Hi there !
As many mentioned there is no way to have a 21:9 resolution for the moment, the customer support asked us to continu this thread in the bug report section as it was supposed to support that type of ratio.
My native resolution is 3440x1440 and the closest I get is 2560x1440 that is a 16:9 ratio. Even in fullscreen bordeless it seems stretch. I also tried to edit the War3Preferences.txt but it set back the 16:9 resolution. Nothing in the regedit to force the resolution either …
I hope we will have a fix for all the people using 21:9 aspect ratio !
PS : the thread where the customer support responded Game doesn't work on 3440x1440
Have a nice day !