This didn't fix the issue for me, but I saw in another post it fixed it for a different user, maybe your luck will be better than mine.

Try

1. Browsing your game files for ffxiv.exe and ffxiv_dx11.exe
For Each:
2. Right click, select "Properties"
3. Go to the Compatibility Tab
4. Check "Disable full screen optimization"

Also, are you using borderless window mode or fullscreen mode? I was using the former, but I will be trying the latter and I will let you know how it goes.