A minor issue but this has irritated me for years so it's time I mentioned it:
I often switch between having XnView MP full screen, and in a smaller window so I can still see it while working elsewhere. Every time I switch, the subwindows get their sizes changed... understandable, they can't all stay the same. It would help enormously though if the full screen sizes were restored to what they were when I go back to full screen. They are not, so I have to resize them every time I do this, which is a lot.
See the following three screenshots to illustrate this. The top is my full screen layout. Second is dropped out of full screen. Third is back to full screen and you can see the subwindows are now different sizes to what they started.