AvalonEdit perf optimization: when the selection changes, don't redraw the whole selection but only the text added/removed from the selection.

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/trunk@6167 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Editing/TextArea.cs

@@ -385,8 +385,29 @@ namespace ICSharpCode.AvalonEdit.Editing
 				if (!object.Equals(selection, value)) {
 					Debug.WriteLine("Selection change from " + selection + " to " + value);
 					if (textView != null) {
-						textView.Redraw(selection.SurroundingSegment, DispatcherPriority.Background);
-						textView.Redraw(value.SurroundingSegment, DispatcherPriority.Background);
+						ISegment oldSegment = selection.SurroundingSegment;
+						ISegment newSegment = value.SurroundingSegment;
+						if (selection is SimpleSelection && value is SimpleSelection && oldSegment != null && newSegment != null) {
+							// perf optimization:
+							// When a simple selection changes, don't redraw the whole selection, but only the changed parts.
+							int oldSegmentOffset = oldSegment.Offset;
+							int newSegmentOffset = newSegment.Offset;
+							if (oldSegmentOffset != newSegmentOffset) {
+								textView.Redraw(Math.Min(oldSegmentOffset, newSegmentOffset),
+								                Math.Abs(oldSegmentOffset - newSegmentOffset),
+								                DispatcherPriority.Background);
+							}
+							int oldSegmentEndOffset = oldSegment.EndOffset;
+							int newSegmentEndOffset = newSegment.EndOffset;
+							if (oldSegmentEndOffset != newSegmentEndOffset) {
+								textView.Redraw(Math.Min(oldSegmentEndOffset, newSegmentEndOffset),
+								                Math.Abs(oldSegmentEndOffset - newSegmentEndOffset),
+								                DispatcherPriority.Background);
+							}
+						} else {
+							textView.Redraw(oldSegment, DispatcherPriority.Background);
+							textView.Redraw(newSegment, DispatcherPriority.Background);
+						}
 					}
 					selection = value;
 					if (SelectionChanged != null)