Make initial DropDown selection work

Fixes #110

samples/XamlTestApplication/Views/MainWindow.paml

@@ -2,13 +2,7 @@
         xmlns="https://github.com/perspex"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
         Title="Perspex Test Application" Height="350" Width="525" SizeToContent="WidthAndHeight" >
-
-  <ListBox SelectedIndex="0">
-    <ListBoxItem>Hello</ListBoxItem>
-    <ListBoxItem>World</ListBoxItem>
-  </ListBox>
-  
-    <!--<Grid RowDefinitions="Auto,*">
+    <Grid RowDefinitions="Auto,*">
         <TabControl Grid.Row="1">
             <TabItem Header="Buttons">
                 <StackPanel HorizontalAlignment="Center" Width="200" VerticalAlignment="Center">
@@ -125,5 +119,5 @@
                 </Grid>
             </TabItem>
         </TabControl>
-    </Grid>-->
+    </Grid>
 </Window>

src/Markup/Perspex.Markup.Xaml/OmniXAML

@@ -1 +1 @@
-Subproject commit 1461be589a5d39d378fd177648c61a780271c742
+Subproject commit 49e6ec001f5873cf2290e0bc1f6f06ca9b9cf808

src/Markup/Perspex.Markup.Xaml/Perspex.Markup.Xaml.csproj

@@ -57,6 +57,7 @@
     <Compile Include="DataBinding\ChangeTracking\PropertyPath.cs" />
     <Compile Include="DataBinding\ChangeTracking\TargettedProperty.cs" />
     <Compile Include="Context\PerspexParserFactory.cs" />
+    <Compile Include="OmniXAML\Source\Glass\AutoKeyDictionary.cs" />
     <Compile Include="OmniXAML\Source\Glass\ChangeTracking\ObservableProperty.cs" />
     <Compile Include="OmniXAML\Source\Glass\ChangeTracking\ObservablePropertyChain.cs" />
     <Compile Include="OmniXAML\Source\Glass\ChangeTracking\Property.cs" />

src/Perspex.Controls/DropDown.cs

@@ -140,6 +140,8 @@ namespace Perspex.Controls
 
             if (control != null)
             {
+                control.Measure(Size.Infinity);
+
                 SelectionBoxItem = new Rectangle
                 {
                     Width = control.DesiredSize.Width,

src/Perspex.Controls/TextBlock.cs

@@ -235,9 +235,9 @@ namespace Perspex.Controls
         protected virtual FormattedText CreateFormattedText(Size constraint)
         {
             var result = new FormattedText(
-                Text,
-                FontFamily,
-                FontSize,
+                Text ?? string.Empty,
+                FontFamily ?? "Ariel",
+                FontSize > 0 ? FontSize : 12,
                 FontStyle,
                 TextAlignment,
                 FontWeight);

src/Perspex.SceneGraph/Media/FormattedText.cs

@@ -30,6 +30,10 @@ namespace Perspex.Media
             TextAlignment textAlignment,
             FontWeight fontWeight)
         {
+            Contract.Requires<ArgumentNullException>(text != null);
+            Contract.Requires<ArgumentNullException>(fontFamilyName != null);
+            Contract.Requires<ArgumentException>(fontSize > 0);
+
             Text = text;
             FontFamilyName = fontFamilyName;
             FontSize = fontSize;