Windows 7 sp1
here is my reproduction steps;
1.) Run a 'clean' XnView MP 0.48
2.) Go to Settings>Keyboard/Mouse and set the 'Zoom in/out' for "mouse wheel"...
3.) ..Now go to Settings>General and disable the setting "Fullscreen when starting with a file"...
4.) Now Press "Ok" on 'Settings'
3.) In the 'Browser', open any image file to open in the view mode
3.) Close 'Browser' tab
4.) Make sure on your system that .jpg files is already associated to open with XnViewMP (file association tool)
5.) In Windows Explorer double-click on the "Am76I8rCQAAFlyj.jpg" file to open...
6.) ..File opens as normal in XnViewMP with XnViewMP window displaying as foreground program window as normal...BUT...
7.) ..Immediately try to Zoom in/out with Mouse-wheel and the Zoom feature does not respond
Make sure that at step 6 (above) you Zoom immediately with your Mouse-scroll as soon as the picture is opened and nothing else.
I have attached the picture as a ZIP ----------------------------------------------------------------------------------
Note: This issue affects any image files opened from Win Explorer/external file manager. I certainly believe that the reason for this bug is simply a fix needed for the file-opening method (file---> xnview.exe)
Note: There is thankfully no bug with the middle-click (Fullscreen command) when doing the same (step 7) reproduction.

Note: Hmm... this same Zoom bug issue does not exist on same XnView MP when opening the same file from "Recent files"

Note: This bug issue does not exist on XnView win 1.9x after doing the same reproduction tests

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So all in all in summary, the Mouse-scroll wheel is not getting detected immediately for the Zoom command(s) (when laucngin picture file from Win Explorer) since a user would have to click on the opened picture first or do some other program action first before the Mouse-wheel function gets detected to be used. So this needs a fix.
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Please investigate and fix.