[plt-scheme] get-text-extent problem

Hi,

I'm using get-text-extent to determine the width a string to be
included in a snip,

(define/override get-extent
      (lambda (dc x y w h descent space lspace rspace)
        ;(for-each (lambda (b) (when (box? b) (set-box! b 0)))
        ;           (list descent space lspace rspace))

        (let-values (((width height b s) (send dc get-text-extent
(format "~A" my-string))))
          (when (box? w) (set-box! w width))
          (when (box? h) (set-box! h height)))))

The problem is the width is wider when get-extent calls
get-text-extent than when get-text-extent is called by the (snip)draw
method.

The problem seems to be related to  spaces; I've attached a simple
example - use click-drag to select multiple snips.

Any advice appreciated,

Thanks,

Stephen
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