Dual monitor setup on a laptop: internal monitor peeking through on external display

Recently I began to use my laptop (Dell Inspiron N5020) with an external display. I use the displays side-by-side, internal on the left, external on the right.

The problem is, however, that the display on the external monitor seems to be shifted to the right by a few pixels, and at the same time a few pixel wide column from the right edge of the internal display is duplicated on the left edge of the external. On a screenshot this is not visible. I illustrated what I'm actually talking about on this image:

The type of the external monitor doesn't seem to matter. Same thing with a Medion 1680x1050 monitor with VGA connector, and a Dell U2311H through HDMI. Also I tried to set various resolution combinations, and it seems to only happen when the internal monitor is at its native resolution (which is 1366x768)

(I have Linux Mint 15 x64 with Cinnamon.)