XnView has an option to 'Use fullscreen on second monitor for previewing', which I find most useful when working on a multimonitor computer.
However, when using more than two monitors, it really becomes necessity to be able to specify exactly which second monitor should be designated for previewing.
The default choice now used by XnView, is not always satisfactory.
I notice that several multimedia software tools these days have implemented an option for specifying monitors.
Hope for the next verions
br
KrisR