How to set new size for X desktop without xrandr?

[Ulrich Norbisrath]

Hi all,
I hope this a the right forum for my question.
I am looking for a possibility to reset the size of my desktop. Hence, I need 
something quite similar to the randr-extension but working without it.
Imagine I would have an X server running at a resolution of 1280x1024.
After that, I would like to set a new desktop size to 1024x768 like doing it 
with xrandr, but not changing the resolution, leaving a border consisting of 
two bars. One with the width of 256 pixels and one with the height of 256 
pixels.

Perhaps, this is just a matter of extracting some code out of xrandr.c, but I 
hope you can point me to the right place.

I need this for a kind of resizable persistent desktop, running in Xvfb and 
connected via x11vnc. Another solution would be to enable the randr extension 
in Xvfb or x4fvnc, but I think the other solution should be easier.

If both approaches fail, I could try to patch the windowmanager.

Regards,
Uli