XnView Software

Middle Button:

By default the middle button is set to switch between
Browser <> Fullscreen | Viewer <> Fullscreen
in Settings > Keyboard/Mouse > Switching mode [tab]

and that's fine, as far as i know it's been that way ever since i started using xnview years ago.

In the keyboard/mouse [tab],
there is also an option to change the middle button,
it's default setting is:
Do Nothing

if one changes it to any of the other three options,
Move picture
Make selection
Zoomed selection

and starts dragging with the middle button down,
then the picture moves or a (zoomed) selection is drawn,

but when one releases the middle button, xnview mp changes
from viewer to fullscreen or the other way around.

Possible solution:

If the middle button is down and it has moved more than a set distance,
then it should probably prevent the switching mode command from
being applied when releasing the middle button.

The set distance the middle button has to move needs to take into account that
it is easy to accidentaly move the mouse when one presses a button on an external mouse,
maybe 2-3 pixels is enough, i doubt a lot of people moves the image that short of a distance
or makes selections that small.

The right button:

As far as i can remember the default behaviour in xnview (not mp),
has been to move the picture with the right button and select with the left button.

But since i started using xnview mp 3 versions ago (i think it was v0.56)
both the right and left buttons have been set to Make selection,
that seams redundant, maybe it's just an oversight or is there another reason?

I also noticed that in the Image > Filter window,
when one puts a [checkmark] in the Preview option and zooms in on a picture,
the right mouse button moves the picture,
so for concistency it's probably better to have the default behaviour for
the right button set to Move picture everywhere.

The first thing i do after installing xnview mp is to change the right button to move picture.