def roll_buffers
    bufs = rollable_buffers
    bufs.last.force_to_top = false
    raise_to_front bufs.first
  end
def roll_buffers
    bufs = rollable_buffers
    bufs.last.force_to_top = false
    raise_to_front bufs.first
  end
we reset force_to_top when rolling buffers. this is so that the
 human can actually still move buffers around, while still
 programmatically being able to pop stuff up in the middle of
 drawing a window without worrying about covering it up.

 if we ever start calling roll_buffers programmatically, we will
 have to change this. but it's not clear that we will ever actually
 do that.