Freeze the shift+backspace key binding.

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

@@ -51,7 +51,7 @@ namespace ICSharpCode.AvalonEdit.Editing
 			AddBinding(EditingCommands.Delete, ModifierKeys.None, Key.Delete, OnDelete(EditingCommands.SelectRightByCharacter));
 			AddBinding(EditingCommands.DeleteNextWord, ModifierKeys.Control, Key.Delete, OnDelete(EditingCommands.SelectRightByWord));
 			AddBinding(EditingCommands.Backspace, ModifierKeys.None, Key.Back, OnDelete(EditingCommands.SelectLeftByCharacter));
-			InputBindings.Add(new KeyBinding(EditingCommands.Backspace, Key.Back, ModifierKeys.Shift)); // make Shift-Backspace do the same as plain backspace
+			InputBindings.Add(TextAreaDefaultInputHandler.CreateFrozenKeyBinding(EditingCommands.Backspace, ModifierKeys.Shift, Key.Back)); // make Shift-Backspace do the same as plain backspace
 			AddBinding(EditingCommands.DeletePreviousWord, ModifierKeys.Control, Key.Back, OnDelete(EditingCommands.SelectLeftByWord));
 			AddBinding(EditingCommands.EnterParagraphBreak, ModifierKeys.None, Key.Enter, OnEnter);
 			AddBinding(EditingCommands.EnterLineBreak, ModifierKeys.Shift, Key.Enter, OnEnter);