Fix failing layout test.

Perspex.Layout/Layoutable.cs

@@ -198,9 +198,11 @@ namespace Perspex.Layout
                 throw new InvalidOperationException("Invalid Arrange rectangle.");
             }
 
-            if (!this.DesiredSize.HasValue)
+            // If the measure was invalidated during an arrange pass, wait for the arrange pass to 
+            // be re-run.
+            if (!this.IsMeasureValid)
             {
-                throw new InvalidOperationException("Arrange called before Measure.");
+                return;
             }
 
             if (force || !this.IsArrangeValid || this.previousArrange != rect)
@@ -233,6 +235,7 @@ namespace Perspex.Layout
             this.IsArrangeValid = false;
             this.previousMeasure = null;
             this.previousArrange = null;
+            this.DesiredSize = null;
 
             if (parent != null && IsResizable(parent))
             {

Tests/Perspex.Layout.UnitTests/FullLayoutTests.cs

@@ -6,12 +6,15 @@
 
 namespace Perspex.Layout.UnitTests
 {
+    using System.Diagnostics;
+    using System.IO;
     using System.Linq;
     using Moq;
     using Perspex.Controls;
     using Perspex.Controls.Presenters;
     using Perspex.Controls.Primitives;
     using Perspex.Controls.Templates;
+    using Perspex.Diagnostics;
     using Perspex.Input;
     using Perspex.Platform;
     using Perspex.Rendering;
@@ -62,6 +65,30 @@ namespace Perspex.Layout.UnitTests
             }
         }
 
+        class TestLogger : ILogger
+        {
+
+            StreamWriter s;
+
+            public TestLogger()
+            {
+                s = new StreamWriter(new FileStream(@"D:\temp\layout.txt", FileMode.Create, FileAccess.ReadWrite));
+            }
+
+            public LogLevel Level
+            {
+                get;
+                set;
+            }
+
+            public void Write(string message, LogLevel logLevel)
+            {
+                if ((int)logLevel < (int)Level) return;
+                s.WriteLine(message);
+                s.Flush();
+            }
+        }
+
         [Fact]
         public void Test_ScrollViewer_With_TextBlock()
         {
@@ -69,6 +96,9 @@ namespace Perspex.Layout.UnitTests
             {
                 this.RegisterServices();
 
+                LogManager.Enable(new TestLogger());
+                LogManager.Instance.LogLayoutMessages = true;
+
                 ScrollViewer scrollViewer;
                 TextBlock textBlock;
 

Tests/Perspex.Layout.UnitTests/Perspex.Layout.UnitTests.csproj

@@ -67,6 +67,10 @@
     <Compile Include="Properties\AssemblyInfo.cs" />
   </ItemGroup>
   <ItemGroup>
+    <ProjectReference Include="..\..\NGenerics\NGenerics.csproj">
+      <Project>{415e048e-4611-4815-9cf2-d774e29079ac}</Project>
+      <Name>NGenerics</Name>
+    </ProjectReference>
     <ProjectReference Include="..\..\Perspex.Animation\Perspex.Animation.csproj">
       <Project>{d211e587-d8bc-45b9-95a4-f297c8fa5200}</Project>
       <Name>Perspex.Animation</Name>
@@ -79,6 +83,10 @@
       <Project>{d2221c82-4a25-4583-9b43-d791e3f6820c}</Project>
       <Name>Perspex.Controls</Name>
     </ProjectReference>
+    <ProjectReference Include="..\..\Perspex.Diagnostics\Perspex.Diagnostics.csproj">
+      <Project>{7062ae20-5dcc-4442-9645-8195bdece63e}</Project>
+      <Name>Perspex.Diagnostics</Name>
+    </ProjectReference>
     <ProjectReference Include="..\..\Perspex.Input\Perspex.Input.csproj">
       <Project>{62024b2d-53eb-4638-b26b-85eeaa54866e}</Project>
       <Name>Perspex.Input</Name>