In recent builds of SharpDevelop 4.2 and AvalonEdit I implemented the "Virtual Space" feature. This feature allows you to place the caret after the end of the line. It was necessary to properly implement rectangle selection.
You can enable it in the TextEditor by setting EnableVirtualSpace = true or in the SharpDevelop options. When a rectangle selection is active virtual space is enabled implicitly.
For a detailed overview/explanation see the following:
I hope you find this useful. If you find any bugs or have a suggestion, please leave a comment!