Fix "interaction problem between code completion and scrolling" (http://community.sharpdevelop.net/forums/t/12215.aspx)

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/CodeCompletion/CompletionWindowBase.cs

@@ -5,9 +5,9 @@ using System;
 using System.Linq;
 using System.Diagnostics;
 using System.Windows;
+using System.Windows.Controls.Primitives;
 using System.Windows.Input;
 using System.Windows.Threading;
-
 using ICSharpCode.AvalonEdit.Document;
 using ICSharpCode.AvalonEdit.Editing;
 using ICSharpCode.AvalonEdit.Rendering;
@@ -159,7 +159,13 @@ namespace ICSharpCode.AvalonEdit.CodeCompletion
 		
 		void TextViewScrollOffsetChanged(object sender, EventArgs e)
 		{
-			UpdatePosition();
+			IScrollInfo scrollInfo = this.TextArea.TextView;
+			Rect visibleRect = new Rect(scrollInfo.HorizontalOffset, scrollInfo.VerticalOffset, scrollInfo.ViewportWidth, scrollInfo.ViewportHeight);
+			// close completion window when the user scrolls so far that the anchor position is leaving the visible area
+			if (visibleRect.Contains(visualLocation) || visibleRect.Contains(visualLocationTop))
+				UpdatePosition();
+			else
+				Close();
 		}
 		
 		void TextAreaDocumentChanged(object sender, EventArgs e)