Fix #311 - AvalonEdit cannot be used in WPF application with multiple UI threads.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit.Tests/ICSharpCode.AvalonEdit.Tests.csproj

@@ -80,6 +80,7 @@
     <Compile Include="Editing\ChangeDocumentTests.cs" />
     <Compile Include="Editing\TextSegmentReadOnlySectionTests.cs" />
     <Compile Include="Highlighting\HtmlClipboardTests.cs" />
+    <Compile Include="MultipleUIThreads.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Document\CollapsingTests.cs" />
     <Compile Include="Document\HeightTests.cs" />

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit.Tests/MultipleUIThreads.cs

@@ -0,0 +1,43 @@
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
+
+using System;
+using System.Threading;
+using System.Windows;
+using NUnit.Framework;
+
+namespace ICSharpCode.AvalonEdit
+{
+	[TestFixture]
+	public class MultipleUIThreads
+	{
+		Exception error;
+		
+		[Test]
+		public void CreateEditorInstancesOnMultipleUIThreads()
+		{
+			Thread t1 = new Thread(new ThreadStart(Run));
+			Thread t2 = new Thread(new ThreadStart(Run));
+			t1.SetApartmentState(ApartmentState.STA);
+			t2.SetApartmentState(ApartmentState.STA);
+			t1.Start();
+			t2.Start();
+			t1.Join();
+			t2.Join();
+			if (error != null)
+				throw new InvalidOperationException(error.Message, error);
+		}
+		
+		[STAThread]
+		void Run()
+		{
+			try {
+				var window = new Window();
+				window.Content = new TextEditor();
+				window.Show();
+			} catch (Exception ex) {
+				error = ex;
+			}
+		}
+	}
+}

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/TextEditor.xaml

@@ -37,7 +37,7 @@
 		</Setter>
 	</Style>
 	
-	<Style TargetType="{x:Type editing:TextArea}">
+	<Style TargetType="{x:Type editing:TextArea}" x:Shared="False">
 		<Setter Property="FocusVisualStyle" Value="{x:Null}"/>
 		<Setter Property="SelectionBrush">
 			<Setter.Value>