Fixed performance bug when pressing Backspace - we were unnecessarily building the visual line for the intermediate state where the previous character is selected but not deleted.

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

@@ -140,6 +140,7 @@ namespace ICSharpCode.AvalonEdit.Editing
 					TextViewPosition oldPosition = textArea.Caret.Position;
 					MoveCaret(textArea, direction);
 					textArea.Selection = textArea.Selection.StartSelectionOrSetEndpoint(oldPosition, textArea.Caret.Position);
+					if (!textArea.Document.IsInUpdate) // if we're inside a larger update (e.g. called by EditingCommandHandler.OnDelete()), avoid calculating the caret rectangle now
 						textArea.Caret.BringCaretToView();
 				}
 			};

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

@@ -59,7 +59,7 @@ namespace ICSharpCode.AvalonEdit.Editing
 			// Work around WPF memory leak:
 			// KeyBinding retains a reference to whichever UIElement it is used in first.
 			// Using a dummy element for this purpose ensures that we don't leak
-			// a real text editor (which a potentially large document).
+			// a real text editor (with a potentially large document).
 			UIElement dummyElement = new UIElement();
 			dummyElement.InputBindings.AddRange(inputBindings);
 		}