Zaphod mode (dual head) with Xfce 4.16?

[Ulli Horlacher]

My problem: I have no longer independant displays with dual head setup.

I am on the way to replace a 10 year old thinkpad notebook running Xubuntu
18 (Xfce 4.12) with a new one running Xubuntu 22 (Xfce 4.16).

Everything runs fine on the new notebook when using just the internal
display, but not with the second, external HDMI monitor.

I do no want the standard configuration with one $DISPLAY, but I want the
so called Zaphod mode with independant displays. The advantage of this
mode is: I can switch die workspaces independantly. For example: workspace
#1 on the internal display (:0.0) and workspace #3 on the external display
(:0.1).
When using the default (non-Zaphod) mode, a workspace switch is done
always on both displays, together.

With the old setup I use a handcrafted xorg.conf for Zaphod mode: 

Section "ServerLayout"
    Identifier     "Layout0"
    Screen         "Screen0" 0 0
    Screen         "Screen1" RightOf "Screen0"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "modesetting"
    Option         "ZaphodHeads" "LVDS-1"
    Option         "AccelMethod" "sna"
    Screen         0
EndSection

Section "Device"
    Identifier     "Device1"
    Driver         "modesetting"
    Option         "ZaphodHeads" "VGA-1"
    Option         "AccelMethod" "sna"
    Screen         1
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
EndSection

Section "Screen"
    Identifier     "Screen1"
    Device         "Device1"
EndSection


I copied this xorg.conf to the Mint system and changed only the names of
the displays ("LVDS-1" to "eDP-1", "VGA-1" to "HDMI-1").
After restarting lightdm I see now only on the internal display windows
and the panel, the external display is just black, BUT I can move the
mouse to it and I can see the mouse pointer! So, the external display is
recognized by X11, but I cannot start there any programs with the mouse.
Nothing is clickable.

I can start an xterm on the external display with:
DISPLAY=:0.1 xterm

Dito this works:
DISPLAY=:0.1 xfce4-settings-manager

But when I click there on "Panel" I get an error message window:
Failed to show the preferences dialog
GDBus.Error:(...)The name org.xfce.Panel was not provided by any .service files


So, I am stuck :-(

How can I have my independant display dual head setup back?


-- 
Ullrich Horlacher              Server und Virtualisierung
Rechenzentrum TIK
Universitaet Stuttgart         E-Mail: horlacher at tik.uni-stuttgart.de
Allmandring 30a                Tel:    ++49-711-68565868
70569 Stuttgart (Germany)      WWW:    http://www.tik.uni-stuttgart.de/
REF:<20230305175302.GA15119 at tik.uni-stuttgart.de>