Share default small change value across controls.

src/Avalonia.Controls/Presenters/ItemsPresenter.cs

@@ -103,7 +103,7 @@ namespace Avalonia.Controls.Presenters
         }
 
         /// <inheritdoc/>
-        Size ILogicalScrollable.ScrollSize => new Size(16, 1);
+        Size ILogicalScrollable.ScrollSize => new Size(ScrollViewer.DefaultSmallChange, 1);
 
         /// <inheritdoc/>
         Size ILogicalScrollable.PageScrollSize => Virtualizer?.Viewport ?? new Size(16, 16);

src/Avalonia.Controls/ScrollViewer.cs

@@ -10,8 +10,6 @@ namespace Avalonia.Controls
     /// </summary>
     public class ScrollViewer : ContentControl, IScrollable, IScrollAnchorProvider
     {
-        private static readonly Size s_defaultSmallChange = new Size(16, 16);
-
         /// <summary>
         /// Defines the <see cref="CanHorizontallyScroll"/> property.
         /// </summary>
@@ -167,13 +165,15 @@ namespace Avalonia.Controls
                 nameof(VerticalScrollBarVisibility),
                 ScrollBarVisibility.Auto);
 
+        internal const double DefaultSmallChange = 16;
+
         private IDisposable _childSubscription;
         private ILogicalScrollable _logicalScrollable;
         private Size _extent;
         private Vector _offset;
         private Size _viewport;
         private Size _largeChange;
-        private Size _smallChange = s_defaultSmallChange;
+        private Size _smallChange = new Size(DefaultSmallChange, DefaultSmallChange);
 
         /// <summary>
         /// Initializes static members of the <see cref="ScrollViewer"/> class.
@@ -543,7 +543,7 @@ namespace Avalonia.Controls
             }
             else
             {
-                SetAndRaise(SmallChangeProperty, ref _smallChange, s_defaultSmallChange);
+                SetAndRaise(SmallChangeProperty, ref _smallChange, new Size(DefaultSmallChange, DefaultSmallChange));
                 SetAndRaise(LargeChangeProperty, ref _largeChange, Viewport);
             }
         }