[racket] 2htdp/image: Silent cropping?

Hi all,

A student of mine noticed yesterday that part of an image was getting
truncated. Some further experimentation in BSL yielded this example:

*(ellipse 5050 100 'solid 'blue)* ; => image with 50 pixels missing at
right end
*(ellipse 100 5050 'solid 'blue)* ; => image with 50 pixels missing at
bottom

For both images, the *image-{width,height}* functions produce 100 and 5050;
the library simply doesn't draw the entire ellipse, although click-and-drag
highlighting does show the image as having 50 extra blank pixels at the end.

I can't find documentation of the size limits. Is this intended behavior?
If so, an error might be more student-friendly.

Cheers,
Jordan
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