The display number that the Xvnc plugin uses to start its displays at is :10;
subsequent displays are at :11, :12, and so on. This is not currently configurable.

At Amazon, our company-wide sshd_config sets its X forwarding ports to start at
an offset of 10, which collides with the X ports used by Xvnc. We can't change
this sshd_config, since that would affect thousands of computers.

Making the Xvnc plugin have a configurable base display offset solves this
problem- when I change the base display number to something else (say, 85) the
sshd X forwarding ports and the Xvnc ports don't collide.

I have submitted a patch that does this to dev@hudson.dev.java.net - it adds the
configuration field to the plugin control panel UI, and changes the plugin and
its display allocator to use the configured field. The default remains 10, if
the user doesn't enter anything in the UI.