As I wrote in coments for bug ID: 2275692 I think I have solved this problem as good as possible. The somewhat older technology of using a drag-rectangle maybe should be given up within some future steps by use of alpha-blending. But first I wanted to enhance the behaviour, especially that was needed for the Vista Aero UI style, where read accesses to the whole desktop are too expensive.
I tried to explain all changed lines by some comments, feel free to forshorten these comments if you mean they are to garrulous.
Thank you very much in advance,