Maximizing iTunes on the secondary monitor causes it to incorrectly fill the screen. What happens depends on where the taskbar is located:
Taskbar on TOP-
Taskbar on LEFT-
RIGHT and BOTTOM act correctly.
What you don't see is that there is black space on the opposite side: So for the TOP configuration, there is black space on the bottom of the screen (where the taskbar would be if it was set to a BOTTOM position) and for the LEFT configuration, the black space is on the RIGHT side.
Interesting! I've confirmed the issue but it actually seems like it's an iTunes bug. I tested the same scenario without DisplayFusion running and just using the Windows taskbar, and iTunes behaved the same when maximized.
Could you try it out on the screen with the Windows taskbar without DisplayFusion running to confirm?
