capi bounding rects question

I'm building a simple 2D graphics editor using CAPI, using pinboard objects.  
I noticed that many of my objects would not redraw correctly.  For example, 
when I slide (using the mouse) another window over the pinboard and slide it 
off, the damaged objects sometimes do not redraw at all.  It looks like I'm 
not causing the bounding rect of capi objects to be set correctly, or that I 
have a basic misunderstanding somewhere.

Here's a lump of code that appears to demonstrate my problem (a slightly 
modified version of example code in the manual).  Test1 and test2 cause two 
3x3 grids of rectangles to be drawn.  The test2 rectangles are the size of 
the "bounding box" (to my current understanding) and the test1 rectangles are 
slightly smaller.  If you grab, say, the Listener window and slide it slowly 
over these two windows, the test1 rectangles redraw, whereas the test2 
rectangles lose parts of their sides (sometimes).  I get the same results in 
LWW and LWL (4.2.7).  I expected the test2 rectangles to redraw fully (since 
their edges are coincident with the "bounding box" and the damage 
rectangles).

Can anyone spot the bug in my understanding or my code?

thanx
pt


(defclass small-rectangle (capi:drawn-pinboard-object)
  ((id :accessor id :initarg :id :initform nil))
  (:default-initargs
   :display-callback 'draw-small-rectangle
   :visible-min-width 30
   :visible-min-height 30))
   

(defun draw-small-rectangle (gp pane x y w h)
  (capi:with-geometry pane
    (gp:draw-rectangle gp
                       (+ 1 capi:%x%)
                       (+ 1 capi:%y%)
                       (- capi:%width% 2)
                       (- capi:%height% 2))))

(defclass big-rectangle (capi:drawn-pinboard-object)
  ((id :accessor id :initarg :id :initform nil))
  (:default-initargs
   :display-callback 'draw-big-rectangle
   :visible-min-width 30
   :visible-min-height 30))
   

(defun draw-big-rectangle (gp pane x y w h)
  (capi:with-geometry pane
    (gp:draw-rectangle gp capi:%x% capi:%y%
                       capi:%width% capi:%height%)))

(defun test1 ()
  (capi:contain
   (make-instance 'capi:grid-layout
                  :description (loop for i below 9
                                     collect
                                     (make-instance 'small-rectangle))
                  :columns 3)))

(defun test2 ()
  (capi:contain
   (make-instance 'capi:grid-layout
                  :description (loop for i below 9
                                     collect
                                     (make-instance 'big-rectangle))
                  :columns 3)))