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The current implementation of the display dialog does mostly support cloning your display to a second monitor. Xrandr and xfconf can do a lot more than that (e.g. extend and adjust resolutions) and this should be made more accessible to users. In fact this is one of the more frequently asked questions, or requested features. (There was even an episode in the 2011 FOSDEM where an Xfce-dev couldn't get the beamer to work from his notebook for his presentation.)
Currently there's only a list of displays and a few combobuttons to change settings. The list is a bit too much in my opinion, because most people won't have many more than 2 displays maximum.
This is the list of features we want to implement in the first stage:
Other features / problems:
The minimal dialog should only be called automatically when a new display is plugged in. It should provide users quick access to the most-used modes:
Furthermore it should contain a button linking to the display-settings dialog (see next section), in case the user wants to configure the new display more in depth (e.g. laptop docking). An additional checkbox should enable/disable presentation mode (see section above).
Theoretically (and according to Ubuntu's Multi-Monitor Spec) it's possible to (heuristically) detect whether the connected device is a TV or a Projector or an external monitor. We could try to use this information to set the window-title (at least).
This design currently features three 128px icons for the three basic modes, a tickbox for presentation-mode and a button to launch the display settings dialog.
This dialog will enable the user to change all the settings of all connected displays. The set of options basically depends on what RandR and the display drivers offer.
To improve Xfce's dialog one would have to implement the “extended desktop”-mode of RandR, i.e. positioning the monitors “left-of” and “right-of”. (Theoretically there's also “above” and “below”, but in most scenarios multiple monitors are aligned horizontally.) The “mirror displays”-checkbox would make the combobox for choosing the position of the screen insensitive.