Simple idea:
I want to show slides with a projector.

Complicating environment:
The projector supports SVGA resolutions e. g. 800x600 or 1024x768 but the laptop
has an UWXGA display with resolutions like 800x512, 960x540, 1280x1024,
1400x1050, 1920x120.
If I edit /etc/X11/xorg.conf and set a fixed resolution to 800x600 or 1024x768
in clone mode my laptop LCD stays black.

Consequence:
From hardware point I cannot use cloned displays with 800x600 or 1024x768.

Solution:
With nVidia display drivers you can define a metamode that includes the internal
LCD e. g. with 1028x1024 and the external projector with 1024x768 in clone mode.
If the projector is marked as "primary display" both screens use only 1024x768
for "full screen" mode.

This works perfect for OO writer, calc, etc. but not for impress.
Impress uses the 1028x1024 resolution when it starts as maximized window and, of
course, in full screen slide show mode. Therefor customers cannot see the
complete presentation - only the left top "edge".

Enhancement:
If you use Option "TwinViewXineramaInfoOverride" "1024x768+0+0,1024x768+0+0"
(see
http://de.download.nvidia.com/XFree86/Linux-x86/169.04/README/appendix-b.html
for details) impress starts respecting the smaller screen size, but still uses
1280x1024 for slide shows.

Steps to reproduce:
 1. Take a laptop with UWXGA display that does not accept 800x600 and/or
1024x768 modes.
 2. Connect a projector to your laptop
 3. switch graphics driver to clone mode
 4. start impress
 5. try full screen slide show

Core question:  
How to present with a UWXGA laptop on a SVGA projector in clone mode?

Side questions: 
Why does Impress behave differently compared with Writer and Calc when started
as application in an maximized window?

Kind regards,
Thorsten