Comment 48 for bug 661450

Dual screen with Unity and Natty, 64-bit version on a Lenovo Thinkpad X201s. Intel graphics chip. Display port used for the external monitor at work, VGA at home, the same symptoms.

With the external monitor logically to the right of the laptop, everything is just fine.
With the external monitor logically above the laptop (corresponds to the physical setup), some rather serious issues.
  Sometimes parts of the monitors are black/inaccessible, similar to the examples in this bug.
  Sometimes the desktop is ok (all accessible and visible), but window resizing at the bottom border results in closing the window, clikcing an icon in the launch bar with several windows open displays selection images, but they do nothing when clicked on.

Testing with the monitors stacked in various ways, including vertically, might shed some light on the underlying problems.