Use a path for the TextBox error icon.

src/Avalonia.Controls/Canvas.cs

@@ -47,6 +47,7 @@ namespace Avalonia.Controls
         /// </summary>
         static Canvas()
         {
+            ClipToBoundsProperty.OverrideDefaultValue<Canvas>(false);
             AffectsCanvasArrange(LeftProperty, TopProperty, RightProperty, BottomProperty);
         }
 

src/Avalonia.Themes.Default/TextBox.xaml

@@ -29,15 +29,12 @@
             </TextBlock>
 
             <DockPanel LastChildFill="True">
-              <Panel Name="error" DockPanel.Dock="Right" Margin="4 0 0 0">
+              <Canvas Name="error" DockPanel.Dock="Right" Width="14" Height="14" Margin="4 0 1 0">
                 <ToolTip.Tip>
                   <ItemsControl Items="{TemplateBinding DataValidationErrors}" MemberSelector="Message"/>
                 </ToolTip.Tip>
-                <!-- TODO: Draw as a path -->
+                <Path Data="M14,7 A7,7 0 0,0 0,7 M0,7 A7,7 0 1,0 14,7 M7,3l0,5 M7,9l0,2" Stroke="{StyleResource ErrorBrush}" StrokeThickness="2"/>
-                <Ellipse Width="14" Height="14" Stroke="{StyleResource ErrorBrush}" StrokeThickness="1"/>
+              </Canvas>
-                <Path Data="M7,4l0,5" Stroke="{StyleResource ErrorBrush}" StrokeThickness="2"/>
-                <Path Data="M7,10l0,2" Stroke="{StyleResource ErrorBrush}" StrokeThickness="2"/>
-              </Panel>
               
               <ScrollViewer CanScrollHorizontally="{TemplateBinding CanScrollHorizontally}"
                             HorizontalScrollBarVisibility="{TemplateBinding (ScrollViewer.HorizontalScrollBarVisibility)}"
@@ -73,10 +70,10 @@
   <Style Selector="TextBox:error /template/ Border#border">
     <Setter Property="BorderBrush" Value="{StyleResource ErrorBrush}"/>
   </Style>
-  <Style Selector="TextBox /template/ Panel#error">
+  <Style Selector="TextBox /template/ Canvas#error">
     <Setter Property="IsVisible" Value="False"/>
   </Style>
-  <Style Selector="TextBox:error /template/ Panel#error">
+  <Style Selector="TextBox:error /template/ Canvas#error">
     <Setter Property="IsVisible" Value="True"/>
   </Style>
   <Style Selector="TextBox /template/ ToolTip">