First of all I recorded a video on the phenomenon, this might be more descriptive than my words: video of the problem
I have a project in which I have to use two displays: one that is the 7 inch touchscreen shipped with the Raspberry (link), and another one connected via HDMI.
The first display is used as a touch control-panel, the second one is used only for displaying some information.
The problem is the following: When using only the touchscreen, everything is fine, the pointer clicks exactly where my finger is. But when I attach and configure the secondary monitor, the virtual "canvas" grows to more than twice the size, and my touches on the touchscreen become highly shifted to the right.
So the touch area now wants to cover the entire virtual canvas, offsetting the pointer from the physical touch, and making it nearly impossible to precisely hit any button on the screen.
I would be glad to receive any help!