If you have configured X.org so that your display spans multiple  monitors, usually when you login to an XFCE session, it will appear as  if your monitors are simple clones of one another. You can use an xrandr  tool to tweak your setup but if this is not called at an appropriate  time in the startup sequence, some functionality may be lost with parts  of your display being inaccessible to the mouse pointer.

A better way is to configure XFCE to match your desired display  arrangement. However, at present (xfce-settings 4.10), there is no tool  available to assist with configuring multiple monitors directly.

The Settings -> Display tool does allow configuration of screen  resolution, rotation and enabling individual monitors; warning: using  this tool to adjust display settings will reset or lose settings made  manually for properties not explicitly offered as buttons in the tool  (see below).

The Settings -> Settings Editor allows manipulation of all  configuration items in particular the displays settings which are saved  in the file displays.xml below

~/.config/xfce4/xfconf/xfce-perchannel-xml

Alternatively, the displays.xml can be edited using your favourite editor.

The main requirement for multiple monitors is their arrangement  relative to one another. This can be controlled by setting the Position  properties (X and Y) to suit; an (x,y) position of 0,0 corresponds to  the top, left position of the monitor array. This is the default  position for all monitors and if several monitors are enabled they will  appear as a cloned display area extending from this point.

To extend the display area correctly across both monitors:

  • for side-by-side monitors, set the X property of the rightmost monitor to equal the width of the left-most monitor
  • for above-and-below monitors, set the Y property of the bottom monitor to equal the height of the upper monitor
  • for other arrangements, set the X and Y properties of each monitor to correspond to your layout

Measurements are in pixels. As an example, a pair of monitors with  nominal dimensions of 1920x1080 which are rotated by 90 and placed  side-by-side can be configured with a displays.xml like this:

<channel name="displays" version="1.0">
 <property name="Default" type="empty">
   <property name="VGA-1" type="string" value="Idek Iiyama 23"">
     <property name="Active" type="bool" value="true"/>
     <property name="Resolution" type="string" value="1920x1080"/>
     <property name="RefreshRate" type="double" value="60.000000"/>
     <property name="Rotation" type="int" value="90"/>
     <property name="Reflection" type="string" value="0"/>
     <property name="Primary" type="bool" value="false"/>
     <property name="Position" type="empty">
       <property name="X" type="int" value="0"/>
       <property name="Y" type="int" value="0"/>
     </property>
   </property>
   <property name="DVI-0" type="string" value="Digital display">
     <property name="Active" type="bool" value="true"/>
     <property name="Resolution" type="string" value="1920x1080"/>
     <property name="RefreshRate" type="double" value="60.000000"/>
     <property name="Rotation" type="int" value="90"/>
     <property name="Reflection" type="string" value="0"/>
     <property name="Primary" type="bool" value="false"/>
     <property name="Position" type="empty">
       <property name="X" type="int" value="1080"/>
       <property name="Y" type="int" value="0"/>
     </property>
   </property>
 </property>
</channel>

Usually, editing settings in this way requires a logout/login to action them.

A new method for configuring multiple monitors will be available in the forthcoming xfce-settings 4.12 release.

This posting was originally posted to Arch Linux Wiki.