Please make sure you run an independent game instance with the commandline -DX12 flag to ensure you are running DX12 for this repro. Also please make sure your graphics card supports DX12
desktop resolution of 1920x1080 or anything you want really, editor closed, loading game as an independent game instance
open console with ~ and type in 1920x1080f
The window will become windowed fullscreen, NOT regular fullscreen, you can tell the difference because the screen never briefly goes black like it does with fullscreen in dx11, and also, if you press ALT tab the desktop OS is still there
Please note this may be related to my other Dx12 bug report:
I was able to reproduce this issue on our end. I have written up a report and I have submitted it to the developers for further consideration. I have also provided a link to the public tracker. Please feel free to check the link for future updates on this issue.