Make stuff get measured properly.

And added some logging to Layoutable to help diagnose this.

Perspex.Controls/Panel.cs

@@ -7,7 +7,7 @@
 namespace Perspex.Controls
 {
     using System;
-    using System.Collections.Generic;
+    using System.Collections.Specialized;
     using System.Linq;
 
     /// <summary>
@@ -24,22 +24,56 @@ namespace Perspex.Controls
                 if (this.children == null)
                 {
                     this.children = new Controls();
+                    this.children.CollectionChanged += ChildrenChanged;
                 }
 
                 return this.children;
             }
-            
+
             set
             {
                 Contract.Requires<ArgumentNullException>(value != null);
 
                 if (this.children != value)
                 {
+                    if (this.children != null)
+                    {
+                        this.children.CollectionChanged -= ChildrenChanged;
+                    }
+
                     this.children = value;
                     this.ClearVisualChildren();
-                    this.AddVisualChildren(value);
+
+                    if (this.children != null)
+                    {
+                        this.children.CollectionChanged += ChildrenChanged;
+                        this.AddVisualChildren(value);
+                        this.InvalidateMeasure();
+                    }
                 }
             }
         }
+
+        private void ChildrenChanged(object sender, NotifyCollectionChangedEventArgs e)
+        {
+            // TODO: Handle Move and Replace.
+            switch (e.Action)
+            {
+                case NotifyCollectionChangedAction.Add:
+                    this.AddVisualChildren(e.NewItems.OfType<Visual>());
+                    break;
+
+                case NotifyCollectionChangedAction.Remove:
+                    this.RemoveVisualChildren(e.OldItems.OfType<Visual>());
+                    break;
+
+                case NotifyCollectionChangedAction.Reset:
+                    this.ClearVisualChildren();
+                    this.AddVisualChildren(this.children);
+                    break;
+            }
+
+            this.InvalidateMeasure();
+        }
     }
 }

Perspex.Layout/Layoutable.cs

@@ -8,6 +8,7 @@ namespace Perspex.Layout
 {
     using System;
     using System.Linq;
+    using Splat;
 
     public enum HorizontalAlignment
     {
@@ -25,7 +26,7 @@ namespace Perspex.Layout
         Bottom,
     }
 
-    public class Layoutable : Visual, ILayoutable
+    public class Layoutable : Visual, ILayoutable, IEnableLogger
     {
         public static readonly PerspexProperty<double> WidthProperty =
             PerspexProperty.Register<Layoutable, double>("Width", double.NaN);
@@ -123,10 +124,22 @@ namespace Perspex.Layout
         {
             availableSize = availableSize.Deflate(this.Margin);
             this.DesiredSize = this.MeasureCore(availableSize).Constrain(availableSize);
+
+            this.Log().Debug(string.Format(
+                "Measure of {0} (#{1:x8}) requested {2} ",
+                this.GetType().Name,
+                this.GetHashCode(),
+                this.DesiredSize));
         }
 
         public void Arrange(Rect rect)
         {
+            this.Log().Debug(string.Format(
+                "Arrange of {0} (#{1:x8}) gave {2} ",
+                this.GetType().Name,
+                this.GetHashCode(),
+                rect));
+
             if (this.DesiredSize.HasValue)
             {
                 this.ArrangeCore(rect);

Perspex.SceneGraph/Visual.cs

@@ -143,6 +143,18 @@ namespace Perspex
             this.visualChildren.Remove(visual);
         }
 
+        protected void RemoveVisualChildren(IEnumerable<Visual> visuals)
+        {
+            Contract.Requires<ArgumentNullException>(visuals != null);
+
+            this.EnsureVisualChildrenCreated();
+
+            foreach (var v in visuals)
+            {
+                this.visualChildren.Remove(v);
+            }
+        }
+
         protected void SetVisualBounds(Rect bounds)
         {
             this.bounds = bounds;

Perspex.Windows/Window.cs

@@ -33,8 +33,6 @@ namespace Perspex.Windows
 
         private IInputManager inputManager;
 
-        private bool layoutPending;
-
         public Window()
         {
             IPlatformRenderInterface factory = Locator.Current.GetService<IPlatformRenderInterface>();
@@ -47,9 +45,8 @@ namespace Perspex.Windows
             this.inputManager = Locator.Current.GetService<IInputManager>();
             this.Template = ControlTemplate.Create<Window>(this.DefaultTemplate);
 
-            this.LayoutManager.LayoutNeeded.Where(_ => !this.layoutPending).Subscribe(x => 
+            this.LayoutManager.LayoutNeeded.Subscribe(x => 
             {
-                this.layoutPending = true;
                 WindowsDispatcher.CurrentDispatcher.BeginInvoke(
                     DispatcherPriority.Render, 
                     () =>
@@ -57,22 +54,18 @@ namespace Perspex.Windows
                         this.LayoutManager.ExecuteLayoutPass();
                         this.renderer.Render(this);
                         this.RenderManager.RenderFinished();
-                        this.layoutPending = false;
                     });
             });
 
-            this.RenderManager.RenderNeeded.Where(_ => !layoutPending)
+            this.RenderManager.RenderNeeded
                 .Subscribe(x =>
             {
                 WindowsDispatcher.CurrentDispatcher.BeginInvoke(
                     DispatcherPriority.Render,
                     () =>
                     {
-                        if (!this.layoutPending)
-                        {
-                            this.renderer.Render(this);
-                            this.RenderManager.RenderFinished();
-                        }
+                        this.renderer.Render(this);
+                        this.RenderManager.RenderFinished();
                     });
             });
         }