Desktop icons and popup notifications partly off screen

Mint 13 Cinnamon 64-bit, clean install. Dual monitors (LH monitor 2560x1600, RH 1920x1080 in portrait orientation) using Nouveau driver on an nVidia dual head card. The desktop just contains the standard Computer and Home icons, but if I right-click on the desktop and choose "Organise desktop by name", the Computer icon gets placed so that about 3/4 of it is missing off the top of the screen. I can drag it down to be completely visible, but if I choose the same "Organise" option from the right-click menu, it gets shoved up off the screen again.

The same problem happens with popup notifications, such as when I send a job to the network printer or when a new e-mail arrives. The notifications are mostly off the top of the screen.

When I maximise windows, the title bar doesn't go off screen, not can I move the mouse pointer above the top of the screen. It's just the desktop icon placement and these temporary notifications that are getting shoved partly off screen.

Attached desktop.png shows a crop of the top left of a screenshot after doing "Organise desktop by name".

The problem has been there since I first installed Mint 13 about a month ago.