Widescreen in windowed mode?

[Azul5]

I might just be overlooking something with the immense power of some massive idiot here, but I can't for the life of me figure out how to play a game in widescreen, while windowed. Fullscreen, widescreen works fine. Otherwise, I can't figure out how to make, say, Sonic 3 and Knuckles widescreen in a window. Hope I'm just overlooking it, since I've been trying to figure it out with no progress at all.