Fix scrollviewer causing lockups.

Add a throttle to showing/hiding scrollbars to prevent them flashing on
and off.

Perspex.Controls/ScrollViewer.cs

@@ -10,6 +10,7 @@ namespace Perspex.Controls
     using System.Reactive.Linq;
     using Perspex.Controls.Presenters;
     using Perspex.Controls.Primitives;
+    using Perspex.Threading;
 
     public class ScrollViewer : ContentControl
     {
@@ -62,7 +63,8 @@ namespace Perspex.Controls
             var extentAndViewport = Observable.CombineLatest(
                 this.GetObservable(ExtentProperty),
                 this.GetObservable(ViewportProperty))
-                .Select(x => new { Extent = x[0], Viewport = x[1] });
+                .Select(x => new { Extent = x[0], Viewport = x[1] })
+                .Throttle(TimeSpan.FromMilliseconds(100), PerspexScheduler.Instance);
 
             this.Bind(
                 VerticalScrollBarViewportSizeProperty,

Perspex.Diagnostics/DevTools.cs

@@ -118,10 +118,8 @@ namespace Perspex.Diagnostics
             {
                 Window window = new Window
                 {
-                    // HACK: Set width and height here as a quick fix as there's a problem with 
+                    Width = 1024,
-                    // the dev tools window hanging when it's set to auto-size.
+                    Height = 512,
-                    Width = 800,
-                    Height = 600,
                     Content = new DevTools
                     {
                         Root = (Window)sender,