When "full screen" / maximized the frame of the application extends outside the bounds of a single monitor. The natural behavior of an maximized window should only extend to the monitor the window is displayed on. 

My configuration left and center monitor 2560x1440 and right monitor 1920x1080

When maximinzed the app will always overflow the bounds of the screen, this has the effect  of pushing UI elements close to the edge as well. 

The portion that is the same color as your window boarder. This suggests your UI anchor point is inside edge the boarder instead of outside edge of the board as all other Windows applications are.