Get effective viewport size from forthcoming arrange.

Instead of using `ScrollContentPresenter.Viewport` which hasn't been updated yet.

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -90,7 +90,7 @@ namespace Avalonia.Controls.Presenters
             this.GetObservable(ChildProperty).Subscribe(UpdateScrollableSubscription);
         }
 
-        internal event EventHandler<EventArgs> PreArrange;
+        internal event EventHandler<VectorEventArgs> PreArrange;
 
         /// <summary>
         /// Gets or sets a value indicating whether the content can be scrolled horizontally.
@@ -241,7 +241,10 @@ namespace Avalonia.Controls.Presenters
         /// <inheritdoc/>
         protected override Size ArrangeOverride(Size finalSize)
         {
-            PreArrange?.Invoke(this, EventArgs.Empty);
+            PreArrange?.Invoke(this, new VectorEventArgs
+            {
+                Vector = new Vector(finalSize.Width, finalSize.Height),
+            });
 
             if (_logicalScrollSubscription != null || Child == null)
             {

src/Avalonia.Controls/Repeater/ViewportManager.cs

@@ -9,6 +9,7 @@ using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Threading.Tasks;
 using Avalonia.Controls.Presenters;
+using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Logging;
 using Avalonia.Media;
@@ -537,10 +538,10 @@ namespace Avalonia.Controls
             return Disposable.Empty;
         }
 
-        private void ScrollContentPresenterPreArrange(object sender, EventArgs e)
+        private void ScrollContentPresenterPreArrange(object sender, VectorEventArgs e)
         {
             var scp = (ScrollContentPresenter)sender;
-            var effectiveViewport = new Rect((Point)scp.Offset, scp.Viewport);
+            var effectiveViewport = new Rect((Point)scp.Offset, new Size(e.Vector.X, e.Vector.Y));
 
             if (effectiveViewport != _visibleWindow)
             {