So if I understand correctly you are playing in Fullscreen because Borderless window mode is borked and you don't like windowed mode (don't blame you on that one). So the issues isn't that mouse won't exit the Fullscreen mode (because DX locks it to the screen automatically), the real issue is that your Borderless window mode doesn't size properly. So in all, you have 2 bugs. One that the hardware mouse doesn't click on the screen (might be a problem on your video drivers ... try updating them if you haven't already) and 2, your borderless querys the windows desktop size and doesn't get the proper resolution. That one is trickier. I could be a code bug in SE's side or it could be a driver problem, or even worse it could be an OS problem. What Videocard, videocard driver and windows version are you currently using?