I installed easy-fancybox, because the default style of the gallery view looks very outdated.
Now, if I click on an image from a gallery the image opens first in easy-fancybox style, but is overlayed by the default image viewer of ngg. I have to close the ngg-viewer to see the fancybox viewer.
So the images are shown in both gallery viewers.
It can be tested at my website, for example here:
BTW: the dia - show does not work as well.