Dual monitor and xrand: cannot move windows to the other display

Both master@d9ea6f0 and next@fc43778 exhibit this behaviour:

.xinitrc:
...
xrandr --output DVI-0 --auto --output VGA-0 --auto --left-of DVI-0 &
exec wmaker

Both display come up correctly, with docks on the left and nothing but the
background image on the right, but when i drag one window from the left
monitor to the right one, at first the window starts showing on the right
display but when i reach a position (say > 25% of display width) the window
wraps around and comes back on the right monitor.
Seems like WindomMaker thinks around 25% of the right monitor width there's
the edge of the workspace and when i drag the window past that point, it
wraps it around.

Did anyone meet this behavirour before?
Is it a bug or a misconfiguration on my part?
-- 
bye,
p.