[plt-scheme] A GUI/font puzzle

Thanks for tracking this down! It's now fixed in SVN.

The bug was in installing a font (either explicitly or implicitly by
`get-text-extent') when the most recent drawing was in smoothed mode.
The mangled text-size information survived across runs within DrScheme,
because `get-text-extent' installs its results in an internal cache
that is used by future calls to `draw-text' and `get-text-extent'.

At Sun, 25 Jan 2009 02:25:31 +0100, Jakub Piotr Cłapa wrote:
> In the good tradition of posting puzzles I have something funny to 
> present. Does not have much to do with denotational semantics but took 
> from me a little more the ten hours nevertheless. ;-)
> 
> Try the following program on Windows (but also read the notes below the 
> code):
> #lang scheme/gui
> (define my-canvas%
>    (class canvas%
>      (super-new [min-width 100]
>                 [min-height 100])
> 
>      (inherit get-dc)
> 
>      (define/override (on-paint)
>        (let* ([txt "Starting"]
>               [backbuf (make-object bitmap% 100 100)]
>               [back-dc (new bitmap-dc% [bitmap backbuf])]
>               [value-font (make-object font% 18 "Verdana" 'default 
> 'normal 'bold #f 'smoothed)])
>          (send back-dc set-background (make-object color% 255 255 255)) 
> ; may be left out
>          (send back-dc clear) ; may be left out
>          (send back-dc set-smoothing 'smoothed) ; critical
>          (send back-dc draw-rectangle 0 0 2 2) ; critical
>          (send back-dc set-smoothing 'unsmoothed) ; may be left out
>          (send back-dc set-font value-font) ; critical
>          (send back-dc get-text-extent txt) ; critical
>          (send back-dc draw-text txt 0 0)
> 
>          (send (get-dc) draw-bitmap backbuf 0 0)))))
> 
> (define frame (new frame% [label "Font test"]))
> (define canvas (new my-canvas% [parent frame]))
> (send frame show #t)
> 
> 
> 
> Some notes:
> 
> - the problem is with font kerning on Windows only (does not seem to
>    depend on the font)
> - it manifests itself at least in 4.1.2 and 4.1.4 (but probably in other
>    versions too) on several different WinXP machines (1 VirtualBox and 2
>    physical)
> - if you remove any of the lines labeled "critical" the problem
>    disappears
> - once you run it in a DrScheme session it will make the font look bad
>    (or good) for all future runs (even if you change the code)
>    [this means the error (or the lack of it) persists between custodian
>     shutdowns]
> - the most impotant operation seems to be the call to get-text-extent
>    before the call to draw-text (adding another draw-text before
>    get-text-extent fixes the bug)
> - this is probably the best testcase I have ever done (I found this bug
>    in a 2KLoS multithreaded program ;)
> 
> -- 
> regards,
> Jakub Piotr Cłapa
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