Double screen: desktops "moving" with the mouse

I am running Linux Mint 17.1 Rebecca, with Cinnamon 2.4.5. I have a double screen: my laptop's¹ screen, and another (Dell) screen, both with resolution 1920×1080. I have a strange bug (maybe is it a feature, that I do not know how to deactivate!), and I have difficulties to explain it, so please tell me if you cannot make sense of my explanations.

Let me precise that my external screen is the primary screen, and the laptop screen the secondary screen. Also, the laptop screen is on the right of the external screen. The bug occurs with mouse moves:

1. When I log in, everything is displayed well. The mouse is on the primary screen.


2. I move the mouse to the right on the external screen, up to the frontier between both screens, everything works well.


3. Then, if I continue to the right (going now to the laptop screen), the laptop screen displays what it should, but the external screens begins to display what is on the laptop screen also. What is displayed on the external screen begins to "slide to the left" (the left part is not visible anymore) and on its right, I see the same thing as the left part of the laptop screen.


4. If I go all the way long to the right of the laptop screen, I end up with two identical screens, both displaying what the laptop is supposed to display.


5. Now if I make the inverse path, going to the left, nothing is going on as long as the mouse is on the laptop screen, and when it attains the external screen, the external screen begins to slide again, to the right this time, to display what it should.


6. When the mouses attains the left edge of the external screen, I am back with the correct display.

I wanted to put here a screenshot, but screenshots are (also...) working strangely, it is probably related.

Do you have any clues?

¹ Dell Precision M4700