When I try to toggle fullscreen mode in game, it still acts as borderless windowed.

I've also tried manually editing GameUserSettings.ini file but i still get borderless windowed:


FullscreenMode=1
LastConfirmedFullscreenMode=1
PreferredFullscreenMode=1

Running 2560x1440 with a RTX2080, Win10, Ryzen 7 3700X.

Exclusive Fullscreen mode is not a thing anymore in Windows 10 or later. Instead you get FullScreen Optimizations mode. See:

https://devblogs.microsoft.com/directx/demystifying-full-screen-optimizations/

To be more precise:

Fullscreen exclusive technically still exists in Windows 10, however, it does NOT exist in Direct3D 12 or Vulkan. Instead, the DXGI flip presentation model is used, which gets you the best of both worlds, fullscreen performance without the alt+tab delay. It also engages g-sync, if supported by your hardware and driver, and if there's nothing else drawing on top of the game window. Flip was available in D3D 10 and maybe even in 9. You can force some games to use it with the Special K program, which I often do in order to get g-sync to work. There's really no reason not to use it if your OS supports it; developers just seemingly didn't know enough about it, leading to even most D3D 10 games not doing so.

Windows 10 will by default attempt to coax most applications using D3D 11 and below (aside from those MS has excluded due to incompatibility) to use the flip model (or some kind of hybrid) instead of true, fullscreen exclusive mode. When you disable fullscreen optimizations for an executable, you are telling Windows not to do that.

There is some room for error in my description; I'm not an expert on this.