Shouldn't work with diablo atm, I started working on it just recently and you would need to compile the diablo_warcraft2 branch on your own if you want to try. It's far from perfect though, main menu doesn't scale, bnet doesn't scale and windowed mode shows multiple windows in the menu - during the gameplay it works perfectly though.
Well, it works if I don't try to set custom resolutions. Here's some screenshots using
and 4xScaleHQ shader that I borrowed from the repository. Yours keeps the game in fullscreen windowed while also showing the texts of bnet in the "real" window to the side. Which is much better than all other ddraw options that I tried so far.
One of the screenshots is jpg since GOG wouldn't let me upload files over 500Kb ...