the only problem with this kinda thing is you dont want the sprite used for the selection box to stretch. If you make a 1 pixel thick hollow square on an image then stretch that sprite to twice its original size then that 1 pixel thick border becomes 2 pixels thick.
Maybe not a huge problem... possibly a very a big problem depending on the project. Either way it looks bad
The way its normally drawn is with frame images. 1 for each corner 1 for each side and 1 for the middle which is usually tiled not stretched. Not an easy solution to setup in c2 tho
I made this capx the first time I read this thread but didn't post it for some reason. It's an expanded version of Nimtrix's capx. I think I had that same problem that you're talking about at one point but I can't remember the cause. If you can't get it working post up your capx or a screenshot anyway.