Windows Version: XP SP2 (Build 2600)
This is hard for me to explain, so I'll let my screen shots do most of the talking, but here are some of the settings that might help explain why this is weird:
View - Auto-Image Size: "No fit"
View - Fullscreen - Hide Scrollbar: checked
View - Fullscreen - Auto-Image Size: "Fit image to window, large only"
Click on each image for the full-sized version!

- I've just opened up the image and nothing is wrong so far

- I've just pushed F11 and entered fullscreen mode
- The image has been scaled to fit the screen, as it should be!


- F11 again, but the image hasn't gone back to "No fit"
- It looks like it's kept the fullscreen property - "Fit image to window, large only"
- There isn't anything to scroll (especially not vertically), so the scrollbar shouldn't be there, should it?

- F11 once again. But wait! Why is that scrollbar there on the right side?
- The image is still scaled! Nothing to scroll, and I told XnView to hide the scrollbars!

Hopefully that made sense? Can anyone reproduce this?
Cheers!
Criftus