I just tried naively installing Diablo II from scratch inside Bottles and sadly I couldn’t get it to work at all this time, at least not without further tinkering but I couldn’t spare more time at this moment. I presume installing it like this and then adding it to Steam to use Proton could work. I know it worked for Warcraft III a few months ago. And then Proton usually handles fullscreen completely seamlessly.
Or more accurately: Finagle’s law, often confused with Murphy’s law. Murphy’s law is more about UI design that shouldn’t allow for mistakes. Finagle’s law is about bad luck and the general perversity of the universe.