Sometimes, developers implement Fullscreen Optimizations incorrectly. You might see a situation where the game claims to be fullscreen, but your mouse cursor drifts onto a second monitor, or the resolution doesn't scale correctly. In these specific bug cases, disabling FSO is the fix.
is Windows' solution. It allows a game to: fullscreen optimizations on or off
If you’ve ever dug into the Properties of a game’s .exe file, you’ve seen it: But should you check that box or leave it alone? fullscreen optimizations on or off
DirectX 12 and Vulkan games do not even support traditional "Exclusive Fullscreen". Disabling FSO for these titles is generally useless and can sometimes cause instability. fullscreen optimizations on or off
while technically running in a borderless window.