Hi Fabian,
In my testing, both SM_CXSCREEN and SM_CYSCREEN both return an int value that you can print out and check manually. I did this when initially figuring out what was going wrong. If you're unhappy, I could try comparing against the desktop windows dimensions instead. Or do you mean I should use the GetSystemMetrics() function? Either way, I thought the simplest method would be to just compare against those values.
I should mention though, that before applying this patch/fix, every game I tested had the issue, and afterwards did not have the issue. Here's a video I uploaded to demonstrate the issue: https://mega.nz/embed#!YU8GRKTS!nK2JGVs6EzAhu1bs-KtH1lajx6y3XRakTFNZUYNyTwo
Tested on both Wine and Wine Staging 4.0 rc4.
Thanks in Advance, Hamish