The size used for a supersized window uses the union of the total area of each screen. This should instead be the union of the working area of each screen.
This makes a difference when the taskbar is at the edge of the union of the area and streches from one side to the other, eg. if the taskbar is at the left of two screens arranged horizontally as in these cases the real area for a supersized window is less than the the total area of the screens.
This leads to 2 problems as reported in https://sourceforge.net/projects/dualmonitortool/forums/forum/1141004/topic/3954045 and https://sourceforge.net/projects/dualmonitortool/forums/forum/1141004/topic/3954061.