel), new FrameworkPropertyMetadata(TabScrollDirection.Unknown, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
+            typeof(PaddedBringIntoViewStackPanel), new PropertyMetadata(TabScrollDirection.Unknown));
 
     public double HeaderPadding
     {
@@ -55,7 +55,6 @@ private static void OnRequestBringIntoView(object sender, RoutedEventArgs e)
                 TabScrollDirection.Forward => panel.HeaderPadding,
                 _ => 0
             };
-            panel.ScrollDirection = TabScrollDirection.Unknown; // Destructive read of panel.ScrollDirection to avoid leaving an invalid value in the DP (and AP).
             var point = child.TranslatePoint(new Point(), panel);
             var newTargetRect = new Rect(new Point(point.X + offset, point.Y), child.RenderSize);
             panel.BringIntoView(newTargetRect);