Comment 16 for bug 693151

Displaying LDM in the primary monitor isn't always what the user will want. His primary monitor might be the one that is turned off, and the (active) secondary monitor then will only display a black background, making it very hard for him to log in.

Instead of patching LDM, another idea is to modify the /usr/share/ltsp/xinitrc.d/I10-xrandr script, so that it autodetects and runs something like:
xrandr --output LVDS1 --auto --scale 1x1 --output VGA1 --auto --same-as LVDS1 --scale-from 1400x1050
...where 1400x1050 is the primary (LVDS1 in this case) monitor resolution.

The result will be a perfectly stretched cloned output, so that the user will be able to see LDM centered in all monitors and use any one of them to login.
He can then keep using that arrangement inside the user session if he likes (which is a very good default when e.g. using a projector to show the teacher screen, even if they have different resolutions), or he could use the gnome monitor control panel to arrange them however he wants.

I'm going to implement that ^ method and discard the proposed LDM patch if there are no objections.