to reproduce:
1.) download & extract a clean XnView MP 0.39 (x86)
2.) Create an 'xnview.ini' in the same extracted program folder(s)
3.) Make sure there is no Database file in default user app folder
4.) Drag & drop an image file to the xnview.exe (or I used an icon file(s) ICO or ICNS)
5.) Program opens with a brief first-time 'wait' (normal behaviour)...
6.) ...Now quickly do some multiple fast left-clicking on the interface during this 'wait'
7.) APP CRASH!!
Right, I think this is a probable 'Fullscreen' bug or 'Browser' command interference.
Furthermore, continuing from above steps
1.) After that App Crash
2.) Drag & drop the same file to xnview.exe
3.) Image file will open in fullscreen (with no program crash on multiple left-click)
4.) While in Image fullscreen....
5.) ...Now double-click on image to go to 'Browser' (still in fullscreen)
6.) User is now perpetually STUCK in Fullscreen 'Browser' - with no escape

Here is the very file I did the test with (Vista aero wallpaper): tested Win 7 x86