Viewport edge flip

[Josh Babcock <jbabcock atlantech net>]

I have viewport edge flip enabled, which is one of my favorite features
in a WM, but there is one thing missing. I have only been able to
configure it to wait a set number of ms before flipping the viewport,
but what I really want is for the viewport to flip after I have moved
the cursor x pixels out of the current viewport. I remember there being
a feature like this in Afterstep, but I haven't found a way to do it in
Sawfish. Any ideas?

Thanks,
Josh