Re-enable TreeView horizontal scroll.

Perspex.Themes.Default/ScrollViewerStyle.cs

@@ -31,8 +31,6 @@ namespace Perspex.Themes.Default
 
         private Control Template(ScrollViewer control)
         {
-            ScrollBar vert;
-
             var result = new Grid
             {
                 ColumnDefinitions = new ColumnDefinitions
@@ -61,26 +59,24 @@ namespace Perspex.Themes.Default
                         Id = "horizontalScrollBar",
                         Orientation = Orientation.Horizontal,
                         [~ScrollBar.MaximumProperty] = control[~ScrollViewer.HorizontalScrollBarMaximumProperty],
-                        //[~~ScrollBar.ValueProperty] = control[~~ScrollViewer.HorizontalScrollBarValueProperty],
+                        [~~ScrollBar.ValueProperty] = control[~~ScrollViewer.HorizontalScrollBarValueProperty],
                         [~ScrollBar.ViewportSizeProperty] = control[~ScrollViewer.HorizontalScrollBarViewportSizeProperty],
                         [~ScrollBar.VisibilityProperty] = control[~ScrollViewer.HorizontalScrollBarVisibilityProperty],
                         [Grid.RowProperty] = 1,
                     },
-                    (vert = new ScrollBar
+                    new ScrollBar
                     {
                         Id = "verticalScrollBar",
                         Orientation = Orientation.Vertical,
                         [~ScrollBar.MaximumProperty] = control[~ScrollViewer.VerticalScrollBarMaximumProperty],
-                        //[~~ScrollBar.ValueProperty] = control[~~ScrollViewer.VerticalScrollBarValueProperty],
+                        [~~ScrollBar.ValueProperty] = control[~~ScrollViewer.VerticalScrollBarValueProperty],
                         [~ScrollBar.ViewportSizeProperty] = control[~ScrollViewer.VerticalScrollBarViewportSizeProperty],
                         [~ScrollBar.VisibilityProperty] = control[~ScrollViewer.VerticalScrollBarVisibilityProperty],
                         [Grid.ColumnProperty] = 1,
-                    }),
+                    },
                 },
             };
 
-            vert[~~ScrollBar.ValueProperty] = control[~~ScrollViewer.VerticalScrollBarValueProperty];
-
             return result;
         }
     }

Perspex.Themes.Default/TreeViewStyle.cs

@@ -40,6 +40,7 @@ namespace Perspex.Themes.Default
                 [~Border.BorderThicknessProperty] = control[~TreeView.BorderThicknessProperty],
                 Content = new ScrollViewer
                 {
+                    CanScrollHorizontally = true,
                     Content = new ItemsPresenter
                     {
                         Id = "itemsPresenter",