Which program does this? And do you have your taskbar set to the left or right side of your monitor?
 
 

Franky Wan820861
2 discussion posts
 
Any program that is windowed fullscreen does this. Only PC Games have windowed fullscreen mode, e.g Counter-Strike, StarCraft, etc. 
My Windows Taskbar is on the left, the Display Fusion Taskbar is on the right. 
Moving the program back to the left monitor also leaves a gap.