Comment 29 for bug 426675

I don't know for the touchpad, but it is simple to reproduce the problem for a normal mouse on the current karmic:

Simply open the configuration editor and activate the left_handed gconf key and click with the left mouse button some item: the contextual menu does not appear, but the item gets selected as if a primary click occurred instead of a secondary click.

However, if I used the Mouse control panel instead of the configuration editor to activate the left handedness of the mouse, the problem does not occur.

Finally, I have been told that the problem is fixed in some 2.29.x version in GNOME, but for the moment I have not been able to verify it with Ubuntu lucid, as lucid does not start up properly here.