I want to suggest a little option that let the user set how many lines/rows (or thumbs) get jumped when the mouse wheel is used to scroll.
At moment I use thumbails 180px in height, so I always loose the orientiation when I scroll, because nearly the whole screen get's scrolled out of sight with only one step of the mouse wheel.

Or, alternatively, if you don't want to get XnViewMP getting too full of options, it could be nice when MP set's the amount of rows which get skipped automatically through comparition of window/screen and thumbnail size. But setting it up manually would be the coolest
