When I drag the file from one xnview window (instance) to another, when I drag it over the folder thumbnail it's moved, when I drag it over the empty area it's copied. Sometimes it's copied even if I drag over the folder thumbnail. Sometime it doesn't work at all - the same as
here.
When dragging the file from the same disk, the standard Windows behaviour is to move, so it should be the same in xnview too IMO, but definitely it should be the same, at least (always move, or always copy, if not using ctrl/shift).
...Now I see how important and useful would be a dual pane browser.

It's not just because of issues like this, but it would be also much faster.
XnView 1.97, Windows Vista SP2.