The docs say displayName is "(default) choosen by random"

The code comments on the variable say "DISPLAY name, if NULL chosen by random."

The actual contents of src/main/java/org/jenkinsci/plugins/xvfb/XvfbBuildWrapper.java L470 say that it will always use executor.getNumber() + displayNameOffset - which always returns displayNameOffset (default 1).

As a result, DISPLAY is always 1.

Using autoDisplayName is not an option, because it requires a version of Xvfb far too new for our builders (1.14 I think?)