Comment 9 for bug 1241895

Created attachment 4894
display the X11 root window property _NET_DESKTOP_GEOMETRY

TL;DR: The pager itself is (presumably) fine. Xfwm needs to be fixed to update the _NET_DESKTOP_GEOMETRY property.

Long story:
I have the same problem as Hans Nieser. I might have located the source of this bug. The Xfce pager uses libwnck, which uses the _NET_DESKTOP_GEOMETRY property to determine the full desktop size. However, it seems that Xfwm does not update the _NET_DESKTOP_GEOMETRY property
According to the page above, this property should be set by the window manager.

I verified that Xfwm indeed does not modify this property using the attached C file, which can be compiled via
    gcc test.c -lX11 -o test

1. Log in and enter xfce desktop.
2. Compile test.c and run ./test to verify that the desktop geometry is correct.
3. Change the screen configuration via xrandr by adding/removing a monitor, or change the virtual desktop size via "xrandr --fb widthxheight" (as along as the new size is big enough to hold the existing screen configuration).
4. Run ./test again and observe that the desktop geometry stays unchanged.