[racket] Text snip bleaches canvas space that does not belong to it

Hello,

I need a text snip on a canvas that paints itself strictly in some defined WxH bounds.
I need no interactive moving or resizing, only interactive editing.
Here is a code I have come up with, which also displays a window with the snip and a grid.
The grid facilitates the visual check of the text snip bounds.

(require racket/gui
          racket/draw)

(define my-pasteboard%
   (class pasteboard%
     (super-new)
     (define/augment (can-select? snip on?)
       #f)
     (define/augment (can-interactive-move? snip)
       #f)
     (define/augment (can-interactive-resize? snip)
       #f)))

(define frame (new frame% (label "Test")))
(define pasteboard (new my-pasteboard%))

(define my-canvas%
   (class editor-canvas%
     (super-new)
     (define/override (on-paint)
       (super on-paint)
       (define dc (send this get-dc))
       ;; Draw a 10x10 grid with cell size 20x20 pixels
       (for ((x (in-range 10))) (send dc draw-line (* x 20) 0 (* x 20) 200))
       (for ((y (in-range 10))) (send dc draw-line 0 (* y 20)  200 (* y 20)))
       )))

(define editor-canvas
   (new my-canvas%
        (editor pasteboard)
        (vertical-inset 0) (horizontal-inset 0)
        (parent frame)
        (style '(no-border no-hscroll no-vscroll))))

(send frame resize 200 200)
(send frame show #t)

(define text-obj (new text%))
(send text-obj insert "Hello" 0)
(define text-snip (new editor-snip%
                      (editor text-obj)
                      (left-margin 0) (right-margin 0) (top-margin 0) (bottom-margin 0)
                      (left-inset 0) (right-inset 0) (top-inset 0) (bottom-inset 0)
                      (with-border? #f)
                      (min-width 40)
                      (max-width 40)
                      (min-height 20)
                      (max-height 20)))

(send pasteboard insert text-snip 40 60)
(send pasteboard set-caret-owner text-snip)


When I click on the snip in the window and try to add a "!" character after
the "Hello" text, I get this:

http://imgur.com/cikwlqh

It is obvious that the text snip (or something associated with it)
expanded its supposed area of rendering by 3-4 pixels in all directions.
And if I do silly things, like pressing "Enter", things get even worse.
Can that be avoided somehow?


Best regards,

Dmitry