fix #452: XAML CC inserts ", " in unexpected places when typing nested markup extension

src/AddIns/BackendBindings/XamlBinding/XamlBinding/MarkupExtensionParser.cs

@@ -74,7 +74,11 @@ namespace ICSharpCode.XamlBinding
 					info.TryAddNamedArgument(argumentName, ParseValue(token.Value, token.StartOffset + offset));
 			}
 			
-			 return info;
+			if (info.EndOffset == 0 && token != null) {
+				info.EndOffset = token.EndOffset + offset;
+			}
+			
+			return info;
 		}
 		
 		static void TryAddNamedArgument(this MarkupExtensionInfo info, string name, AttributeValue value)

src/AddIns/BackendBindings/XamlBinding/XamlBinding/Utils.cs

@@ -126,7 +126,7 @@ namespace ICSharpCode.XamlBinding
 			MarkupExtensionInfo tmp = info;
 			
 			foreach (var item in info.PositionalArguments) {
-				if (item.StartOffset < offset && offset < item.EndOffset)
+				if (item.StartOffset < offset && offset <= item.EndOffset)
 					tmp = item.IsString ? tmp : GetMarkupExtensionAtPosition(item.ExtensionValue, offset);
 			}
 			

src/AddIns/BackendBindings/XamlBinding/XamlBinding/XamlCompletionItemList.cs

@@ -92,7 +92,7 @@ namespace ICSharpCode.XamlBinding
 						AttributeValue value = MarkupExtensionParser.ParseValue(valuePart);
 						
 						if (value != null && !value.IsString) {
-							var markup = Utils.GetMarkupExtensionAtPosition(value.ExtensionValue, context.Editor.Caret.Offset);
+							var markup = Utils.GetMarkupExtensionAtPosition(value.ExtensionValue, xamlContext.ValueStartOffset);
 							if (markup.NamedArguments.Count > 0 || markup.PositionalArguments.Count > 0) {
 								int oldOffset = context.Editor.Caret.Offset;
 								context.Editor.Caret.Offset = context.StartOffset;