Full screen problem

Hello, i've been retired from playing dekaron for about 3 years but i am here now, log onto my acc. Everything is great but when i change window mode off to full screen mode, half of my screen is frozen and half of it is full black, i can't get the full screen mode to work properly. After it happens i have to change back mode in options.txt to false because in other case i can't even log into game, because 'start' button on the bottom is under big black bar. How to fix it?