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

The drawing model for editors is that an editor is allowed to draw
directly to the canvas, without going through `on-paint`, and typically
updating only a part of the canvas.

To draw a background behind the editor content, the intent is that you
use `on-paint` at the editor level (i.e., in `pasteboard%`). It's
called both before and after the editor content is drawn.

At Mon, 08 Sep 2014 22:17:07 +0400, Dmitry Pavlov wrote:
> 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
> 
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users