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I have googled this quite a few times, and I know you can resize the Pvp.net window smaller to an extent.
I play with a Nvidia 570, and on a Samsung 40inch lcd using HDMI 1900x1080? resolution standard.
The Pvp.net screen even at smallest size is cut off at the bottom of the TV. the start menu taskbar is positioned correctly, so I know it is not an overscan issue etc.
I finally tried hooking my PC up to a regular 20 inch monitor, and I have actually seen what the chat buttons look like etc, instead of just the top border (had to learn which was which by trial and error).
When I start LoL it is already at the smallest size window, and when I try to move it up a bit, it locks itself back down where it was.