Anomalous drawing whilst scrolling :without-bar (XP, Vista)

Hi again, fellow LispWorkers,

The test code below demonstrates a problem I am having with my application,
relating to :without-bar scrolling (I am only
working on WinXP currently). Notice that the top pane, when scrolled, seems to
'bleed' the just-exposed region. With
clipped drawing the problem does go away, but even though my actual application
draws only to the just-exposed region
the problem still occurs. I am just wondering what type of problem this might
be, as I've been fighting with it for a few
weeks, to no avail. Is it simply an issue relating to the amount of data being
drawn? I have known (and would expect)
unoptimized drawing to produce slow scrolling, but not an effect like this. Any
thoughts? 

I have tested this on two XP boxes, and a Vista laptop, with the same results.
My app draws waveforms, and the code
below is a very, *VERY* crude approximation to my actual drawing code, which
does quite a bit of pre-drawing calculation. I get
the problem when combining scrolling my waveform data pane simultaneously with a
ruler, which measures time, placed above it
within a larger grid-layout. The waveform exhibits the identical effect
demonstrated here, but interestingly - and oddly - if
the ruler is placed below the waveform the waveform scrolls perfectly, but the
ruler is then affected.

(defpackage :test
  (:add-use-defaults t)
  (:use :capi))
  
(in-package :test)

(progn
  (defparameter *max* (make-array 2048
                                  :initial-contents
                                    (loop repeat 2048
                                          collect (+ (random 150) 150))))
  (defparameter *min* (make-array 2048
                                  :initial-contents
                                    (loop repeat 2048
                                          collect (random 150))))
  nil)

(define-interface test  ()
  ()
  (:panes
    (pane-1 output-pane
	    :scroll-width 2048
	    :horizontal-scroll :without-bar
	    :visible-max-height 300
	    :display-callback 'draw-data)
    (pane-2 output-pane
	    :scroll-width 2048
	    :horizontal-scroll t
	    :scroll-callback 'on-scroll
	    :visible-max-height 300
	    :display-callback 'draw-data)) ; Clipping here removes the problem.
  (:layouts
    (main-layout column-layout
	         '(pane-1 pane-2)
	         :default t))
  (:default-initargs
    :best-width 800
    :best-height 630))
	
(defun draw-data (pane x y width height)
  (declare (optimize (speed 3) (safety 0) (fixnum-safety 0))
           (ignore x y width height))
    (loop for i fixnum from 0 below 2048
          for min fixnum across *min*
          for max fixnum across *max*
	  append (list i min i max) into coords
	  finally (gp:draw-lines pane coords)))

; This does clipped drawing.
(defun %draw-data (pane x y width height)
  (declare (optimize (speed 3) (safety 0) (fixnum-safety 0))
           (ignore y height))
    (loop for i fixnum from x below (+ x (1- width))
          for min fixnum = (aref *min* i)
          for max fixnum = (aref *max* i)
	  append (list i min i max) into coords
	  finally (gp:draw-lines pane coords)))
		  
(defun on-scroll (pane dimension operation value
		  &key &allow-other-keys)
  (declare (ignore dimension operation value))
  (with-slots (pane-1) (top-level-interface pane)
    (set-horizontal-scroll-parameters pane-1
      :slug-position (get-horizontal-scroll-parameters pane :slug-position))))
	  
(defun test ()
  (contain (make-instance 'test)))
  
(compile 'draw-data)
(compile 'on-scroll)
(compile 'test)