That doesn't work.

You mention Windowed mode in that thread, but it's literally impossible to run Diablo in Battle.net in Windowed mode. dll files that give the functionality to the game, mess with the server's ability to register your copy as legitimate or even the same game.
It's like having French speak to English. Without extra work to translate, it won't work, simply put.

Also Natively with no addons via dll files, Diablo 1 was made before -w or "Windowed mode" even existed in games.

Solved my issue, it was caused by LogiOptions a mouse and kb software. Might be the overlay displaying messages like Caps Lock on/off etc. If you don't have it be on the lookout for anything employing an overlay!

Run diablo 1 gog classic in window on windows 10

https://www.youtube.com/watch?v=QQ71nq10vO8

Strange but true, killing the LogiOptions process has helped me too, I hope that it will help others.

OMG thank you! This completly fixed the problem for me. I swear is also runs better. Dont know if this had anything to do with it, but i also disabled the windows 10 game overlay. With both apps gone everything is working very well.