two-pixel trace left when move overlay

[Drew Adams]

I have an overlay in buffer *Completions* to highlight the current candidate
during cycling of candidates in the minibuffer. I use this code to
reposition the overlay:

(if (symbol-value overlay)
      (move-overlay (symbol-value overlay) start end buffer)
    (set overlay (make-overlay start end buffer))
    (overlay-put (symbol-value overlay) 'face face))

In Emacs 20, 21, and a CVS snapshot of June 26, 2005, this works OK. In a
CVS snapshot of 2006-03-20, when I move the overlay, a two-pixel (I believe)
trace is left behind. See attached screenshots.

bug-emacs-22-Completions-pixel.jpg
Description: JPEG image

bug-emacs-22-Completions-pixel-closeup.jpg
Description: JPEG image