XnView Software

Both.
Actually I found that if you restart the program it gets applied properly. Strange behavior.

F3 can't be reassigned, always used by 'Associate Program'
Yes, but can we please be allowed to do that in the future, because I use F1-4 for my various zooming across all programs.

could you explain me more?
When I want to disable a shortcut I set it to NONE in the settings, that appears to ...

I don't know if this is very new, but I think it wasn't a problem in old versions.
I want to use the F3 key but it conflicts with "Open with > Associated Program" and that one is not present in the settings, so it cannot be changed.
Also on a side note if I set a shortcut to "None" it does NOT ...