Some time ago after some windows 7 update, any window I try to maximize, won't fully maximize and leaves gap between task bar and that window. If I restart or turn off/on my PC sometimes this problem disappears, but then after other restart it can randomly appear again. I uploaded screenshot how my opera browser looks maximized now. Also this happens with any window, not just particular browser or program, so it is definitely windows 7 problem.
http://s14.postimg.org/64pops4ch/window_gap.png
http://s14.postimg.org/64pops4ch/window_gap.png