I have two monitors. One is 1920x1080 (the left screen), and the other is 1440x900 (the right screen). I am using Ubuntu 12.04.
Right now, I have it set up such that the tops of both monitors are aligned. That way, when I have my mouse at the very top of the left screen and I move my mouse to the right, it will move seamlessly to the right screen. However, if my mouse is on the bottom 180 pixels of the left screen and I move my mouse to the right, it won't move to the right screen, so I have to move my mouse up before I move it to the right because the right screen is smaller than the left screen.
My desired functionality: if I am at the bottom of the left screen and I move my mouse to the right, it should appear at the bottom of the right screen.