In manual window placement mode, the window is nailed
down when I left-click, but the ensuing mouse movements
and button-release events are not eaten.  This means
that when I try to place a window over, say, a terminal
window, the terminal window (in my case generally an rxvt
but I don't think it matters) sees mouse movement with
the button pressed and converts it into selection.  This
doesn't happen with other window managers.