If you have a desktop that spans multiple displays, but not multiple heads, then the programs that center themselves in the screen show up half on one display and half on the other, which is in the middle of the entire screen, but not in the middle of any one display.
If you have a desktop that spans multiple displays, but not multiple heads, then the programs that center themselves in the screen show up half on one display and half on the other, which is in the middle of the entire screen, but not in the middle of any one display.