I am having similar problems on my notebook, but it is an AMD. The desktop becomes bigger than the screen, and the mouse disappears. It works fine when using a lower resolution. Still, when applying something like 1024x768, it reduces desktop size to the actual pixels, and on a 1920x1080 screen, that is like using a small centered square to play the game.
On 1920x1080, the mouse is unusable.
You may try to lower the resolution to see what happens. Maybe it is not an ATI problem but some late update that affects win 8.1. I still haven't found a solution.
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