you may want to try setting a fixed size for the navigation arrows and
display them permanently
in other words, arrows (and clickable navigation areas) will be just
as big as the size of the (arrow) icon and won't conflict with other
clickable areas inside the box
the reason to display them permanently is because otherwise visitors
won't know where to mouse hover to navigate the gallery
AFTER you linked the fancybox css file, add this css declaration in
your <head> section:
<style type="text/css">
#fancybox-left,
#fancybox-right {
width: 30px; height: 30px; top: 45%
}
#fancybox-left:hover span,
#fancybox-left-ico{
left: -10px;
}
#fancybox-right:hover span,
#fancybox-right-ico {
right: -10px;
left: auto;
}
</style>
you can play with the values to fit your needs
btw, leave the z-index values as they are