I post this problem/solution here in the hope that it will be useful, because it drove me crazy...
After starting Guild2 in POL in fullscreen-mode, it changed my fullscreen-resolution to 1024x768. After changing it back to 1920x1080, my screen had an offset which caused the mouse to be trapped in a small rectangle-shaped area at the left upper corner of the screen when a game run in fullscreen-mode.
Solution: execute xrandr without parameter in console. You will get some output like: DVI-D-0 connected 1920x1080+400+900 (normal left inverted right x axis y axis) 400 and 900 are examples for the not wanted offset of the screen.
xrandr --output DVI-D-0 --auto will reset it to DVI-D-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) Replace DVI-D-0 if your device name is different.