If you choose "Select area" item, and then the included effects of compiz, get the screenshot trace of folding translucent window: http://static.itmages.ru/i/10/0912/h_1284264531_86c86b0383.png
Thank you for your bug report, but xfce4-screenshooter is doing its job: get a picture of the current screen state. If compiz adds some visual artifacts, there is no way to filter them out.
When it was without black window, then this problem does not existed. I think that nothing prevents allow the user to choose which method to allocate the region. Better able to allocate a pieces of the dotted line on the perimeter than to have permanent artifacts on screenshot.And now the problem has become more urgent. Thank you.
I will update screenshot http://static.itmages.ru/i/10/1129/h_1290980091_4d9a7da3c0.png
Created attachment 3300 Patch for fix bug 6696 I propose my solution to this problem. Patch in the attachments. Thank you!