I am having this issue as well.
Just started happening the last 2 times I have played game. I think a patch or recent change in the game files has caused this
Changing the res and/or trying windowed-mode do not fix the problem.
I had to move the powers bar and chat window up so I was not accidentally clicking out of game, then game loses focus, I cannot move my character, etc...
Really annoying when on a big team and trying to mow down baddies.
It is almost game-breaking for me.