Currently, I run WoW off a laptop connected to 3 of the same 4K displays. If my main display is set to 4K, when trying to run WoW after a fresh install, the game only renders in the top left corner of the window or screen. However, the UI is still functional, if invisible, mapped 1:1 to the full area of the display.
Through trial and error and careful manipulation of menu options that aren’t mapped to the display, switching the graphics API to DX11 from DX12 causes the game to behave normally. There is some error with the DX12 implementation vs. the DX11 implementation with this extremely high resolution configuration, and the game defaults to DX12 on a fresh install.
As far as I know this can’t be changed outside of the in-game settings, unless there’s an ini file or such to edit.
This applies to retail/BfA and Classic WoW.
However it’s a bigger problem in Classic WoW, as there is no option switch the graphics API in the options. The only solution there is to change the display resolution specifically to 1920x1080. Currently, playing in a window or in a high resolution is not possible for Classic, if my display is set at a resolution above 1920x1080.
I can provide screenshots of the issue and further technical specifics and specifications of my hardware/drivers/etc. if necessary.