Fluxbox

I have my certain keys bound to moving windows around the screen. When I
move a window off the screen using these keys, the window does not move to
the next desktop.

As an example, say leftwards movement is bound like this:

~/.fluxbox/keys:
Mod1 Control h : Move -8 0

Then when I hold down Alt+Ctrl+h the window will move left 8 pixel at a
time. When the window moves off the screen, it won't transfer to the next
desktop, even if the window travels all the way off the screen.