If the second screen has a smaller horizontal resolution that the main screen, you cannot move Renoise onto that screen, although you can work around it by resizing the Renoise window so that its fits horizontally on the other screen, and then move it. This worked automatically in Renoise 2.8, it would resize the window to fit automatically.
Not a very serious bug, but confusing if you don’t know about the workaround.
I’m using Ubuntu 12.04 in 2D mode.