light blue overlay selection rectangle

Question

Anyone have a clue how to do this? I read an article someone wrote about transparent controls, and you can use one of these (resize, relocate, etc.) to get it to work. But it flickers despite all double-buffering attempts and doesn't look great. Any thoughts on how to maybe do this? I'm thinking that perhaps you could show a window that is mostly transparent and move and resize this over the current window...