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If I hook an external monitor to my Linux laptop and configure it for dual head with the Gnome system tools, I do get them configured with the proper resolution and position (one to the left of the other)

The mouse pointer, however, will apparently use the greatest of the two vertical resolutions. This way, when I move up to the top border, the mouse exits the visible screen, traveling in empty space, and this is quite annoying, because you can't go to the upper panel just by going up very fast.

Is there a way of enforcing the right resolutions for the mouse, too?

Thank you

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This is a default behaviour of X. When two screen have different size (vertically) the Virtual desktop created by merging theses two screens is as high as the maximum vertical size :(
I ran in the same trouble with triple screens (one 1600x1200 at the center and two 1280x1024 left and right ... creates a 4160x1200 Virtual / Merged screen. You can check in the logs of X)

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This was also reported to xorg as a bug: and to Ubuntu: As of November 2010, there is a tentative patch in place for xorg: In the Ubuntu bug report, there is an interim workaround:

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Try xcursorclamp. Works great for me.

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Please can you add more detail about this. Why does this work? – ChrisF Jan 15 '13 at 10:27
I don't know why or how it works, but I don't have "dead area" anymore. – domen Feb 5 '13 at 14:53

I had this same issue a couple of years ago on my Ubuntu 10.04 desktop with multiple heterogeneous monitors, and solved it by creating a script that would run on startup that would do something like this:

killall XCreateMouseVoid
xrandr (parameters to create the monitor arrangement I want)
XCreateMouseVoid (parameters) &

Occasionally the mouse would get stuck in the dead area, so I would then open a terminal with the keyboard and call the script to get it out.

This requires a program called XCreateMouseVoid, which you can find on github. I think it's the same program that @Warnaud referenced above.

However, on newer versions of X, the issue has been fixed so that your virtual desktop no longer has to be rectangular.

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